Lead-free Design Guide

Section Number

Section Title Status Review Date

1 Introduction For final Review2 Document Scope Finished3 Background For comment 3.1 General Solder Alloy Characteristics For comment 3.2 Tin Whiskers For comment 3.3 Silver Dendrites For comment 3.4 Manufacturing Processes (Background) For comment

Soldering Processes – Through Hole For comment Soldering Processes SMT For comment Rework For comment

3.5 Substrate Issues (Background) For comment De-lamination For comment SIR and Dendrites For comment CAF For comment Substrate Passivation (ENIG) For comment Substrate Passivation (ENIPIG) For comment Substrate Passivation (HASL) For comment Substrate Passivation (OSP) For comment Substrate Passivation Immersion Silver For comment Substrate Passivation Immersion Tin For comment Cleaning For comment Conformal Coating Needs words + expansion

3.6 Component Selection For comment Material issues For comment Device lifetime For comment Obsolescence issues For comment

3.7 Component Storage For comment 3.8 The Supply Chain For comment words4 The Application For comment 4.1 Control Level Classification For comment but needs additions 4.2 Classification Control Level 1 For comment but needs additions 4.3 Classification Control Level 2a For comment but needs additions 4.4 Classification Control Level 2b For comment but needs additions 4.5 Classification Control Level 2c For comment but needs additions 4.6 Classification Control Level 3 For comment but needs additions5 Equipment Design 5.1 Architecture Needs additional words 5.2 Technology N/A 5.3 Component Qualification Needs words6 Manufacturing For High Reliability N/A 6.1 Tin Whisker Self Mitigation For comment

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6.2 Manufacturing Processes For comment 6.3 Supply Chain Control For comment 6.4 Obsolescence Management For comment?7 Design Process Flow Needs inputs8 Terms and Definitions To be updated9 Abbreviations To be updated10 References To be updated11 Appendix 1 To be completedSection Number

Section Title Status Review Date

1 Introduction For final Review2 Document Scope Finished 16/07/143 Background For comment 3.1 General Solder Alloy Characteristics Requires rework 3.2 Tin Whiskers For comment 3.3 Silver Dendrites For comment 3.4 Manufacturing Processes (Background) For comment

Soldering Processes – Through Hole For comment Soldering Processes SMT For comment Rework Needs additional words?

3.5 Substrate Issues (Background) Needs additional words De-lamination For comment SIR and Dendrites For comment CAF For comment Substrate Passivation (ENIG) For comment Substrate Passivation (ENIPIG) For comment Substrate Passivation (HASL) For comment Substrate Passivation (OSP) For comment Substrate Passivation Immersion Silver For comment Substrate Passivation Immersion Tin For comment Cleaning For comment Conformal Coating Needs words + expansion

3.6 Component Selection For comment Material issues Needs more words Device lifetime For comment Obsolescence issues For comment

3.7 The Supply Chain Needs additional words4 The Application N/A 4.1 Control Level Classification For comment but needs additions 4.2 Classification Control Level 1 For comment but needs additions 4.3 Classification Control Level 2a For comment but needs additions 4.4 Classification Control Level 2b For comment but needs additions 4.5 Classification Control Level 2c For comment but needs additions 4.6 Classification Control Level 3 For comment but needs additions5 Equipment Design N/A

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5.1 Architecture Needs additional words 5.2 Technology N/A 5.3 Component Qualification Needs words6 Manufacturing For High Reliability N/A 6.1 Tin Whisker Self Mitigation For comment 6.2 Manufacturing Processes Needs words 6.3 Supply Chain Control Needs words 6.4 Obsolescence Management Needs additional words?7 Design Process Flow Needs words and chart8 Terms and Definitions To be updated9 Abbreviations To be updated10 References To be updated11 Appendix 1 To be completed

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Lead-free Design Guide

Contents1. Introduction................................................................................................................................5

2. Document Scope......................................................................................................................8

3. Background...............................................................................................................................9

3.1. General Pb-Free Solder Alloy Characteristics................................................................9

Processing Temperature...............................................................................................9

Microstructure...............................................................................................................10

3.2. Tin Whiskers......................................................................................................................11

3.3. Silver Dendrites.................................................................................................................15

3.4. Manufacturing Processes................................................................................................17

Background...................................................................................................................17

Soldering Processes – Through Hole Technology (THT).......................................17

Soldering Processes – SMT........................................................................................19

Re-work..........................................................................................................................19

3.5. Substrate Issues................................................................................................................20

Background...................................................................................................................20

De-lamination................................................................................................................20

Surface Insulation Resistance and dendrite growth................................................20

Conductive Anodic Filaments (CAF)..........................................................................21

Substrate Passivation..................................................................................................21

Cleaning.........................................................................................................................25

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Conformal Coating........................................................................................................25

3.6. Component Selection.......................................................................................................26

Material Issues..............................................................................................................26

Device Lifetime.............................................................................................................26

Obsolescence Issues...................................................................................................26

3.7. Component Storage..........................................................................................................27

3.8. The Supply Chain..............................................................................................................28

4. The Application.......................................................................................................................29

4.1. Control Level Classification.............................................................................................29

4.2. Classification Control Level 1..........................................................................................29

4.3. Classification Control Level 2a........................................................................................30

4.4. Classification Control Level 2b........................................................................................30

4.5. Classification Control Level 2c........................................................................................31

4.6. Classification Control Level 3..........................................................................................31

5. Equipment Design..................................................................................................................32

5.1. Architecture........................................................................................................................32

Multiple channels and voting?.....................................................................................32

5.2. Technology.........................................................................................................................32

Hybrid Implementation.................................................................................................32

Conventional Packaged Components.......................................................................32

Commercial Off The Shelf (COTS) Assemblies.......................................................32

5.3. Component Qualification..................................................................................................32

6. Manufacturing for High Reliability.........................................................................................33

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6.1. Tin Whisker Self-Mitigation:.............................................................................................33

6.2. Manufacturing Process.....................................................................................................33

Storage Prior to Manufacture......................................................................................33

Specialist Build – Hybridisation...................................................................................33

Soldering – THT............................................................................................................33

Soldering – SMT...........................................................................................................33

PCB Surface Finish......................................................................................................33

6.3. Supply Chain Control........................................................................................................34

6.4. Obsolescence Management............................................................................................37

7. Design Process Flow..............................................................................................................39

8. Terms and Definitions............................................................................................................40

9. Abbreviations...........................................................................................................................41

10. References..............................................................................................................................42

11. Appendix 1...............................................................................................................................44

Collation of Suggested Best-Practice..............................................................................................44

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Lead-free Design Guide

1. Introduction[1.1.] This guide is intended to aid Electronics and Systemsall Design design engineers

(e.g. System, Hardware, Circuit, Component, and Manufacturing) in the design of high reliability electronic circuits and assemblies that make use of Lead-free materials, components and processes.

1.1.[1.2.] It is here assumed that the designers themselves are competent in all aspects of circuit and or system design, but that they may not be fully conversant with the detailed vagaries of Lead-free (LF) manufacturing processes, materials or components or how these impact on equipment reliability or longevity.

[1.3.] Whilst a great deal of historical knowledge exists relating to Tin/Lead based systems and circuits, the rapid evolution of the electronics industries towards Lead-free has, virtually at a stroke, removed changed that experience base.

1.2.[1.4.] The impact of Lead-free technology, when compared with Tin/Lead processes and materials, is most apparent in the following areas. Note that this is not a comprehensive list and that these areas are listed in no particular order of importance: High manufacturing process temperature are usually required for LF

manufacturing processes and this increase in thermal stress can introduce significant component degradation and use-life issues.

Generally the poor wetting characteristic of Lead-free solders has resulted in the need to use more active (acidic) fluxes. select flux compatible with the Pb-free solder and the cleaning process if needed

In order to properly activate the appropriate fluxes a more stringent control of the processing temperatures is required. OR Lead-free solders have higher melting temperatures requiring increase solder process temperatures with more stringent process temperature controls being required.

More active flux residues that are more difficult to clean off. OR Fluxes and flux residues for Lead-free solder processes require improved cleaning chemistries/procedures/processes. Subsequently, the compatibility of flux and cleaner needs to be checked as well.

The effect of the cooling cycle on the joint crystallography is enhanced.

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More brittle solder joints which have a much shorter use-life than that expected from the use Tin/Lead solder. This is particularly the case for circuits used in the more demanding mixed thermal and mechanical environments.

Joint embrittlement tends to increase with time ultimately resulting in an open circuit joint failure due to brittle fracture. Note that this is an additional dominant failure mode over the creep mechanism that occurs with Tin/Lead solders.

The solder joints grow (Tin) whiskers.

A significant number of LF components have Tin passivated leads which over a long product lifetime will grow (Tin) whiskers if subjected to certain conditions (e.g. high heat, humidity, localized static stress, etc.).

The incompatibility material and strength properties of some LF materials can result in lower lifetime joints, poor thermal / mechanical performance and / or the earlier onset of whisker growth.

The printed circuit board (PCB) substrate thermal characteristics create greater joint stress at the LF process temperatures. OR Greater stress due to higher lead-free soldering process temperature results in the need to select appropriate PCB (e.g. materials) to withstand the temperature gap (vs. Sn-Pb process). Do we add something on considering PTH reliability and failure modes change from SnPb to LF. The second choice is provided based on Catherine’s dis-agreement with the original statement.

