Achieving High Reliability for Lead-Free Solder Joints ...€¦ · Ref: Joel Flumerfelt and Tom...

1

1

Achieving High Reliability for Lead-Free Solder Joints- Materials Consideration

Dr. Ning-Cheng Lee

Indium Corporation

Achieving High Reliability for Lead-Free Solder Joints – Materials ConsiderationOutline

1. Main stream lead-free soldering practices1) Prevailing lead-free alloys2) Leaded component surface finishes

2. Surface finishes issues1) Some issues with lead-free surface finishes2) Issue of ENIG3) Issue of ImAg

3. Mechanical properties1) Shear and pull strength2) Creep

4. Intermetallic compounds1) Interaction of Ni, Ge with P2) Interaction of Cu and Ni3) Effect of Cu content in SAC4) Effect of alloy additives5) Effect of phase transition on Intrinsic IMC cracking

5. Failure modes1) Grain boundary sliding and cavitation2) Grain coarsening3) Grain orientation4) Lead contamination5) Mixed alloys6) Effect of Cu pad on grain size

6. Reliability – thermal cycle1) Effect of thermal cycle test condition2) Effect of PCB surface finish (Cu vs Ni) and solder composition3) Polymer core solder ball

7. Reliability - fragility1) Effect of dopant on IMC growth2) Effect of additive on grain size and IMC type

8. Reliability – rigidity and ductility9. Reliability – electromigration

1) Influence of current, polarity and metallization on solder joint microstructure2) Effect of current density and metallization on IMC thickness3) Effect of solder composition

10. Reliability – tin whisker11. Reference

2

2

3

Part 1Main Stream LF

Soldering Practices

4

Prevailing Lead-Free Alloys

170

180

190

200

210

220

230

Tem

p (

C)

SnCu (+ dopants, e.g. Ni, Co, Ce)

SnAg (+ Cu, +Sb, + dopants, e.g. Mn, Ti, Al, Ni, Zn, Co, Pt, P, Ce)

SnAg (+ Bi, + Cu, + In, + dopants)

SnZn (+ Bi)

Reflow Wave Hand

√

√√

√ √

√

√

√

√

Most prevailing alloys: SnAgCu, with Ag 3-4%. Trend toward further reduced Ag.

BiSn(+Ag) (mp 140°C) on the rise.

3

5

Leading Component Surface Finishes

• Finishes– Survey results indicate top three finishes used – (1)

pure Sn, (2) SnBi, and (3) NiPdAu

– For leadframe packages, Ni/Pd/Au is favorite for pre-plated finishes

– For passives, pure Sn favored

6

Part 2Surface Finishes

Issues

4

7

Some Issues with Lead-Free Surface Finishes

• PCB– OSP: prone to oxidation & attack by corrosive

atmosphere– NiAu: interface brittle. Black pad with ENIG– ImAg: Champagne bubbles & Cu diffusion (reduce

wetting)– Sn: oxidation & IMC formation

• Components– Sn: prone to tin whisker formation, oxidation– ImAg: Ag migration– NiPdAu: Cu creep corrosion through cracks on finish

8

Issues of ENIG

5

9

ENIG

• Advantages – Depositing EN does not require electrical connections– EN has superior “throwing power” over electroplated nickel

offering superior coverage in, for example, plated through holes.

• Issue: Black Pad • Potential Causes

– Etching of the nickel surface immediately prior to and during the deposition of IG leading to interfacial tarnishing and bond separation,

– Phosphorus compounds at the interface of the IG and EN?

Ref: Joel Flumerfelt and Tom Schleisman, " Black Pad and Revisiting Methodologies", APEX, S02-02, Feb. 20-22, 2007, Los Angeles, CA

10

Black Pads

(1) Zequn Mei, Samuel K. Liem, and Allan Shih, “A Failure Analysis and Rework Method of Electronic Assembly on Electroless Ni / Immersion Au Surface Finish”, SMTA, Chicago, IL, 1999. (2) Nicholas Biunno, “A Root Cause Failure Mechanism For Solder Joint Integrity of Electroless Nickel/Immersion Gold Surface Finishes”, SMTA, Chicago, IL, 1999

Caused by a hyperactive “corrosive” IG process. This progress changes the near surface microstructure of P-Ni into a black band, with a marginal to a total non-wetting state

6

11

XPS (X-ray Photoelectron Spectroscopy) depth profile plot of IG over low phosphorus EN

panel showing presence of oxygen and carbon at and below

the gold-nickel interface.

