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The three biggest issues in lead-free assembly are which lead-free solder to chose, whether to go mixed (lead-free BGA with SnPb solder) and figuring out how to rework. The initial hope was that Sn3.0Ag0.5Cu (SAC305) would be the accepted replacement to SnPb. That has fallen apart in the last 12-18 months, as portables have asked for SAC105 (more shock resistant), wave solder manufacturers have asked for SNC, and every solder manufacturer is adding their own secret ingredient (SNCX and SACX). For mixed assemblies, keep peak temperature above 220C. A defect-free mixed assembly does well under thermal cycling, but less is known about vibration and mechanical shock. Copper dissolution is the reduction or elimination of surface copper conductors due to repeated exposure to Sn-based solders. It is a significant concern for industries that perform rework. Research has determined that contact time is the major driver, with some indications of a 25 second limit on contact with molten solder. This creates a limit of 1X rework. Copper dissolution is already having a detrimental effect as some major OEM unable to repair ball grid arrays (BGAs) Lead Free: Predicting and Ensuring Reliability in Military Avionics -400 -300 -200 - 10 0 0 10 0 200 300 400 0 2 4 6 8 10 12 Time (sec) Wetting Force ( µ N/mm) 0 Months Shelf HASL 6 months Shelf HASL 0 months Shelf ImmAg 0 months Field ImmAg 6 months Shelf ImmAg 1.Copper thickness = 2OZ use material listed on column 260 ℃ 2.Copper thickness >= 3OZ use Phenolic base material or High Tg Halogen free materials only 3.Twice lamination product use Phenolic material or High Tg Halogen free materials only (includes HDI) 4.Follow customer requirement if customer has his own material requirement 5.DE people have to confirm the IR reflow Temperature profile TBD – Consult Engineering for specific design review ≧161mil PhenolicTg170 + Filler IS415, 370 HR, 370 MOD, N4000-11 HF material - TBD ≧161mil Phenolic Tg170 + Filler IS415, 370 HR, 370 MOD, N4000-11 HF –high TG materials OK ≧131mil Phenolic Tg170 IS410, IT180, PLC-FR-370 Turbo TU722-7 HF –high TG materials OK 121~160mil Phenolic Tg170 IS410, IT180, PLC-FR-370 Turbo, TU722-7 HF –middle and high TG materials OK 93~130mil Tg150 Phenolic + Filler IS400, IT150M, TU722-5 Tg 150 HF –middle and high TG materials OK 93~120mil Tg150 Phenolic + Filler IS400, IT150M, TU722-5, GA150 HF –middle and high TG materials OK 73~93mil Tg170 Dicy, NP150G-HF HF –middle and high TG materials OK 73~93mil Tg170 Dicy HF –middle and high TG materials OK 60~73mil Tg150 Dicy NP150, TU622-5 All HF materials OK 60~73mil Tg150 Dicy HF- middle and high Tg materials OK ≤ 60mil Tg140 Dicy All HF materials OK ≤60mil IR-260℃ Board thickness IR-240~250℃ Board thickness No 6 months -45C SAC305 (Nihon, Sweatman, JEDEX 2005 No 24 months -40C SAC (IBM, Kang, ECTC 2003 No 3 months -42C SAC305 (RIM, Christian, IPC/JEDEC 2005 No 1.5 months -78C Sn0.2Ag (Murphy) Pest? Time Temp Alloy y = 301550x -2.7427 0.1 1 10 10 100 1000 Change in Temperature ( o C) Normalized Characteristic Life Introduction By now, everyone in military electronics, from the designer to the manufacturer, from the engineer to the executive, and from the lowliest sub-contractor to some of the highest reaches of the Pentagon, knows about Lead (Pb) Free. Is lead-free inevitable? Unfortunately, yes. While certain aspects may never be acceptable, such as tin plating in mission-critical applications, there are strong indications that markets not required to go lead-free, such as telecom, automotive, industrial controls, even medical and avionics /military , are transitioning right now, with timelines ranging from mid- 2007 to late 2009. So what is the reality of lead-free? The reality is that most transitions to lead-free in the commercial world were “successful”. Successful means that most companies are reporting similar to lower levels of field returns for Pb-free products. But… • Time required was often 50 to 100% longer than planned • Use environment tends to be more