The quality of the substrates has degraded and in a large minority of cases failure due to Conductive Anodic Filaments (CAF) is now apparent. OR The increased process temperatures of Lead-free solder processing and the current state of laminate material characteristics increase the potential of having Conductive Anodic Filament (CAF) failures on printed circuit assemblies. AJR follow-up: Soften it up….refer to CAF section, discuss approaches

Lack: an increase of the tombstoning phenomena has been observed with lead-free solder. Appropriate pad design and improvement of assembly process can decrease it. (Dale Lee comment 16 June 2015: The reverse of this is true. We less tombstone issues with lead free solder due to paste range during reflow process with all other thing being equal).

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Lack: Lead-free assembly create more voids than leaded solder. These voids can lead to the solder joint fissure and/or decrease the thermal dissipation. (Dale Lee comment 16 June 2015: With proper soldering process profile development, voiding is similar to tin/lead.)

1.3.[1.5.] The normal evolutional drivers of the electronics industries, commercial pressure and political expediency, has also contributed in making the design of high reliability equipment much more demanding. Whilst this trend continues, the interactions between components and materials need more than ever to be considered holistically. OR [“Genoa” proposed change] Above all the transition to Lead free technology, mainly achieved via Tin based alloys which have been known for decades, is not a total revolution, but therefore we have to face with both:

The re-apparition of old failure mechanisms (which have historically disappeared with improvement of processes and materials)

The apparition of new failure mechanisms linked to the new Lead free technology limits

As today there is no real Lead free field experience return for harsh environment, it is important to remind the main potential failure mechanisms that can occur and to introduce or propose adequate mitigation solutions, guidance or best practices that can be used during: design phases

selection of components, material, PCB

implementations of the assembly process

1.4. to reduce the risks of Lead free electronics used in ADHP products (Aerospace,Defense and High Performance

1.5.[1.6.] This document is intended to facilitate the implementation of a justifiable design process.

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2. Document Scope [2.1.] This document identifies those processes that may be used for designing

electronic equipment and in mitigating the potential deleterious effects of Lead-free materials, processes etc. within electronic systems

2.1.[2.2.] This document is applicable to Aerospace, Defence and High Performance (ADHP) electronic applications and which includes equipment that may have historically contained Tin/Lead materials and made use of the associated processes but now have, or are about to be, migrated to a Pb-free status.

[2.3.] In addition to design guidance, Tthe guidelines contained herein may be used by ADHP manufacturers (at all levels) Original Equipment Manufacturers (OEMs) and / or maintenance facilities to develop and implement the methodologies they have chosen to useneeded to assure the performance, reliability, airworthiness, safety, and / or certifiability of their products, in accordance with associated performance specifications/standards. Document GEIA-STD-0005-1, reference 1. [Mods per Jeff Rowe recommendation.]

2.2.[2.4.] This document, in part, is based on contains lessons learned from previous experience with Pb-Free systems in a variety of applications. This experience gives specific references to solder alloys and other materials, and their expected applicability to various operating environmental conditions. They are intended for guidance only and cannot be considered guarantees of success in any given application.

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The following is suggested for replacing the current Introduction and Scope sections of the design guide. It removes much of the detail information that was included for various issues. That detail should be moved to appropriate sections of the guide. The proposed text focuses on the “why, what and who”. The listed key areas can become the basis for the following sections of the document. Section 3 Background should be renamed. (Provided by Jeff Rowe.)

The European Union’s Restriction on Hazardous Substances (RoHS) legislation, which went into effect July 2006, has had a profound impact on the electronics industry. One of the restricted substance, lead (Pb), is commonly used in alloys with tin for component finishes, PWB finishes and solders. While a great deal of historical knowledge exists relating to tin-lead based electronic systems and products, the rapid transition of the electronics industries towards lead-free has, virtually at a stroke, changed that experience base.

Pb-free materials directly impact product performance, reliability and service life in many ways. There have been numerous, documented failures of electronics due to Pb-free materials in both commercial and Aerospace, Defense and High Performance (ADHP) products. The failure mechanisms for Pb-free materials are significantly different than tin-lead based materials. Some failure mechanisms have not been experienced before. The ADHP industry has tried to avoid the use of Pb-free materials to the extent possible but the cost for doing so is increasing for product design, materials and manufacturing. Eventually it will be cost prohibitive to remain tin-lead based.

The design guide serves to address the following key areas affecting performance, reliability and service life and offers guidelines on dealing with the risks.

Pb-free solders and solder joints Tin Whiskers Printed Wiring Board (PWB) defects Product qualification Manufacturing processes Supply chain control Obsolescence management

Background information will be provided for each of the areas to give the designer a basic understanding of the underlying issues. References are included for those designers looking for more in-depth understanding. The design guide includes recommendations for addressing the limitations and risks associated with Pb-free materials.

The intent is to assist design engineers in developing electronics that are completely Pb-free and meet the demanding requirements of ADHP systems and products. It is assumed that the design engineers using this guide are competent and experienced with tin-lead based electronics but may not be fully conversant with the detailed vagaries of Pb-free materials, components or manufacturing processes or how these impact equipment reliability and longevity.

ADHP systems and products have a broad range of performance requirements, operating environments and service life. The guidelines presented in this document may not provide solutions for all products. Currently, not all Pb-free material risks have solutions that reduce the risk to acceptable levels. On-going research targeted for the ADHP industry and experience with Pb-free materials in commercial and industrial products continue to improve the knowledge base.

Lead-free Design Guide

3. Background

Add some introductory remarks here about the intent of this section, e.g. may include some manufacturing experiences (but not all inclusive) and how they may be impacted by design. Provided solely as information to the designer. ALSO: ED. NOTE: Consider consolidating these next sub-sections into short paragraphs that summarize the “delta” of CAF between SnPb and pb-free.

3.1. General Pb-Free Solder Alloy Characteristics3.1.1. The Tin-Silver-Copper (SAC) and Tin-Copper (Sn-Cu) Pb-free alloys have

generally been found to be stronger and more creep resistant than Tin-Lead (Sn-Pb) alloys [3] but conversely exhibit an additional failure (wear-out) mechanism, that of brittle fracture. OR Tin-Lead alloys differ greatly from Pb-free materials with respect to material and physical properties. These changes manifest themselves in behavior under such service/environmental conditions such as thermal cycling, mechanical shock, vibration, tensile strength and shear strength.

3.1.2. Lead-free solders tend not to ‘wet’ as well as the traditional Sn-Pb alloys requiring a more active (acidic) flux to be used in the soldering process. (Dale Lee comment 16 June 2015: “I don’t agree with this statement, lead free still wets well, depending on attachment finish, it does not spread like Sn-Pb.”) OR Likewise, Tin-Lead and Pb-free materials will differ in manufacturing/production process behaviors, e.g. wetting, reflow, etc.

[3.1.3.] Whilst in-service Lead-free solders, in assembly, have the propensity to grow Tin whiskers in certain use environments. (M. Miller/Polina input 8-20-2015 WE NEED DAVE HILLMAN TO COMMENT ON THISWhilst in-service Lead-free )solders have the propensity to grow Tin whiskers.

[3.1.4.] The melting point of the heritage Sn-Pb eutectic alloy solder is 183 °C (361 °F) whilst that of some Pb-free solders (SAC family of solders for example) is can be typically 30+°C higher (221°C to 227°C). Other families of Pb-free solders can have different melting point ranges as well.

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Processing Temperature 9/2/2015 LEFT OFF HERE[3.1.5.] This The higher melting temperature of Pb-free alloys results in a typical 30 to

40 ºC (54 to 72 °F) increase in the required processing temperature *(or higher temperatures depending on type of soldering process or lead free alloy ) as compared to that used with Sn-Pb alloys. [*Parenthetical addition recommended by Dale Lee on 16 June 2015.]

3.1.3.[3.1.6.] The melting points of the common eutectic solder alloys are collated in the table (1) below.

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Table 1. Melting Points of Sn-Pb and some Pb-free Alloys

Solder Alloy Proportions Melting point temperature

Reference

Sn-Pb 63 - 37 183 °C (361 °F) 10

SAC Sn - 3.5Ag - 0.9Cu 217.2 ± 0.2 °C (423 ± 0.36 °F)

8

Sn-Cu Sn-0.7Cu 227 °C (441 °F) , 7

Sn-Ag Sn-3.5Ag 221 °C 7

Sn 100% 231.9 °C (449.4 °F)

Table 1: Melting points.

Microstructure3.1.4.[3.1.7.] The Pb-free alloy microstructure differs substantially from the

lamellar/colony structure of eutectic Sn-Pb.

3.1.5.[3.1.8.] The microstructure of Sn-Ag and Sn-Ag-Cu is comprised of relatively large β phase Sn dendrites, in-between which there are lamellar arrays of β-Sn, Ag3Sn and Cu6Sn5 Error: Reference source not foundphases.