Ref: Joel Flumerfelt and Tom Schleisman, " Black Pad and Revisiting Methodologies", APEX, S02-02, Feb. 20-22, 2007, Los Angeles, CA

XPS depth profile plot of IG over medium phosphorus EN

panel showing presence of oxygen and carbon at and

below the gold-nickel interface.

XPS depth profile plot of IG over high phosphorus EN

panel showing undetectable levels of oxygen and

carbon at the gold-nickel interface and in the bulk

EN.

ENIG system with a bulk P concentration of 9.5- 13% P is showing advantages compared to a lower P ENIG system, whereas the solder joint reliability is much superior and the Gold wire bondability is unique for an ENIG system. The recommended high P ENIG thickness is as followed:

-Electroless NiP: 5.5 (5 - 6) µm

-Immersion Gold: 0.05 (0.03 – 0.07) µm

Sven Lamprecht, Kuldip Johal, H.-J. Schreier, Hugh Roberts (Atotech Deutschland GmbH – Atotech USA), “Phosphorus in Electroless Nickel Layers – Curse or Blessing?”, Apex, S10-3, Anaheim, CA, Feb, 2004

12

SO2 Gas Results According to P Content of Electroless Ni Layer

Masahiro Nozu, Akira Kuzuhara, Atsuko Hayashi, Hiroshi Otake, Shigeo Hashimoto, and Donald Gudeczauskas (C.Uyemura & Co.,Ltd.), “High Phosphorous Electroless Nickel Process for Mobile Phone PWBs”, Apex, S10-1-1, Anaheim, CA, Feb, 2004

P ≥ 9% causes SO2 resistance

7

13

Comparison of Force Length Diagrams of 600µm SMD Pads, with Lead Free Solder Balls (SnAgCu) Determined with Three Different Bulk P Contents in the E’less Nickel Layers, after 1000 TC –55°C / +125°C


The force length diagrams for 8.0% P and 11.2% P show a narrow shape of all curves, indicating similar force length strength for all tested SMD BGA pads. Whereas the Nickel layer with 4.2% P show a wide spread of force length curves, indicating a wider spread (10N –16N) of total shear force for the tested SMD BGA pads.

Merge on 31 curves

Merge on 31 curves

Merge on 31 curves

14

SEM Image of a X-Section (40 000x mag.) of the NiSn IMC on an 8.0% P Nickel Layer Using a SnAgCu Solder Alloy


At IG process, Nickel is solved and Gold is deposit, and the P gets enriched at the interphase between the e’less Nickel layer and the Gold layer.

Assuming that a corrosion resistant e’less Nickel layer is less attacked by the immersion Gold reaction, at same Gold bath parameters, the Gold thickness will be lower compared to a less corrosion resistant Nickel P layer.

Quantitative P measurement shows, for a bulk P content of 8.0%, an increase of P of 80% at the interphase between Nickel layer and NiSn IMC. Whereas the Nickel layer with 11.2% P shows only a P increase of 50% at the interphase.

More corrosion resistant → less Ni removed by Solder → less P enrichment

NiNiSn

8% P

11.2% P

8

15

IG Thickness on Medium P (7-9%) Nickel Layers Compared two high P (9.5-13%) Nickel Layers, at Constant Immersion Gold Bath Parameters


A higher Gold thickness, using a medium P Nickel layer. This also means, the medium P Nickel layer has a stronger P enrichment at the Nickel Gold interphase compared to a high P Nickel layer. (See Figure below.)

The second P increase at the interphase occurs during soldering, when the molten solder solves Nickel and forms the NiSn IMC. Again P gets enriched at the surface of the e’less Nickel layer.