benign (consumer / computer) • Design life tends to be limited (3 yr - 5 yr) • Some failure mechanisms of concern have been recently noted In addition to these words of warning, there has been a recent divergence in lead-free solders (see below). These variations, from changes in tin-silver-copper (SAC) composition to widespread acceptance of tin-nickel-copper (SNC), often come with little reliability information and can lead to confusion and consternation. What to do? Focus on your parts, focus on your boards, and focus on your solder. SAC405 SAC305 SAC105 SACX SNC SnAg SNCX SnCu SnAgCu ?? SAC405 SAC305 SAC105 SACX SNC SnAg SNCX SnCu SnAgCu ?? Be aware that a limited number of components can become damaged after exposure to lead-free reflow temperatures (240°C - 260°C peak). They include • Aluminum electrolytic capacitors • Ceramic chip capacitors (wave soldering) • Surface mount connectors (nylon housing) • Specialty components (RF, optoelectronic, etc.) What to do? Do not assume that lead-free/RoHS compliant parts will survive lead-free reflow. Identify the components at risk and measure their temperature during reflow and rework. Compare the results to manufacturers’ specifications. Parts The simple answer here is to pay attention to moisture sensitivity levels (MSL). MSLs are well defined in J-STD-020D (see below). Two additional words of warning • Avoid any component with a MSL > 4 • Pay particular attention to plastic encapsulated capacitors (tantalum, tantalum polymer, aluminum organic) PECs are often overlooked and are sometimes not appropriately labeled (no MSL marking or obsolete version of J-STD-020). Reduce your risk, and wasted resources, by realizing that matte tin over copper tends to have a finite length (< 0.5 mm). This will tend to limit your focus to components with 0.8 mm pitch or less (some companies focus on less than 0.65 mm pitch). What about whiskers breaking off? Prior research strongly indicate this only occurs during handling (table on right). Finally, be aware of your options. If you are slightly concerned, request that your suppliers follow JESD22A121 and JESD201. Require variable data (no pass/fail). If you are concerned, request mitigation at the part manufacturer (nickel underplate, annealing, minimum tin thickness, palladium). Of the four options, palladium is the only guarantee (and is increasingly popular). Very concerned? Follow GEIA STD- 0005-2 and consider OEM mitigation (solder dipping or conformal coating). Robustness Popcorning Tin Whiskering 35 microns 3000 cycles -55C/85C 275 microns 450 days 30C/60%RH Dittes (E4) [8] 34 microns 3634 hours 51C/85%RH Romm (TI) [7] 40 microns 3000 cycles -55/85C 100 microns 8500 hours Room Conditions 450 microns 5000 hours 60C/93%RH Hilty (Tyco) [6] 250 microns 500 cycles -40C/90C Brusse (NASA) [5] 35 microns 3000 cycles -55C/85C 60 microns 3000 hours 60C/95%RH Gedney (iNEMI) [4] 85 microns 2200 cycles -40C/85C Okada (Murata) [3] 130 microns 500 cycles -60C/60C Hashemzadeh (Linköping) [2] 70 microns 3000 hours 60C/93%RH Peng (Freescale) [1] Maximum Length Time Period Environment Reference 50 1 2000 G 60 sec / frequency 6 G 50, 100, 200, 250 Sinusoidal N/A 20 G 10 – 2000 Sinusoidal Dunn 18 100 Events 0.3 6 Pulse Width (ms) 3000 G 500 G 1000 G Maximum Acceleration Mechanical Shock 22 hours Duration 18 Grms Maximum Acceleration 10 - 2000 Frequency (Hz) Random Type Extended Duration N/A 5 minutes Duration 20 G 3.5 Grms Maximum Acceleration 10 – 2000 10 – 4500 Frequency (Hz) Sinusoidal Random Type Resonance Sweep Ando Hashemzadeh References’] Tin whisker are probably the number one concern of military and avionic companies. Why? Because the current state of knowledge is relatively limited, with uncertainty as to root-cause (plating chemistry, contaminants, etc.) and how to accelerate this mechanism. N/A [6] QFP IBM Sn, SnCu, or SnPb QFP AMD SnBi TSOP Pd QFP Matsushita/Panasonic Sn TSOP Micron Sn, SnBi, or NiPdAu QFP / TSOP NEC Sn [5] QFP / TSOP Freescale SnBi TSOP Hynix NiPdAu TSOP 100% Sn QFP Philips/NXP Pd or SnPb QFP / TSOP Sony Mostly