In some studies large Ag3Sn platelets have been observed. The solidification behavior strongly influences the solid microstructure. The Sn-Pb eutectic solder joint requires only 2ºC of undercooling to begin the solidification of the Pb onto the Cu or Ni substrate. In contrast, the eutectic Sn-Ag-Cu system begins solidification with the formation of the Ag3Sn. Unfortunately, the presence of Ag3Sn does not facilitate the nucleation of the β-Sn and significant under-cooling can occur. An undercooling of 18 °C (32 °F) was reported for β-Sn. The formation of large Ag3Sn intermetallic plates in liquid Sn was observed during slower solidification rates for SAC alloys having 3.5 and 3.8 wt% Ag and not with the 3.0 wt% Ag alloy. These plates are expected to change the mechanical response of the system. The Ag3Sn intermetallic is not brittle,

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and the plate may stop or re-direct the crack. If the plate is in the same direction of the shear load, life can be reduced, but it is not common to see plates oriented parallel to the PCB or piece-part pads. The presence of Ag3Sn plates is of greater concern for flip chip and wafer scale chip pack solder joints. The volume fraction of β-Sn dendrites in the solidified solder is dependent upon cooling rate and alloy composition. The grain size of the β-Sn is relatively large with respect to the solder joint size. A BGA solder joint can be comprised of as few as 10 to 30 β-Sn grains and even fewer for wafer level chip scale package and flip chip joints. Since dispesed intermetallics in a SAC alloy tend to increase the hardness and stiffness of the solder, a greater volume fraction of Sn dendrites generally results in a solder joint with decreased stiffness. Reduced solder stiffness can be beneficial in some high stress shock applications because the solder does not impart as much stress on the pad intermetallic or the pad laminate interfaces. Presently, some investigators are evaluating SAC alloys with reduced Ag and Cu content (SAC-L) in an effort to obtain improved drop shock performance of BGA assemblies. Unfortunately, the melting temperatures of SAC-L alloys are greater than the traditional SAC alloys and their thermal cycling characteristics require evaluation.

The three main Pb-free solders are based on the Tin rich Sn-Cu, Sn-Ag or the Sn-Ag-Cu (SAC) families of alloys. Sometimes small alloy additions of Ni, Ge, In, and Sb, are made to these basic alloys in an effort to alter dissolution, solidification, mechanical properties or wetting characteristics. The melting point of pure Sn is 231.9 °C (449.4 °F) and the addition of 37%Pb to the Sn reduces the melting temperature to the eutectic point of 183 °C (361 °F). Similarly, the addition of Ag and Cu to Sn reduces the melting temperature but not to the same extent as Pb. The Sn - 3.5Ag - 0.9Cu ternary eutectic SAC alloy melting temperature is 217.2 ± 0.2 °C (423 ±0.36 °F) [4], the Sn-0.7Cu eutectic alloy melts at a temperature of 227 °C (441 °F) [5], and the Sn-3.5Ag eutectic melts at 221 °C [5]. These Pb-free solder melting temperatures are considerably higher than Sn-Pb eutectic. The higher melting temperature of Pb-free alloys, results in a 30 to 40 ºC increase (54 to 72 °F increase) in processing temperature as compared to the temperatures used to process heritage Sn-Pb alloys. Higher melting temperatures result in increased amounts of base metal dissolution (see section 10.2 Copper Dissolution) and increased shrinkage stresses on components during cooling. An additional consideration is that the SAC alloys have generally been found to be stronger and more creep resistant than the heritage Sn-Pb solders at typical electronic use temperatures [2] [3].The Pb-free alloy microstructure differs substantially from (ED. NOTE. INCOMPLETE STATEMENT)

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3.2. Tin Whiskers3.2.1. Pure Tin is used as the material surface finish of choice by numerous electronic

component manufacturers for component leads, housings, cases etc. It appears in a variety of forms and finishes.

3.2.2. Tin is compatible with the vast majority of solders and thus is considered as a ‘safe option’ with respect to material compatibility by the component manufactures.

3.2.3. All types of Tin will eventually grow whiskers, although in some instants the whiskers will take longer to appear than in others exhibiting an initial dormancy period of up to several years.

3.2.4. There is no such thing as a whisker free Tin surface finish. Over a (typically) 30 year lifetime all Tin will grow whiskers though in the majority of cases these are expected to be small and thus would not impact on the associated circuit functionality.

3.2.5. The photograph in Figure 1 below shows a whisker of just under 2mm in length growing out of a component lead having a plating passivation of Tin with >4% Lead as measured by X-ray Fluorescence (XRF). The board of which this was a part was manufactured in 2002. It was then used extensively for testing before being stored in an uncontrolled laboratory environment until inspected and whiskers found in 2010.

3.2.6. A large minority of Tin whiskers will be long, that is ≥1mm in length, whilst a very few will be much longer. The NASA website (Reference 9) displays or records examples of whiskers the longest of which are currently up to >32mm long and still growing.

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Figure 1: A Tin whisker out of a 12 year old component termination having 4+% Lead (Figure courtesy of ???)

[3.2.7.] Although a vast amount of research has been undertaken over the past decade on the propensity of Tin to grow whiskers we still do not have a complete understanding of what drives their growth, their physical or other characteristics has not been established.

[3.2.8.] We do however believe It is an industry concensus that Tin whiskers grow as a stress release mechanism and that the stress causing this growth can be caused by one or more of the following: Internal compressive stress caused by the growth of intermetallic layers at

the interface of the Tin layer and base material or under-plate. External compressive stress caused by bending a Tin plated lead (or

termination) or for example fitting a Tin plated press fit connection, or similar. Corrosion produced stress. Shear stress caused by different temperature coefficients between materials

or even material grains of different sizes. Stress caused by and during the plating process and electrolyte additives or

contaminants. Something else as yet unknown.

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3.2.7.[3.2.9.] Whiskers grow from their base and whilst the force driving them is likely to come from a source below the nucleation point, at least in those cases exhibiting significant internal compressive stress, the material for the whisker comes from the plating around or to the side of the whisker growth site.

3.2.8.[3.2.10.] The dormancy time, that is the time before a whisker nucleates, can (is likely to) be several years. However if a particularly bad plating chemistry has been used then there can be virtually no dormancy period.

3.2.9.[3.2.11.] Whiskers can stop growing for long periods then start again at apparently a random interval. This can happen repeatedly.

3.2.10.[3.2.12.] The optimum temperature for whisker growth will depend in the driving force. High temperatures have been shown to inhibit growth rate, it can anneal the coating, whilst temperatures around 60oC have been shown to encourage growth. There is therefore no guaranteed whisker growth acceleration temperature that can used to test the reliably (justifiably) or qualify the performance of a Tin plated layer.

3.2.11.[3.2.13.] Although Tin is conductive a whisker will have an insulating Tin Oxide layer both at its tip and along the length. These oxide layers will be of different thicknesses relating to the age, dormancy time and growth rate of the whisker.

3.2.12.[3.2.14.] A mechanical whisker shorting of 2 electrical nodes of different relative potential will not therefore necessarily cause an immediate electrical short circuit.

3.2.13.[3.2.15.] The Tin oxide will break down only in the presence of a large enough voltage, a time element or an applied mechanical force.

3.2.14.[3.2.16.] The diameter of a Tin whisker can vary by several orders of magnitude. So along with varied whisker lengths the resistance of a ‘shorting’ whisker can also vary, typically this can be from a very few ohms to many 10s of ohms.

3.2.15.[3.2.17.] Electric fields will not affect the whisker growth rate but could at large field strengths attract the whisker causing it to bend.

3.2.16.[3.2.18.] Tin whiskers are very strongly attached unless growing from a corrosion center, so loose whiskers are not usually a problem.

3.2.17.[3.2.19.] The photograph figure 2 below shows a loose whisker which whilst easily long enough to cause a problem (>4mm long), it has not, so far, done so. Note that the picture is a 2-D representation and the whisker lies at a compound

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angle to the plane of the lens. The height (Z-axis) movement of the microscope adjusted to focus the two end allows a more accurate (longer) value of length to be calculated.

Figure 2: A loose Tin whisker in excess of 4mm in length (Figure courtesy of ???)

3.2.18.[3.2.20.] They are strong enough to grow through any (conformal) coating currently marketed.

3.2.19.[3.2.21.] A Tin whisker short circuit will ‘blow’ when sufficient current is passed to raise the whisker temperature sufficiently high. This can be at a few milli-ampers or several 10s of milli-amperes.

3.2.20.[3.2.22.] If a Tin whisker shorting 2 electrical nodes fuses, and the source impedance associated with the electrical nodes is low then a Tin plasma can form. This can happen at relatively low voltages and at normal atmospheric pressure.

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3.2.21.[3.2.23.] Tin whiskers therefore represent a significant threat to equipment reliability over a long term use-life.

3.3. Silver Dendrites3.3.1. The SAC solders can grow Silver dendrites. An example of this is shown as

Figures 3 and 4 below.

Figure 3: A SAC 305 solder joint with Silver dendrites (Figure courtesy of ???)

3.3.2. To enable dendrite growth an ionic contaminant, moisture and an electric field is required.

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3.3.3. The (active) flux residue can provide both the source of contamination and moisture. Growths of the dendrites can be very fast depending on field strength and the degree of contamination.