16

Issues of ImAg

- Microvoiding Mechanism

9

17

Muffadal Mukadam, Norman Armendariz*, Raiyo Aspandiar, Mike Witkowski, Victor Alvarez, Andrew Tong, Betty Phillips, and Gary Long (Intel Corporation), " PLANAR MICROVOIDING IN LEAD-FREE SECOND-LEVEL INTERCONNECT SOLDER JOINTS", SMTAI, September, 2006, Chicago, IL

Four (4) different LF PCB lots with increasing amounts of planar microvoidsthat reduce the number of temperature cycles the solder joint can survive.

Shows OM top views of PCB Cu surface after shearing ball (L) and an x-sectional view (R) of samples processed without ImAg and exhibited planar microvoid phenomena direct from a Cu (oxide) surface.

18


Cross-sectional view at a location where ImAg remained after SMT assembly, because it was not covered with solder paste and did not dissolve during SMT. Cu erosion was observed near the soldermask edge in the form of cavities or “caves.” under ImAg.

Shows an SEM image of a FIB prepared x-section (L), which clearly outlines PCB cave and ImAg roof features as compared to a conventionally prepared metallographic cross-section on the same sample (R).

10

19


Shows a X-sectional illustration of the mechanism from Cu-oxide corrosion of Cu “caves” in the PCB (L) to planar microvoid

formation during SMT reflow (R).

18

20


SEM X-sectional image (L) and EDS elemental analysis spectra (R) characterizing the PCB Cu cave feature with an Ag roof and Cu in an oxidized condition prior to SMT assembly.

Shows that an increased cave density (linear density cave length-μm/ length inspected-μm) was found in pre-SMT samples corresponding to increased microvoid (%) found in-parallel to SMT assembled X-sectioned samples.

11

21

Muffadal Mukadam, Norman Armendariz*, Raiyo Aspandiar, Mike Witkowski, Victor Alvarez, Andrew Tong, Betty Phillips, and Gary Long (Intel Corporation), " PLANAR MICROVOIDING IN LEAD-FREE SECOND-LEVEL INTERCONNECT SOLDER JOINTS", SMTAI, September, 2006, Chicago, IL 20

22


SEM cross section image of a bare PCB with ImAg prior to SMT assembly. Cu cave observed near the soldermask edge, demonstrating the crevice “path” galvanic corrosion mechanism with Cu oxide detected on the cave floor and relatively thin Ag near SM edge.

12

23

Part 3Mechanical Properties

24

Shear & Pull Strength

13

25

Fubin Song and S. W. Ricky Lee, “Investigation of IMC Thickness Effect on the Lead-free Solder Ball Attachment Strength: Comparison between Ball Shear Test and Cold Bump Pull Test Results”, 56th ECTC Proceedings, P. 1196-1203, San Diego, CA, May 30-June 2, 2006

Sensitivity toward IMC thickness is comparable for pull & shear test.

Pull Shear

26

Creep

14

27Ref: S. Wiese, M. Roellig, K.-J. Wolter, " Creep of Eutectic SnAgCu in Thermally Treated Solder Joints", 55th ECTC, P.1272-1281,May 31-June 3, 2005

Cooling rate effect not significant at 0.7

- 2/sec range

Joints on Ni creep slower than on Cu.

Perhaps dissolved Ni& Au refines grain size of

SnAg & hardens

SnAg

Cu

Ni


Thermally aged

As cast

A short thermal storage softens SAC significantly.

Longer thermal storage does not further change the creep behaviour of SAC

15


Cu and Au IMC significantly strengthen SA and SAC joints, thus creep slower.

Smaller solder volumes (FC) undercool less, & higher conc of dissolved base metal, thus creep slower than PWB & bulk.

SA behaves similar to SAC

n

A

stress

Creep rate

Solder (Joint) Volume Effect

30

Ref: Ahmer Syed, " Accumulated Creep Strain and Energy Density Based Thermal Fatigue Life Prediction Models for SnAgCu Solder Joints", 54th ECTC, P.737-746, June 1-4, 2004, Las Vegas, Nevada.