NiPdAu, with some Sn-Cu, Sn-Bi [4] TSOP Mostly Sn-Cu, Sn-Bi; some NiPdAu [3] QFP Renesas Technology NiPdAu TSOP Sn or SnPb QFP Infineon NiPdAu [2] QFP / TSOP STMicroelectronics NiPdAu or SnAg or SnBi TSOP (LSI) SnAg or SnCu TSOP (Memory) NiPdAu TSOP (Discretes) Toshiba NiPdAu QFP / TSOP Texas Instruments NiPdAu QFP / TSOP Samsung Sn [1] QFP / TSOP Intel Plating Package Company Printed Circuit Boards While typically overlooked by personnel preparing for lead- free, understanding lead-free printed circuit boards (PCBs) and their inherent risks is critical, as most issues experienced by consumer/computer was related to their PCBs. First, chose your laminate wisely. The glass transition temperature (Tg) should be appropriate. Too low, and you’ll experience delamination, warpage, and plated through hole (PTH) cracking. Too high and you’ll experience drilling issues and pad cratering (and pay higher costs). And don’t forget thermal stability (either time to delamination, T-260 or T-288, or temperature to decomposition, Td). Second, chose your solderability plating. No one plating is universal, as each plating has its risks and its benefits. • Electroless nickel/immersion gold (ENIG) can provide long storage life and prevents copper dissolution, but comes at risk of black pad, dewetting, crevice corrosion, poor shock performance, and poor adhesion with large BGAs • Immersion silver (ImAg) can also provide long storage life, but is susceptible to planar voiding, can cause degradation of plated through holes, and is very sensitive to sulfur gases (both in storage and in the field) • Immersion tin (ImSn) and organic solderability plating (OSP) are lower cost options, but have limited storage life (6-12 months) and OSP can cause poor hole fill during wave soldering • Lead-free hot air solder level (HASL) has seen increasing market share, primarily because of long-term storage and resistance to copper dissolution, but may not be compatible with some BGAs and thick (>90 mil) PCBs Finally, do not forget to qualify. This includes performing tests to assess conductive anodic filament (CAF) formation (IPC TM-650 2.6.25) and PTH fatigue (most commonly with interconnect stress testing) and construction analysis. 18% Solder Manufacturing Solder Long-Term Reliability Environment: Cold Temperature Failure Mechanism: Cold Pest Risk: Extremely Low Environment: Temperature Cycling Failure Mechanism: Creep, Elastic/Plastic Fatigue Risk: Finite element modeling based on material behaviors (creep equation) and an epidemiological study of results from accelerated life testing suggest that will be a minimal statistical difference in time to failure between SnPb and SAC305. These findings assume realistic worst-case environments (Tmax < 90C) and take into account the influence of long dwell times. Environment: Vibration Failure Mechanism: Elastic/Plastic Fatigue Risk: Additional data required, but initial results imply similar performance to SnPb when subjected to loads similar to field conditions. Environment: Mechanical Shock Failure Mechanism: Solder or Intermetallic Fracture Risk: Lead-free does perform worse than SnPb, but an even bigger driver is the board plating (nickel- intermetallics vs. copper intermetallics). Chai, ECTC 2005 Qualcomm B. Willis, SMART Group IBM Gold Circuits P. Biocca, Kester S. Zweigart, Solectron 2512 SMT Resistor Avoiding thermal shock cracks in ceramic capacitors • Orient terminations parallel to wave solder • Reduce maximum case size for wave soldering from 1210 to 0805 • Maintain a maximum thickness for wave soldering of 1.2 mm • C0G, X7R preferred for wave soldering wave • Use manufacturer’s recommended bond pad dimensions or smaller for soldering • Adequate spacing from hand soldering operations • Room temperature to preheat (max. 2-3oC/sec.) • Preheat to at least 150oC • Preheat to maximum temperature (max. 4- 5oC/sec.) • Cooling (max. 2-3oC/sec.) • Make sure assembly is less than 60oC before cleaning • Maintain belt speeds to a maximum of 1.2 to 1.5 meters/minute • Eliminate touchup or rework with solder irons