3.3.4. The dendrites exhibit a weak attachment strength and are conductive. They will break off easily and if long enough have the capability to introduce electrical short circuits to the associated equipment.

3.3.5. The joints captured in the pictures shown in figure 3 and 4 have only been powered up for the duration of a qualification test. This shows evidence of the very rapid dendrite growth rate.

Figure 4: Detail of a SAC 305 soldered joint growing Silver dendrites. (Figure courtesy of ???)

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3.4. Manufacturing Processes

Background3.4.1. The commercial manufacturing processes are more than adequate for the vast

majority of applications on which they are used.

3.4.2. It should be noted however that the move to Pb-free technology has made electronic equipment manufacture that much more demanding. See Section 3.6 on Component Selection.

3.4.3. CAUTION: The higher processing temperatures of most Pb-free solders have reduced the thermal window within which some components can be safely soldered. Designer needs to be aware of component selection and whether they are adaptable to a Pb-free soldering processes. Also, account for the potential for numerous reworks.

3.4.4. The commercial manufacturing processes are more than adequate for the vast majority of applications on which they are used.

[3.4.2.] It should be noted however that the move to Pb-free technology has made electronic equipment manufacture that much more demanding.

[3.4.3.] The higher processing temperatures of most Pb-free solders have reduced the thermal window within which components can be safely soldered.

[3.4.4.] In addition these higher temperatures exacerbate the problems already apparent with new generation packaging materials and flame retardants, [Dale Lee 6-16-15: What flame retardants is this in reference to? Halide Free ?] creating an increase in the potential for substrate de-lamination and package splitting (popcorning). [Dale Lee 6-16-2015: With lead free there is a decrease in MSL level sensitivity versus tin-lead. Generally this is an increase in sensitivity of 1 level so some components that were not MSL sensitive with tin-lead are with lead free.]

3.4.5. The need for improved cleaning of PCBs after population and of improved conformal coating application, where applicable, has similarly imposed stricter manufacturing process constraints. [Dale Lee 6-16-2015: This is true for tin-lead and lead free. No a lead free only DFM issue .] This would apply to SnPb soldering processes as well.

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3.4.6. It is essential that the manufacturing processes reflect the requirements of the application both in controlling the applied manufacturing stress and in the economic justification of the product.

3.4.7. In addition these higher temperatures exacerbate the problems already apparent with new generation packaging materials and flame retardants, creating an increase in the potential for substrate de-lamination and package splitting (popcorning).

[3.4.5.] The need for improved cleaning of PCBs after population and of improved conformal coating application, where applicable, has similarly imposed stricter manufacturing process constraints.

[3.4.6.] It is essential that the manufacturing processes reflect the requirements of the application both in controlling the applied manufacturing stress and in the economic justification of the product.

Soldering Processes – Through Hole Technology (THT)3.4.8.[3.4.7.] The main soldering processes associated with THT components. These

are:

a. Hand soldering

b. Wave soldering

c. Selective wave soldering

d. Pin in Hole (pin in paste) or intrusive reflow

e. Laser

3.4.9. Soft-beamThere are 4 main soldering processes associated with THT components. These are:

[a.] Hand soldering

[b.] Wave soldering

[c.] Selective wave soldering

[d.] Pin in Hole (pin in paste) or intrusive reflow.

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Hand Soldering3.4.10.[3.4.8.] Arguably the hand soldering option is the process that applies the least

stress to the equipment being manufactured, however it is also likely to be the least controlled. OR Hand soldering has the greatest degree of thermal variability and requires strict controls as it can severely damage components. It should be used only when special conditions apply (e.g. a water intolerant component, etc.) [ED. NOTE: This latter statement may need more work.]

Wave Soldering3.4.11.[3.4.9.] Wave soldering is widely used in commercial industry as an extrapolation

of Tin/Lead manufacturing processes but with different solder bath containers.

[3.4.10.] Since through hole components are normally shielded from the (solder) heat source by being on the opposite side of the boards, this could also be thought of as a low stress process however since heat is applied across the whole of the substrate in a progressive process the substrate will be subjected to high thermal stress.Since through hole components are normally shielded from the (solder) heat source by being on the opposite side of the boards, this could also be thought of as a low stress process however since heat is applied across the whole of the substrate in a progressive process the substrate will be subjected to high thermal stress.

3.4.12.

Selective Wave Soldering3.4.13.[3.4.11.] This is a relatively new process where a reduced length of solder wave

is applied to the parts of the circuit to be soldered. If applied properly this presents a much lower substrate stress, equivalent to that of hand soldering but with better repeatability of process. An example of this is robotic soldering.

3.4.14.[3.4.12.] Unfortunately since this takes longer to process a board than ‘normal’ wave soldering and is only a batch or single board process rather than the continuous flow achieved by wave soldering, selective wave is not widely used in commercial industry. selective wave is not widely used in commercial industry.

3.4.15.

Pin in Hole Reflow3.4.16.[3.4.13.] This is inherently a high stress process when compared to other

through-hole soldering processes.

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[3.4.14.] Solder paste is applied to the top side (component side) of the board. A portion of the paste is pushed into the hole (PCB via) by the application of the component lead. The component is soldered using Surface Mount Technology (SMT) reflow soldering equipment. The components are thus subjected to direct heating.Solder paste or solder pre-form is applied to the top side (component side) of the board. A portion of the paste is pushed into the hole (PCB via) by the application of the component lead. The component is soldered using Surface Mount Technology (SMT) reflow soldering equipment. The components are thus subjected to direct heating

Compliant Pin or Press Fit or Square Pin Round Hole

3.4.17. Placeholder.

Soldering Processes – SMT[3.4.15.] There are numerous types of SMT reflow processes in use today. The vast

majority of these processes can be characterized as being inherently ‘high-stress’ in nature. That is they are adjusted during the set-up of the SMT oven to be able to accommodate the thermal requirements of the component with the largest thermal mass, this invariably means that any components having a (much) smaller mass will be subjected to temperatures during solder reflow that are (much) higher than their manufacturers stated maximum. As with Sn-Pb systems, there are numerous types of SMT reflow processes in use today. The vast majority of these processes can be characterized as being inherently ‘high temperature” making processing of some parts (e.g. large BGAs may have increased warpage) more challenging. That is they are adjusted during the set-up of the SMT oven to be able to accommodate the thermal requirements of the component with the largest thermal mass, this invariably means that any components having a (much) smaller mass will be subjected to temperatures during solder reflow that are (much) higher than their manufacturers stated maximum. In general, the designer should be aware of the potential for a smaller process window (i.e. maximum upper limit and achieving sufficient minimum temperatures to attain proper wetting) if Pb-free technology is selected.

[3.4.16.] The result of the high process temperatures will be to use up some portion of the diffusion based wear-out of the components so stressed. Modern small feature sized semiconductors are particularly susceptible to this type of degradation..

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[3.4.17.] Whilst commercial applications having a short equipment lifetime requirement may not be noticeably affected, those equipment’s requiring a longer lifetime or having a more demanding application most certainly will be.

[3.4.18.] Note that diffusion based wear-out mechanisms invariably result in the growth of brittle intermetallics which can in turn introduce the potential for equipment common mode failure from an external event (such as mechanical shock) some time before an individual joint will wear-out.Note that diffusion based wear-out mechanisms invariably result in the growth of brittle intermetallics which can in turn introduce the potential for equipment common mode failure from an external event (such as mechanical shock) some time before an individual joint will wear-out.

Re-work3.4.18.[3.4.19.] Every thermal cycle of the manufacturing process adds thermal stress to

the assembly and this has the potential to cause damage to the PCB substrate, the components being re-worked, or those that are mounted immediately adjacent to them.

3.4.19.[3.4.20.] This is particularly the case with Hot Air re-work which tends to drive significant amounts of heat into localized areas of the board.

3.4.20.[3.4.21.] The use of Lead-free solder technologies has exacerbated this problem. The problem is also prevalent when de-soldering a component, i.e. the potential for pad lifting or other pad defects is increased. Also, note, a characteristic of Pb-free solders is the potential for copper dissolution exacerbated to one or multiple re-work cycles.

3.5. Substrate Issues

Background3.5.1. The continuing evolution of the European Union’s REACh legislation as

well as commercial pressures has led to the further degradation of substrate (specifically FR4) performance.

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3.5.2. Whilst this is not strictly a Pb-free issue, the higher soldering process temperatures required with Pb-free solder does mean that the substrate is now much more vulnerable to damage.

De-lamination3.5.3. The changes in materials have led to the FR4 laminate being able to absorb

moisture much more readily than previously and at a greater rate than before.

3.5.4. When combined with the potential for being exposed to the higher processing temperatures associated with Lead-free technologies it is now much more likely that the substrate will sustain damage through delamination during the manufacturing cycle.

3.5.5. Such delamination may or may not be physically visible to the naked eye and may not immediately affect the associated circuit functionality; it will however create a weakness that could cause early equipment failure in service.

Surface Insulation Resistance and dendrite Dendrite growthGrowth[3.5.6.] Lead-free solders generally do not wet as well as the Tin/Lead equivalent. When

designing hardware for stringent and harsh use conditions, a more active flux may be necessary to ensure optimal performance. Consult applicable aerospace, industry, and defense standards and handbooks for guidance. As a result of this a more active flux is required to be used.