SAC creep rate curve cross over SnPb

16

Creep Behavior

31

Low stress High stress High stress

Resist changeSlow deformation

Stress beyond threshold value

Stress beyond threshold valueMuch Faster deformation

System with Long Range Order

System without Long Range OrderSlow deformation Fast deformation

Low stress High stress

Ning-Cheng Lee, Lead-free Reliability Course, 2014

32

Part 4IMC

17

Interaction of Ni, Ge with P

33*Keith Sweatman, Takatoshi Nishimura and Takuro Fukami, (Nihon Superior), “The Effects of Phosphorus in Lead-Free Solders”, SMTA International, Rosemont, IL, September 27 - October 1, 2015

Ni Impart Fluidity, P Erase it in SnCu


When added to Sn-0.7Cu-0.05Ni solders phosphorus reacts with the nickel to form nickel phosphide Ni2P thus reducing the beneficial effects that it has on solder fluidity and Cu6Sn5 stability. Shiny Surface: The unmodified Sn-0.7Cu alloy solidifies in two stages, with primary tin dendrites growing first with Sn-Cu6Sn5 eutectic finally freezing in the interdendritic spaces. This two stage solidification results in the dull cracked surface of the cast alloy (Figure 2(a)). The smooth bright surface of Sn-0.7Cu-0.05Ni cast under the same conditions indicates that the two stage solidification has been replaced by single stage pseudoeutectic solidification.

18

P Offset Ni & Ge Benefit in SnCu


When adding P to SnCuNiGe, Wetting of copper is slowed. Erosion of iron plated soldering tool tips is accelerated. The rate of dross production in tin-copper-nickel alloy that already have germanium as their antioxidant is increased, presumably by forming germanium phosphide GeP.

36

Interaction of Cu and Ni

C. Robert Kao, “Cross-interaction between Cu and Ni in lead-free solder joints”, TMS Lead Free Workshop, San Antonia, TX, March 12, 2006.

19

37C. Robert Kao, “Cross-interaction between Cu and Ni in lead-free solder joints”, TMS Lead Free Workshop, San Antonia, TX, March 12, 2006.

Ni 0 Ni 0.02-0.11 Ni 0.04-0.11 Ni 0.04-0.15

Increasing Ni conc cause increase in Ni inclusion in (Cu,Ni)6Sn5.

Ni inclusion result in a thicker IMC than Cu6Sn5

Gibbs free energy of formation: (Cu,Ni)6Sn5 > Cu6Sn5Hao Yu, Vesa Vuorinen and Jorma Kivilahti, “Effect of Ni on the Formation of Cu6Sn5 and Cu3Sn Intermetallics”, 56th ECTC Proceedings, P. 1204-1209, San Diego, CA, May 30-June 2, 2006


(Relatively large solder volume)

Cu

0.2

0.4

0.5

0.6

Increasing Cu conc cause shift of IMC from Ni3Sn4 to Cu6Sn5 on Ni

* Chemistry of Materials, 14, p.949, 2002. ** J. of Electronic Materials, 31, p.548, 2002

20


Kirkendall


Additional Cu migrated to Ni

side

Ni consumption suppressed by Cu migrated

SnPb ball

21

41

Effect of Cu Content in SAC

42Ref: Henry Y. Lu, Haluk Balkan, Joan Vrtis, and K.Y. Simon Ng, " Impact of Cu Content on the Sn-Ag-Cu Interconnects", 55th ECTC,P.113-119, May 31-June 3, 2005

Die shear strength not affected by the Cu content.

• Generally more solder joint voiding was seen in the LF paste assembled devices than that was seen in the dip flux assemblies.

• More voiding failures occurred in the 0.0Cu group than in any other alloy groups. There is a trend of reduction of voiding failure as the Cu content increases.

• Formation of Kirkendall voiding can be suppressed by high Cu content in SAC alloys, thus improve the reliability of solder joints on Cu pad.

• Presumably a higher conc. Cu in solder suppresses the diffusion of Cu from Cu3Sn toward solder.

22

43

Evolution of Interface with Increasing Cu Conc.