[3.5.7.] CAUTION (to the process engineer): A Invariably the more active Lead-Free flux will leave a more active flux residue on the surface of the board after the soldering application(s) .

3.5.6.[3.5.8.] This residue has usually not been formulated to be cleaned off (no-clean or non-rosin based flux) although some of the more modern fluxes have now been produced with the ability to be cleaned. (s this NEEDED? Is this a process note appendix item? .

3.5.7.[3.5.9.] In service if the substrate is likely to be subjected to a humid environment, then that, combined with the ionic contamination and the electric field due to being powered up, can cause an electro-migration based surface failure mechanism resulting in reduced Surface Insulation Resistance (SIR) and in the extreme case dendrite growth. Subsequently, the designer should impose design aspects to mitigate risks associated with humidity, e.g. conformal coat, controlled atmosphere (where feasible), etc. ED. NOTE: Consider a preface to the guide that points to previous works on Pb-free experience/concerns, e.g.

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LFMP PH I and II, GEIA docs, etc. Does the preface also need to identify for whom the guide is written, i.e. an experiences electronics designer with little Pb-free experience or a brand new designer in general (prefer not the latter !!!!)

Conductive Anodic Filaments (CAF)

ED. NOTE: Consider consolidating these next six sub-sections into a short paragraph that summarizes the “delta” of CAF between SnPb and pb-free. See ED. NOTE immediately following next six sections.

[3.5.10.] CAF Conductive Filament Formation (CFF) is a needle-like growth exhibiting dendritic growth characteristics. CAF is a specific form of CFF in that it forms within the layers of a PWB.s.

3.5.8.[3.5.11.] It was first encountered many decades ago when multi-layered circuit substrates made using fiberglass laminate were first trialed.

3.5.9.[3.5.12.] Close control of the laminate quality all but removed this as a failure mechanism for Tin/Lead based equipment until the use of Lead-free materials and the associated higher process temperatures along with an apparent reduction in laminate repeatability due to the extensive use of ‘new’ Asian manufacturers’ re-introduced the phenomena.

3.5.10.[3.5.13.] At the time of writing this document a large minority of laminates are potentially susceptible to CAF growth. When designed with a narrow track to pad spacing such that the resulting field strength is high and when manufactured with Lead-free materials and processes boards have been reported as having failed in less than 4 years use-life.

3.5.11.[3.5.14.] It is apparent that voids within the laminate associated with the glass strands will exacerbate the potential for CAF growth. Also that ionic contamination exists within the substrate as a normal part of the laminate manufacturing process.

3.5.12.[3.5.15.] Given the propensity of modern laminates to absorb moisture from the atmosphere the likelihood of susceptible laminate to grow CAF continues to

increase.

ED. NOTE: The text box, below, represents alternative wording for the section on CAF

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3.5.10. Conductive filament formation (CFF), also referred to as metallic electromigration or conductive anodic filament (CAF), is an electrochemical process which involves the transport (usually ionic) of a metal across a non-metallic medium under the influence of an applied electric field. NOTE: Conductive Anodic Filament (CAF) is a specific type of CFF addressing directionality within the layers of a PWB. The information provided here applies to CAF as well as CFF.

Lead-free Design Guide

Figure ??. A conductive anodic filament bridging two layers (photo courtesy ?? of Dr. Michael Osterman, University of Maryland, CALCE)

3.5.13.[3.5.16.] From a designer’s aspect, selection of the PWB materials (e.g. laminates) should include directions to the supplier to implement appropriate IPC (or other) quality assurance tests. Also, in considering power (12 V and above) requirements, spacings between layers and traces can have an impact

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3.5.10. Conductive filament formation (CFF), also referred to as metallic electromigration or conductive anodic filament (CAF), is an electrochemical process which involves the transport (usually ionic) of a metal across a non-metallic medium under the influence of an applied electric field. NOTE: Conductive Anodic Filament (CAF) is a specific type of CFF addressing directionality within the layers of a PWB. The information provided here applies to CAF as well as CFF.

Lead-free Design Guide

on filament formation. Similarly, high density interconnects can be vulnerable to filament formation. In summary, any opportunity to optimize these design parameters (including ground layer spacings) should be exercised when feasible.

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Substrate Passivation Board Finishes [M. Miller comment: This section should really provide designer info on what are the appropriate finishes for Pb-free for material selection.

The Pb-free printed circuit finishes being used by the consumer industry do not have the same may differ in system level performance as heritage tin-lead solder finishes. In particular, A&D needs finishes with a long shelf life to support high mix low volume manufacturing and repair. Many of the finishes that have been developed for consumer electronics may be acceptable as long as they are fully covered with solder during assembly. Unfortunately, there are many instances where the original printed circuit board finish will not be soldered during assembly (e.g. press fit connectors, test points, etc.). There are

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also failure mechanisms that are exacerbated with the increased rigidity of the Pb-free solder and the intermetallic compounds that can form with some of the Pb-free alloys being considered. In addition, an assessment of the surface finishes and development of test methods for A&D applications is needed. NOTE: The information in this section, while not unique to Pb-free, is emphasized here due to the propensity to have more conditions of 1) higher temperature reflow, 2) non-traditional visual appearance of the solder joint system, 3) specific finishes that may be vulnerable to corrosion, vibration, etc. and 4) possibly others

The design should consider the following lead-free finish attributes after assembly: robustness in harsh environments, solderability after aging, corrosion resistance, and their resistance to forming tin whiskers. There is no known PCB plating finish that meets or exceeds SnPb assembly and reliability performance. Some important differences from heritage tin-lead design practices are:

Existing heuristic rules on the solderability of finishes used by electrical and mechanical designers based on experience with SnPb programs, may be insufficient to ensure reliability of Pb-free product.

The Pb-free material corrosion rates and how they correlate to service environments is not fully understood, may result in increased reliability risk. Further investigation and analysis is required. Investigation and analysis should include corrosive interactions with solder, PCB finishes, and component finishes, under powered and off storage conditions within A&D environments.

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For further information see:

IPC-7093 section 5.3

IPC-7095 section 5.3

IPC-AJ-820 section 4.6

IPC-4552 ENIG Electroless Nickel Immersion Gold

IPC-4553 Immersion Silver

IPC-4554 Immersion Tin

IPC-4556 ENEPIG Electroless Nickel Electroless Palladium Immersion Gold

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Embrittlement considerations3.5.14. Gold and palladium form a brittle intermetallic phase when combined with tin.

The final concentration of Au or Pd in the final solder joint must be validated to ensure that the brittle IMC phase is not formed. Caution, the smaller pad diameters tend to build up thicker Au or Pd during deposition. The feature size defined by the IPC plating standards maybe larger than the smallest solder joint pad diameter. Electrodeposited gold (over elecrodeposited Nickel) can be thick and susceptible to embrittlement.

Plating defects 3.5.15. There are two major plating defects impact solder joint reliability. Champagne

voids in immersion Ag and black/gray pad in ENIG. Partnering with board manufacturers to ensure proper process controls are in place is important to avoid these defects.

Corrosion3.5.16. Creep corrosion can occur when the product use environment containes

sulfur, chlorine, or other ionic contaminates. Small defects in the Imm Ag layer (at the solder mask to pad interface) results in crevice corrosion that can reduce the copper trace cross-sectional area and produce corrosion products resulting in electrical shorting.

Another consideration is corrosion in mixed flowing gas environments (Hannigan etal JEM V41 No 3 pp 611-623 2011). Finishes such as OSP and Imm Ag are more prone to corrosion in mixed flowing gas environments than Imm Sn and ENIG (when no porosity is present).

Corrosion can be reduced by the use of coated by conformal coatings.

Tarnish3.5.17. Imm Ag when exposed to sulfur or chlorine sources such as paper products,

rubber bands, hydrocarbon fuel combustion results in a surface tarnish. The level of tarnish determines the extent of remediation.

Thermal excursion3.5.18. OSP and Imm Sn may have insufficient solderabilty robustness during any

manufacturing process that requires thermal exposure (e.g. surface mount reflow, adhesive curing, touch-up, rework, etc).

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Tin Whiskers3.5.19. Imm Sn can have varying degrees of whisker susceptibility. Characterization

and evaluation of any Imm Sn finish is recommended prior use. Tin whiskers on immersion Sn finishes can be mitigated by the use of properly evaluated conformal coatings. (There is an abundance of industry literature/data that will aid in conformal coat selection.) In addition to coatings, whisker risk can also be mitigated on test points by applying solder.

Press fit connector considerations 3.5.20. Imm Sn in press fit connector applications is discoraged because of whisker

growth concerns.

RF concerns 3.5.21. ENIG and ENEPIG finishes when used in high frequency circuits must be

evaluate do ensure that the nickel does not attenuate the high frequency signal transmission.

HASL concerns 3.5.22. Hot air solder leveled lead-free finishes (e.g. SAC305, SnCuNi) have the same

concerns as tin-lead in terms of coplanarity, solderability and shelf life.

Shelf life3.5.23. Imm Sn and HASL can have solderability issues due to Cu-Sn IMC growth to

the surface.