Cu suppress dissolution of Ni. Hence, Ni3(P,Sn) disappear first, followed by NiPSn. But it also promotes more IMC formation on PCB (Ni) side & nucleation of Ag3Sn plates.

Cu (die side)

Ni (PCB side)

SAC

0, 0.5

0% Cu

0.5, 2

1

(Ni,Cu)3Sn4

NiPSn (500 nm)

Ni3(P,Sn) (300 nm)

Ni (3.84 um)

SnAg

(Cu, Ni)6Sn5

Cu

NiSnP + Ni3(P,Sn) (250 nm)

(Cu, Ni)6Sn5 (2.8 um)

(Cu, Ni)6Sn5 (2.7 um)

Cu

SAC305

Ni (4.68 um)

NiSnP (240 nm)

(Cu, Ni)6Sn5

(Cu, Ni)6Sn5

Cu

SAC3010

Ni

(Cu, Ni)6Sn5

(Cu, Ni)6Sn5

Cu

SAC3020

Ni

Cu conc.

Crack location

High Cu result in more ductile failure (bulk solder) than brittle failure (IMC interface) at die shear test

No Ag3Sn plates in any locations of the solder joints for the 0.0Cu and 0.5Cu at time zero

High Cu lead to flourishing growth of Cu-Sn IMCs, which promotes the growth of Ag3Sn platelets.

Ref: Henry Y. Lu, Haluk Balkan, Joan Vrtis, and K.Y. Simon Ng, " Impact of Cu Content on the Sn-Ag-Cu Interconnects", 55th ECTC, P.113-119, May 31-June 3, 2005

43

44

(125C)

(-40~125C)

(-55~125C)

125C isothermal

TS

TC

Luhua Xu and John H.L. Pang (Nanyang Technological University), “Effect of Intermetallic and Kirkendall Voids Growth on Board Level Drop Reliability for SnAgCu Lead-free BGA Solder Joint”, 56th ECTC Proceedings, P. 275-282, San Diego, CA, May 30-June 2, 2006

TC(-40 ~ 125C, 15min high T dwell, 1 hr/cycle) and TS(-55 ~ 125 C, 5 min high T dwell, 17 min/cycle) aging, were conducted on Sn-3.8Ag-0.7Cu/Ni-Au BGA specimen for 500, 1000, 1500 and 2000 cycles. The IMC growth behavior measured for TC, TS and isothermal aging at 125oC are plotted in Fig 4.

Both high temp and stressaccelerate IMC growth. Effect of TS > TC

Effect of Heat History on IMC

23

45

Effect of Alloy Additives

46Masazumi Amagai (Texas Instruments, Japan), " A Study of Nano Particles in SnAg-Based Lead Free Solders for Intermetallic Compounds and Drop Test Performance", 56th ECTC Proceedings, P. 1170-1190, San Diego, CA, May 30-June 2, 2006

CuCu

Baseline

24

47

Masazumi Amagai (Texas Instruments, Japan), " A Study of Nano Particles in SnAg-Based Lead Free Solders for Intermetallic Compounds and Drop Test Performance", 56th ECTC Proceedings, P. 1170-1190, San Diego, CA, May 30-June 2, 2006

0.03Ni

Effective in retaining the fine IMC structure

Found in IMC

(A) Top view of IMC after 1X reflow

(A) IMC after 1X reflow

IMC elemental analysis (wt%) for Sn 3.0Ag 0.03Ni

(C) Top view of IMC after 4X reflow

(D) Cross-section of IMC after 4X reflow

(B) IMC after 4X reflow

(B) Cross-section of IMC after 1X reflow

IMC for Sn 3.0Ag 0.03Ni

47

48


0.03Co


Found in IMC



IMC elemental analysis (wt%) for Sn 3.0Ag 0.03Co





IMC for Sn 3.0Ag 0.03Co

48

25

49


0.05Pt


Found in IMC



IMC elemental analysis (wt%) for Sn 3.0Ag 0.05Pt





IMC for Sn 3.0Ag 0.05Pt

49

Effect of Phase Transition on Intrinsic IMC Cracking

50

50

26

Cracking and Phase Stability in Reaction Layers between Sn-Cu-Ni Solders and Cu Substrates

K. Nogita, G.M. Gourlay, and T. Nishimura, “Cracking and Phase Stability in Reaction Layers between Sn-Cu-Ni Solders and Cu Substrates”, Vol. 61 No. 6, p45-51, JOM, June 2009

Brittle Cu6Sn5 IMC is often associated with the presence of a large number of microcracks. Ni stabilizes the hexagonal allotrope of (Cu,Ni)6Sn5 at RT., minimize solid state transformation at service.