Electroless Nickel, Immersion Gold (ENIG)[3.5.17.] This substrate finish has been in use for many years. It offers one of the best

co-planarity (flattest) finishes that is available. It does however suffer from the potential for Interfacial Fracture (IFF) or ‘black pad’ as it is called.

[3.5.18.] The failure mechanism is that of a weak solder bond to the Nickel layer on the board. Mechanical stress introduced during board population, test shipping or just general handling can result in the joint cracking away from the board.

[3.5.19.] A close control of the plating process can stop the problem from occurring, however since it can only be found by a destructive test of the joint no guarantee can be given of complete immunity.

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Electroless Nickel, Electroless Palladium, Immersion Gold (ENIPIG)[3.5.20.] It is believed that an ENIPIG finish will provide an equivalent co-planarity to

that of ENIG but without the potential for join failure due to Black Pad.

[3.5.21.] It should be noted however that the palladium layer makes this pad passivation unsuitable for use with Tin/Lead solder systems.

Hot Air Solder Levelling (HASL)[3.5.22.] This was the most commonly used PCB finish with Tin/Lead solders. Boards

are dipped in molten solder then subjected to solder flattening using hot-air knives.

[3.5.23.] The finish is never perfectly flat since the solder will always have a meniscus.

[3.5.24.] Lead-free solder HASL is available but the higher temperatures associated with these solders tends to degrade the PCB substrate.

Organic Solder Preservative (OSP)[3.5.25.] OSP comes in various forms. It is invariably a very thin layer (angstroms).

Historically it had a poor shelf life and could only stand one or two thermal cycles. Todays OSP’s are much better being more robust being designed for a Lead-free environment. They can handle multiple heat cycles and have a typical shelf life of one year.

Immersion Silver[3.5.26.] This is a common Lead-free surface finish having good co-planarity,

reasonable shelf life and a lower cost than the Gold finishes.

[3.5.27.] Unfortunately under the right atmospheric conditions it can grow Silver dendrites.

Immersion Tin[3.5.28.] This is a historically popular surface finish. It has good co-planarity but a limited

shelf life. Tin whiskers are not thought to grow from this plating but this is not proven.

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Cleaning (ED. NOTE: info in these nex four sub-sections is very similar to that already presented in 3.5.6 through 3.5.8)

[3.5.29.] It is necessary to clean off any soldering manufacturing process residue before either completing the unit Production Testing or initiating the application of a conformal coating.

3.5.24.[3.5.30.] Failure to clean the board assembly can introduce reduced Surface Insulation Resistance (SIR) or surface dendrite growth through life. Residues create corrosion, which forces whisker growth. Depending on the use condition, humidity may reactivate corrosive activities of flux residues, even under conformal coating.

3.5.25.[3.5.31.] Since the Lead-free solders do not wet well, the solder flux is invariably very active (acidic) and with a flux residue present after soldering that can introduce significant amounts of ionic contamination to the board and components.

3.5.26.[3.5.32.] The residue can be extremely problematical to clean off. The right combination of flux and cleaner has to be defined. Components must be suitable for cleaning process.

Conformal Coating (ED. NOTE: Shouldn’t it be at CCA level?)

3.6. Component Selection (Just note the Pb-free deltas here, e.g. higher temp-qualified packages?? Need to re-write Section 3.6 to include the Pb-free specific issues only)

3.6.1. The implementation of a successful design will be dependent on the suitable choice of components for the application.

[3.6.2.] Those cComponents must be able to meet not just the parametric requirements of the application but must also its’ and use-life and reliability requirements.

3.6.2.[3.6.3.] The availability through the lifetime of the application must also be considered and an appropriate obsolescence or through-life management plan generated. Component metallization has to be /should be lead-free. Depending on the whisker mitigation strategy, the components metallization has to be defined. The MSL (Moisture sensitivity level) may be increased for lead-free components leading to reduced open-times out of the packaging in

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production. Pb-free component metallization should be defined based on the tin whisker mitigation strategy selected. The MSL (Moisture Sensitivity Level) may be increased for lead-free components leading to reduced out-of-package exposure times.

Material Issues

Interconnection3.6.3.[3.6.4.] Whilst the majority of components have leads or terminations passivated

with pure Tin, a large minority of components make use of other passivation materials.

[3.6.5.] Pure tin is compatible with all common LeadPb-free solder types but it will introduce storage lifetime issues and the potential for tTin whisker growth.

PlasticsPackaging3.6.4.[3.6.6.] Be aware that many Pb-free alloys require higher (with respect to Sn-Pb)

reflow temperatures that can have adverse effects on plastic encapsulated microcircuits, connectors, and other electronic packaging applications. (Concept of compatibility)

Device Lifetime[3.6.7.] Modern Iintegrated circuits (ICs), utilizing Pb-free technology, invariably include

design and material compromises that would warrant an assessment on that limit their use-life. (ED. NOTE: Is this statement necessary, i.e. is this any different than with SnPb? Consider keeping and/or wordsmithing)

[3.6.8.] The actual achieved performance can (will) additionally be adversely affected by the manufacturing stress associated with LeadPb-free materials and processes.

3.6.5.[3.6.9.] Note that the lifetime of a component in storage can be very different from that in service. (ED. NOTE: Is this statement necessary, i.e. is this any different than with SnPb? Consider keeping and/or wordsmithing)

Obsolescence Issues3.6.6.[3.6.10.] Other than the initial change of components to a Pb-free state when vast

numbers of component types were rendered obsolete virtually overnight, Pb-free does not directly impact the component obsolescence threat.

3.6.7.[3.6.11.] It should be noted however that indirectly Pb-free component technology has affected some obsolescence mitigation strategies.

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a. For example a typical through-life management plan might include the activity of monitoring the status of critical component parts and then making a ‘last time buy’ in order to support the equipment either to the end of the planned use-life or until a suitable redesign and qualification can be undertaken.

[3.6.12.] If the component leads, case or terminations are tTin plated, a common Pb-free occurrence, then the storage lifetime will invariably be limited by the intermetallic growth at the plating interface.

b.[a.] After some years, as little as 1 year in the worst case, the intermetallic will begin to break through at the plating surface.

c.[b.] When this happens the terminations will no longer wet and thus cannot be soldered without mechanical or chemical abrasion of the termination. The component is thus effectively scrap and cannot be used.

[c.] In addition if the storage temperature is reduced so as to slow down the intermetallic growth rate there is an increased risk of damage to the component due to Tin tin Pestpest.

3.7. Component Storage [ED, NOTE: Consolidate this]3.7.1. Components are usually shipped in suitably ‘safe’ ESD packaging. In many

cases however the shipping package is unsuitable for the long term storage of components.

3.7.2. The ESD packaging can, in some cases, outgas Chlorine or other acid residue. This can in turn cause degradation of the component metallic interconnections and in some cases the components themselves.

3.7.3. Only packages or materials that have been proved to be stable over time should be used for long-term component storage.

3.7.4. Storage environments must always be a compromise reducing some wear-oput mechanisms without exacerbating others.

3.7.5. Section 6.2 of this document includes more detail of the environmental compromises that should be considered for the different control level of application.

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3.8. The Supply Chain [ED. NOTE: Consolidate this]3.8.1. The supply chain is out of control, at least if you need a particular component

(or components) that is exactly the same as was procured previously.

3.8.2. Changes in legislation, the global position of the supply chain and the economic pressure resulting in greater out sourcing of supply have all combined to make components, substrates et al, more susceptible to Pb-free manufacturing stress.

[3.8.3.] Some rare and or/ expensive components are dispatched with a Certificate of Conformity (CoC). Experience has shown that is a small but significant number of cases this paper qualification is worthless but does engender a (false) sense of security.

3.9. Mixing/Multiple Alloys

Note: Caution should be practiced when mixing different Pb-free alloys. Such practice could impact solder joint integrity. This can be an obsolescence concern if an original Pb-free alloy, used for repair, is terminated (or discontinued) by suppliers. Use of multiple alloys should be avoided or must be managed to minimize risks of inadvertent mixing.

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4. The Application

4.1. Control Level Classification (ED. NOTE: Do we really need this section since this guide is focused on “having to use Pb-free?” Maybe a high-level summary only for TWs? Agenda item for PERM 26 workshop?)

4.1.1. The extent to which the design process must be qualified, and the resulting product controlled, will be predominantly due to the criticality of the application. [What are the impacts? Why consider this? Will these be answered in subsequent sections?]

[4.1.2.] It is recommended that the classification of the application be categorised using the same nomenclature and guidance as is used to support the risk mitigation of Tin whiskers set out in GEIA-STD-0005-2 (the GEIA standard, Reference 2).

a. Adapting terminology from GEIA-STD-0005-2, control level is defined as the amount of attention that should be paid to the risk of using Pb-free in electronics; 1) no restrictions on Pb-free use (focus of this guide), 2) some restrictions on Pb-free use, and 3) prohibition of Pb-free use

[4.1.3.] The designer must consider discuss the potential impact of the different various levels of classification with respect to each specific customer applications or requirements to be designed with the customer.

[4.1.4.] The different levelsDesignating a level of classification will affect the overall cost, timescale to design the equipment, and its in-service life and performance of the design, . tThe discussion to adopt a classification level for each part of the design must be completed before the commencement of any design work or even the placement of a firm order.

[4.1.5.] Note that ultimately the customer is responsible for determining the control level. they are seeking but that However, this should only be done after consultation with the designer(s) on the associated impacts.

[4.1.6.] Note that the classification of the equipment for all applications is not just about the risks associated with Tin whiskers, all aspects of the application , the

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manufacturing process(es) as well as the materials used must all be considered. This may not be needed at all, tends to be confusing.

4.2. Classification Control Level 1 4.2.1. Equipment applications will be allocated this control level if they are of a

commercial nature and are expected to have a short use-life. That is, that they are expected to remain operational for less than a nominal 5 calendar years from manufacture.

4.2.2. The guarantee period is very likely, of course, to be much shorter than the use-life.

4.2.3. In the extreme case a level 1 application will have a minimum of, or even no mechanisms or processes to control the repeatability of the manufactured product.

4.2.4. It is likely that equipment or items of a Control Level 1 will be produced in quantity on a high volume production process line.

4.2.5. Examples include but are not limited to:

Mobile (cell) phones

Games consuls consoles?

Personnel Personal Computers (PCs)

Domestic equipment

4.2.6.

4.3. Classification Control Level 2a [Refer to GEIA-STD-0005-2 for Level 2a definition and applicability]

4.3.1. This classification level (2a) will be applied to those applications that (need to) have some small degree of Safety Related performance.

4.3.2. Typically the applications could be categorised as being equivalent to Safety Integrity Level (SIL) 1; the definition of SIL is detailed in the standard 61508 [reference 11].

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4.3.3. Examples include but are not limited to:

a. Some equipment status monitoring systems

b. Some non-critical control systems

c.

4.4. Classification Control Level 2b4.4.1. Applications can be considered at Level 2b if they meet the criteria of being

Safety Related as defined at SIL 2 in reference 11.

4.4.2. Equipment having a required FIT rate of <10-4

4.3.4. Examples include but are not limited to:

a. Some equipment monitoring and control systems that are critical

4.4.3.

4.5. Classification Control Level 2c4.5.1. Applications can be considered at Level 2c if they meet the criteria of being

Safety Critical as defined at SIL 3 in reference 11.

4.5.2. Equipment having a required FIT rate of at least 10-4.

4.5.3. Examples……….

4.6. Classification Control Level 34.6.1. Applications can be considered at Level 3 if they meet the criteria of being

Safety Critical as defined at SIL 3 in reference 11 and or have a long use-life requirement.

4.6.2. Equipment also having a required FIT rate of >10-4.

4.6.3. Examples include: Space, nuclear reactor control,………

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5. Equipment (or Product) Design

5.1. Architecture

Multiple channels and voting? 5.1.1. Diversity of component technology, but still leaves manufacturing processes

and materials as a potential common mode problem to be addressed?

5.2. Technology

Hybrid Implementation5.2.1. The correct hybrid requirements specification can mitigate many of the Lead-

free issues and process problem areas. This does however come at a cost.

Conventional Packaged Components

Commercial Off The Shelf (COTS) Assemblies

5.3. Component Qualification

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6. Manufacturing for High Reliability

6.1. Tin Whisker Self-Mitigation:6.1.1. Within a Tin/Lead system, some smaller SMT components would have their

(Tin) coated terminations fully converted to Tin/Lead during the normal soldering process.

6.1.2. For Lead-free manufacturing the lack of Lead within the solder means that no (Tin whisker) self-mitigation can be achieved by the soldering process.

6.2. Manufacturing Process

Storage Prior to Manufacture

Specialist Build – Hybridisation

Soldering – THT6.2.1. It is accepted that some components will always have to be hand soldered,

however it is also accepted that the reliability claimable for a joint soldered using an automated process will always be better that that of a hand soldered one.

6.2.2. It should be noted however that wave soldering can create or aggravate problems with PCB substrates, as such a selective wave soldering process is recommended for use with higher reliability, long lifetime applications.

Soldering – SMT6.2.3. For use with equipment intended to support high reliability, long lifetime

application, it is recommended that a Condensation Soldering process (Vapour Phase) be used. This will ensure that no component is over (thermally) stresses irispective of its thermal mass.

PCB Surface Finish6.2.4. There is no single preferred substrate finish for use in Lead-free applications.

The preferred finish will be dependent on that product, the component selection, assembly process and the product design.

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6.2.5. However since it is likely that most modern equipment will consist of predominantly SMT components, it is recommended that Electroless Nickel, Electroless Palladium, Immersion Gold (ENEPIG) be used. This will give a very high degree of surface co-planarity without the potential problems associated with ‘black-pad’ joint weakness.

6.2.6.

Level of Ionics (add some info here)

6.3. Supply Chain Control6.3.1. It is accepted that Pb-free processes and materials are not the only source of

concern, the general evolutionary trend within the components will also affect circuit performance and through life costs etc.

6.3.2. It is therefore essential that some form of additional control be implemented as a part of the component supply acceptance process in order to achieve a continuing acceptable standard of Assembled equipment supply.

6.3.3. The level of the application will determine the effort, cost and time that must be expended to prove that what you have received is what you actually want or need and believe that you ordered.

6.3.4. In order to meet the requirements of the different application categories it is recommended that a degree of control is implemented as collated in the table figure 3 below:

Application Category

State requirements on purchase

order

Monitor PCNs

Test materials

(XRF)

Full test of component / die by de-cap

Life test

1 Yes

Dependent on product

lifetime[note 1]

No No No

2a Yes Probably Possibly No No

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[note 2]

2b Yes Yes Probably[note 3]

For critical components

Possibly if new or

different and critical

2c Yes Yes

Yes for older

designs[note 4]

Yes for new designs

Yes if new or different

3 Yes Yes Yes Yes if new or different

Figure 5: Suggested requirements for different application categories

Note 1: Even though this is likely to be an application that does not justify component control, the cost of a redesign if a critical component is no longer available may make limited monitoring economically sensible.

Note 2: Whilst this is desirable it is debatable how much of an economic case could be made to do this properly.

Note 3: XRF testing will catch material incompatibility with the termination finishes, It will not catch other changes in materials, flame retardants etc. It is a minimum test for critical areas of the circuitry.

Note 4: This is a pragmatic choice. It is unlikely that existing products will have the associated funding required to undertake a full de-cap and baseline program, even if this is a desirable option.

PROPOSED SUBSTITUTE: Replace previous Section 6.3 with the following (J. Rowe of LMCO)

Supply Chain Control

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Pb-free risk management must be flowed down to suppliers at all levels. Because of the unique characteristics and interdependencies associated with Pb-free materials the strategies and methods for supply chain control must be adjusted to ensure ADHP systems will meet their performance requirements. This starts with the lowest level piece parts and extends up through purchased assemblies and subsystems. The further up you are in the supply chain the more challenging it is to ensure adequate controls are applied to the lowest level which could be 4 or 5 suppliers down in the chain.

Supply chain control applies to anything that is purchased including; COTS piece parts defined by vendor data sheet or drawing (electrical and mechanical) Custom piece parts defined by user specification or procurement control drawing COTS assemblies Custom build to print assemblies defined by user detailed drawing and parts list Custom assemblies defined by user specification and/or SOW

The key areas of supply chain control for Pb-free risk management are; Pb-free specific requirements for design, materials and manufacturing Supplier selection and qualification Configuration Management Receipt of Material

Pb-free Specific RequirementsThe systems engineer or design engineer may identify specific actions necessary to ensure the product will meet performance requirements. Examples are: Control Level selection for tin whisker risk management Prohibition of specific materials or finishes that are not compatible with other materials used in the

design Pb-free solder alloy selection tailored to the specific application

o Rare earth additives are prohibited from solder balls Conformal coat type and minimum coverage requirements if it is used as a tin whisker risk

mitigation strategy Qualification testing or analysis protocols that are unique to Pb-free materials Peak temperature and MSL rating declarations

These requirements must be identified and documented in the purchase order, SOW or drawing and flowed down, as appropriate to dub-tier suppliers.

Supplier Selection and QualificationItems should be procured from sources that recognize the Pb-free material risks and who actively select and control material content to manage the risks. The supplier must have a documented system for flowing down appropriate requirements to sub-tier suppliers and for the receipt of material. Supplier selection becomes more critical for COTS items as the user has little if any control over the items design or specifications. It is important the supplier have a good track record for integrating Pb-free risk

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management into the design, procurement and manufacturing of their product. For instance a supplier of an electronic component having a Pb-free tin finish should have successfully passed industry standard tests for tin whisker growth and have necessary controls over the tin plating process. A supplier of a COTS assembly should have a Pb-free Risk Management Plan in place.

When qualifying a supplier, Pb-free risk management should be a key aspect of the evaluation. The supplier must have a demonstrated history of understanding the risks and the role their product has in meeting system level performance requirements.

Configuration Management Meeting performance requirements often means specifying specific materials, finishes and processes. Some changes may be considered as form, fit or function interchangeable under historical guidelines but can result in unacceptable performance. For instance, a change in the Pb-free solder alloy can make a significant difference in service life as can changes to the solder reflow process. A change in the type of conformal coat used may invalidate tin whisker risk mitigation. The impact of these changes demands a higher level of configuration management control and oversight. Suggestion on what to include for markings w/r board layout. This may include Pb-free material content marking on the item in accordance with J-STD-609.