2.15% lower in volume

8.448 g/cm3

8.270 g/cm3

2.15% higher in volume

51

51

ReflowedBGA


after dissolving the tin phase in a solution of 35 g ortho-nitro-phenol and 50g NaOH in 1 L of water at 80'C.

Ni addition reduced crack no.

when tin is soldered onto Cu-xNi substrates, Nadditions suppress the formation of Cu3Sn in the reaction zone, thus no Cu3Sn seen.

grain size of Ni-containing(Cu,Ni)6Sn5 is significantly smaller than that of Cu6Sn5

52

52

27

In Ni-free Cu6Sn5, peaks around 186°C (heating) and 170'C (cooling), indicating the transformation from hexagonal ↔ monoclinic. No peaks seen with (Cu,Ni)6Sn5,

indicating no phase transformation occurs in the (Cu,Ni)6Sn 5 samples.


Cu6Sn5 (Cu,Ni)6Sn5

No phase transformation obsd

Phase transformation obsd

186C

170C

monoclinic

hexagonal

53

53

54

Part 5Failure Modes

28

55

Grain Boundary Sliding & Cavitation

56A.R. Zbrzeznya, P. Snugovskya, (Celestica) D.D. Perovicb (Univ. of Toronto), “Reliability of Lead-Free Chip Resistor Solder Joints Assembled on Boards with Different Finishes Using Different Reflow Cooling Rates”, IPC/JEDEC 5th International Conference on Lead Free Electronic Components and Assemblies, San Jose, CA, March 18-19, 2004

Aging and Accelerated Thermal Cycling (ATC) have been performed on 2512 chip resistors assembled with Sn3.8Ag0.7Cu (by weight percent) solder. During early stages of thermomechanical fatigue, a formation of pores and wedge microcracks occurred preferentially at the grain boundaries and the matrix/intermetallics interfaces. They only appeared in the highly stressed areas of the joints. As the cyclic deformation progressed, these small cracks were eventually linked up by an intergranular crack or multiple cracks.

29


The Pb-phase seemed to have a profound effect on the reliability of the joints. Close examination of the microstructures containing dispersed Pb-phase regions revealed that this ductile phase most likely participated in arresting the cracks.

58

Damage Mechanism

A.R. Zbrzeznya, P. Snugovskya, (Celestica) D.D. Perovicb (Univ. of Toronto), “Reliability of Lead-Free Chip Resistor Solder Joints Assembled on Boards with Different Finishes Using Different Reflow Cooling Rates”, IPC/JEDEC 5th International Conference on Lead Free Electronic Components and Assemblies, San Jose, CA, March 18-19, 2004

Grain boundary sliding, as evidenced on the surface of the fatigued joints, indicated that creep played a dominant role in the overall damage mechanisms. However, the presence of fatigue striations on the surface of the solder demonstrated that not only creep but also fatigue played a role in the joint failure. Consequently, similarly to the Sn-Pb eutectic solder, the creep-fatigue interaction was found to be responsible for the fractures of the Sn-Ag-Cu solder joints.

30

59

Grain Coarsening

60

James Oliver, Margareta Nylén, Olivier Rod, Christofer Markou, “FATIGUE PROPERTIES OF Sn3.5Ag0.7Cu SOLDER JOINTS AND EFFECTS OF Pb-CONTAMINATION”, SMTA International, September 22-26, 2002, Chicago, IL

SAC

SAC

SAC

SnPb

10 μm

31

61Gordon Gray (Tessera), “Lead-free soldering for CSP”, IPC/JEDEC 5th International Conference on Lead Free Electronic Components and Assemblies, San Jose, CA, March 18-19, 2004

SAC305 > SnPb

62

Gordon Gray (Tessera), “Lead-free soldering for CSP”, IPC/JEDEC 5th International Conference on Lead Free Electronic Components and Assemblies, San Jose, CA, March 18-19, 2004

Coarser grain of Sn63 than LF may be responsible for poorer TC results of Sn63.

32

63

Grain Orientation

T.R. Bieler and H. Jiang, L. P. Lehman, T. Kirkpatrick, and E. J. Cotts, " Influence of Sn Grain Size and Orientation on the Thermomechanical Response and Reliability of Pb-free Solder Joints", ECTC, p.1462-1467, May 30-June 2, 2006, San Diego, CA

64

Grain Orientation• Small joint is more vulnerable to

shock, due to – Reduced compliance due to

increasing IMC contribution– The joint may contain only one

grain. The anisotropic nature of grain orientation can cause uneven stress distribution from joints to joints.

– When C-axis is parallel to pad surface (organge color), crack more prone to happen.

• Desire– Dopants such as Ti which

suppress undercooling hence suppress instantaneous large crystal formation

– Dopants such as Ni, Co which refine the grain size.

(Bieler et al)

C-axis parallel to pad more prone to crack, due to large CTE mismatch & high modulus

Single grain

T.R. Bieler and H. Jiang, L. P. Lehman, T. Kirkpatrick, and E. J. Cotts, " Influence of Sn Grain Size and Orientation on the Thermomechanical Response and Reliability of Pb-free Solder Joints", ECTC, p.1462-1467, May 30-June 2, 2006, San Diego, CA

Crystal direction maps of joints from a row in a thermomechanically cycled package

33

65

Lead Contamination

66

C. Key Chung, Raiyo Aspandiar, K. Foo Leong, Cheng Siew Tay, “The Interactions of Lead (Pb) in Lead Free Solder (Sn/Ag/Cu) System”, 52nd ECTC, S04-P7, San Diego, CA, May 28-31, 2002.

voiding was observed due to the dissolution of Pb into the SnAgCu solder system.

34

67

James Oliver, Margareta Nylén, Olivier Rod, Christofer Markou, “FATIGUE PROPERTIES OF Sn3.5Ag0.7Cu SOLDER JOINTS AND EFFECTS OF Pb-CONTAMINATION”, SMTA International, September 22-26, 2002, Chicago, IL

68James Oliver, Margareta Nylén, Olivier Rod, Christofer Markou, “FATIGUE PROPERTIES OF Sn3.5Ag0.7Cu SOLDER JOINTS AND EFFECTS OF Pb-CONTAMINATION”, SMTA International, September 22-26, 2002, Chicago, IL

35

69

Mixed Alloys

70

C. Key Chung, Raiyo Aspandiar, K. Foo Leong, Cheng Siew Tay,“The Interactions of Lead (Pb) in Lead Free Solder (Sn/Ag/Cu) System”, 52nd ECTC, S04-P7, San Diego, CA, May 28-31, 2002.

Backward Compatibility solder joints showed severe grain growth if the peak soldering temperature was below the lead free solder ball melting point.

36

71

Non-homogenized Joint failure occurred after thermal cycles tests

• Thermal cycle Failures• The mix of lead + lead free

does not work for BGA.• First failure 137 cycles• Joint Failure average of 260

cycles to failure.• Normally under similar tests

1000-2000 cycles should be achieved .

• Temperatures cycle is -55ºc + 125ºc, dwell time 11 minutes.

Paul Wood, “Lead-Free Array Rework - What's Important In Lead Free Rework”, Nepcon Shenzin, August 30th2005 71

Effect of Cu Pad Grain Size

72

37

Cu Grain Size Increase with Increasing Plating

Current Density

73

Cu surface roughness increased rapidly as plating current density increased (Fig.3).

From the FIB picture (Fig.4), the grain become larger with plating current increasing, the Cu grain size of F1 is 1-2μm, but the Cu grain size of A1 is 0.2-0.3μm.