Receipt of MaterialThe Pb-free material content of delivered items must be verified. Any product features that are specifically incorporated to meet performance requirements may also require verification. This may require increased levels of incoming inspection, screening and lot sampling. This may include specialized testing or analysis to validate a supplier’s certificate of compliance. The critical attributes associated with meeting product performance requirements with regards to Pb-free material content must be identified and appropriate inspection plans put in place. The specific requirements may need to be flowed down to sub-tier suppliers via the purchase order, SOW or drawing.

Traceability (no different than current standard processes)

6.4. Obsolescence Management6.4.1. Many component types as well as (Pb-free) materials have a significantly

limited commercial availability. If the application is specified to have a long use-life it will therefore be necessary to develop an Obsolescence Management Strategy, early on in the design phase.

6.4.2. There is nothing unique about such a strategy for use with Lead-free technologies though arguably the availability period for LF components is shorter since they are predominantly aimed at the commercial market.

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6.4.3. Since the majority of Lead-free assemblies will have a shorter justifiable use-life than previous (Leaded) technologies, the strategy must plan for a shorter redesign cycle, or else to store components and materials in greater quantities so as to support a larger number of more frequent re-builds.

6.4.4. The most common component lead (termination) passivation is Tin and thus wear-out in storage is more likely since with a Copper base material, for example, the Tin/Copper intermetallic growth at the termination or lead base metal interface can be very fast even at low temperatures.

[6.4.5.] When the intermetallic brakes breaks through the surface of the (Tin) passivation the lead will not wet and thus it cannot be soldered.

6.4.5.[6.4.6.] An Obsolescence Management Plan should address all aspects of the design but not necessarily with the same rigor. For example:

a. The plan should identify the definition of a ‘critical component’ for that particular design. A critical component might be defined by:

i. its rarity, does it come from a single source?

ii. is it particularly critical to the correct operation of the circuit?

iii. is it significantly expensive?

iv. does it have an overly long lead time?

b. It should specify how critical parts are handled:

i.

c.

6.4.6.[6.4.7.]

SUGGESTED REPLACEMENT FOR 6.4

6.4 Obsolescence Management

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A robust obsolescence management plan is critical when using commercial parts and assemblies whose continued availability is often significantly shorter than the production and support life of typical ADHP equipment.

Given that different Pb-free materials can have significantly different performance in long term storage and use, the obsolescence management plan must include a review of the recommended replacement part or assembly for Pb-free material content. Any changes must be evaluated to ensure performance and service life will not be impacted.

The evaluation must also consider impacts to any Pb-free risk mitigation strategies implemented in the design. A change to Pb-free material content or other characteristics may invalidate the mitigation.

Examples of changes with potential adverse impacts include Change from one Pb-free solder alloy to another Change to a Pb-free tin finish Change to the conformal coat or encapsulation of an assembly Change to component spacing or component lead pitch used on an

assembly

Care must be taken when using “life time buy” as a mitigation strategy for parts that are going obsolete. Parts with Pb-free finishes may have different storage requirements and/or have shorter storage times. (ED. NOTE: Put some specific examples for previous statement????) Pure tin finishes may grow tin whiskers during storage that must be addressed prior to use.

In summary, when at all possible, use of multiple Pb-free alloys should be qualified to show compliance to performance requirements. A design note can be used to communicate this to suppliers. Obviously, use of multiple Pb-free alloys should be avoided when possible or convenient.

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7. Design Process Flow7.1.

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8. Terms and Definitions8.1. For purposes of this document, the following terms and definitions apply

Alloy Composition - All alloy compositions are stated as weight percent. For instance 63Sn-37Pb corresponds to a mixture of 63 % by weight of Tin(Sn) and 37 % by weight of Lead (Pb).

Alloy 42 refers a nickel-iron controlled-expansion alloy containing 42% nickel that is often used as a lead-frame material in electronic packages.

Assemblies are electronic items that require electrical attachments, including soldering of wires or component terminations; examples include circuit cards and wire harnesses. This may include soldered assemblies.

Critical item or function, if defective, will result in the system’s inability to retain operational capability, meet primary objective, or will directly affect personnel safety.

Tin Pest is the allotropic transformation of the silvery, ductile metallic allotrope of β-form White-Tin to a brittle, non-metallic, α-form Grey-Tin. It occurs at, or below, 13.2oC.

A failure rate of 1 FIT represents one failure in 109 device (operational) hours.

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9. Abbreviations

ADHP Aerospace, Defence and High Performance AFE Authorisation For ExpenditureAg SilverAu GoldAVSI Aerospace Vehicle Systems Institute

Bi Bismuth

CAF Conductive Anodic Filaments Cu Copper

ENEPIG Electroless Nickel, Electroless Palladium, Immersion GoldENIG Electroless Nickel, Immersion Gold.

FIT Failures in Time sometimes called Failure UnIT FR4 Flame Retardant 4, (a type of fibreglass laminate)

ICs integrated circuits IFF Interfacial Fracture

LF Lead-free

OEMs Original Equipment Manufacturers

Pb LeadPCB printed circuit board

REACh Registration, Evaluation, Authorisation & restriction of Chemicals ROHS Restriction of Hazardous Substances

SAC Tin Silver CopperSIL Safety Integrity Level SIR Surface Insulation Resistance SMT Surface Mount TechnologySn Tin

THT Through Hole Technology

XRF X-ray Fluorescence

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10. References

Reference 1. GEIA-STD-0005-1, “Performance Standard for High Performance Electronic Systems Containing Pb-Free Solder”.

Reference 2. GEIA – Std 0005 – 2: Standard for Mitigating the Effects of Tin Whiskers in Aerospace and High Performance Electronic Systems, current issue.

Reference 3. H. Becker, "On the Quality of Gray Tin Crystals and Their Rate of Growth," J. Appl. Phys., Vol. 29, No. 7, pp. J1110-1121, July 1958.

Reference 4. W.L. Williams, "Gray Tin Formation in Soldered Joints Stored at Low Temperatures," ASTM Spec. A. Bornemann, "Tin Disease in Solder Type Alloys," ASTM Spec. Techn. Publ. 189, pp. 129-148, 1956

Reference 5. ASTM Spec. Techn. Publ. 189, pp. 149-159, 1956.

Reference 6. Binary Alloy Phase Diagrams," T.B. Massalski Editor-in-Chief, ASM Intl., October 1986.

Reference 7. .Solberg, V., "No-Lead Solder for CSP: The Impact of Higher Temperature SMT Assembly Processing," Proc. NEPCON West 2000 Conf. (Feb. 28 - Mar. 2, 2000) Anaheim, CA (Source: Indium Corp.) from NIST, "Database for Solder Properties with Emphasis on New Lead-free Solders". http://www.boulder.nist.gov/div853/lead%20free/part2.html

Reference 8. Mei, Z., “Microstructural Evolution and Interfacial Interactions in Lead-Free Solder Interconnects”, in Lead-Free Solder Interconnect Reliability, Shanguan, D., Ed., ASM International, Materials Park, OH, (2005)

Reference 9. http://nepp.nasa.gov/whisker/

Reference 10. Solder Alloys: Physical and Mechanical Properties. Paul V. Bolotoff. Release date: April 2010

Reference 11. IEC 61508 “Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems (E/E/PE, or E/E/PES)”.

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Reference 12.

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G24

IEC/TS 62239-1 Electronics Component Management

SAE ARP 5890 Reliability

Obsolescence SAE STD-0016

Parts Management SAE EIA-4899

COTS Assemblies SAE EIA-933

Counterfeit

The GEIA – STD – 0005 – 1 has a compliance matrix therefore LFCPs need updating accordingly.

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11. Appendix 1

Collation of Suggested Best-Practice

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A.1.1. This appendix collates the suggestions made in the body of this document for best practice choices for the different application control levels.

A.1.2. Note that these choices can only be considered as being a generalised guide to the options available; other specific design drivers could easily be shown to modify significantly the design, material and or process outcomes.

A.1.3. Note also that these are only suggestions and whilst they may be considered to be ‘best practice’ at the time of writing the document changes in technology, materials and processes will invariably modify future design decisions.

Control Level 1

Control Level 2a

Control Level 2b

Control Level 2c

Control Level 3

Component feature size

N/A N/A or Dependent on lifetime

Dependent on lifetime

170 nm 270 nm

Component long-term storage

N/A Qualified ESD Safe

Low temp’, dry Nitrogen ESD Safe

Low temp’ dry Nitrogen ESD safe watchdog samples

Low temp’ dry Nitrogen ESD safe watchdog samples

Solder type Any LF dependent on cost and short term performance

Any LF dependent on short term performance

Any qualified LF

Any qualified LF or Tin/Lead

Tin/Lead

Flux type Any dependent on solder requirements

Rosin based of an appropriate activity level

Substrate type

Any

Substrate passivation

Any ENEPIG ENEPIG ENEPIG or Tin/Lead horizontal

Tin/Lead horizontal

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HASL HASL

Cleaning

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