Kenny Cao, KH Tan, CM Lai, Li Zhang (Jiangyin Changdian Advanced Package Company), “Solder Joints Reliability with Different Cu Plating Current Density in Wafer Level Chip Scale Packaging (WLCSP)”, 2009 International Conference on Electronic Packaging Technology & High Density Packaging (ICEPT-HDP), p.819-823

Atom Diffusion at Grain Boundary Faster Than at Grain

• In polycrystal Cu, the grain boundary would be as the direct path for atom diffusion, which have much greater diffusion rate than that in grain itself. The Cu grain size for larger plating current density is much greater than that lower plating current density, grain boundary supplies the easier diffusion path for fine grain structure.

• Kirkendall Voids: In fine grain microstructure vacancy should start forming at the site of grain boundary and Cu3Sn interface, then the vacancies were accumulated together and formed the void, when the grain size is very small, voids will be linked and the initial crack generate.

74H.F. Zou, “Morphologies, orientation relationships and evolution of Cu6Sn5 grains formed between molten Sn and Cu single crystals,” Acta Materialia, No. 56 (2008), pp. 2649-2662.

38

Relation Between Plating Cu UBM Microstructure,

IMC Layer Growth and Cu Plating Process

• The growth rate of total IMC layer and Kirkendall voids increased with decreasing Cu grain and increasing thermal aging time. After long aging time, there will be delamination between IMC and UBM layer.

75

Kenny Cao, KH Tan, CM Lai, Li Zhang (Jiangyin Changdian Advanced Package Company), “Solder Joints Reliability with Different Cu Plating Current Density in Wafer Level Chip Scale Packaging (WLCSP)”, 2009 International Conference on Electronic Packaging Technology & High Density Packaging (ICEPT-HDP), p.819-823

Cu3Sn

Cu6Sn5+Cu3Sn

Smaller Cu grain

Smaller Cu grain

76

Part 6Reliability –

Thermal Cycle

39

77

Effect of Thermal Cycle Test Condition

78

Michael Osterman, Abhijit Dasgupta, and Bongtae Han (University of Maryland), “A Strain Range Based Model for Life Assessment ofPb-free SAC Solder Interconnects”, Bala Nandagopal, Zequn Mei and Sue Teng (Cisco Systems, Inc.), “Microstructure and Thermal Fatigue Life of BGAs with Eutectic Sn-Ag-Cu Balls Assembled at 210°C with Eutectic Sn-Pb Solder Paste”, 56th ECTC Proceedings, P. 884-890, San Diego, CA, May 30-June 2, 2006

Reliability:

At low medium temp,

LF > SnPb (L)

At high medium temp,

SnPb > LF

Reliability decreases with increasing medium temp (ave of low & high) & increasing dwell time.

Increasing medium temp & dwell time

LF

L

LFL

25

7550

40

79

Effect of PCB Surface Finish (Cu vs Ni) and Solder Composition


Solder joints to copper showed a significantly higher number of cycles to first failure than the joints on nickel. Better reliability of the copper joints will be explained in terms of the copper content in the bulk. (Cu > Ni) SAC387

First Failure:

Board finish: Cu > Ni

Component finish: Sn8Pb > Sn

41


High Ag High TCT Life

• Changes in Ag content can have significant impact on thermal fatigue reliability

• Terashima et al. found that a decrease of Ag content from 4% to 1% decreases the thermal fatigue life (first failure) by a factor of about 2

– -40/125°C, 10 min dwell.– All alloys had 0.5% Cu– Performance relative to

eutectic Sn-Pb not reported

• Addition of other alloying elements which affect undercooling, formation of various IMCs, matrix properties & microstructure not well understood

S. Terashima, et al., J. Elec. Mater., Vol. 32, No. 12, p.1527 (2003). 82

42

Effect of Ag Content

83

Richard Parker, Richard Coyle, Gregory Henshall, Joe Smetana, Elizabeth Benedetto, “iNEMI Pb-FREE ALLOY CHARACTERIZATION PROJECT REPORT: PART II - THERMAL FATIGUE RESULTS FOR TWO COMMON TEMPERATURE CYCLES”, SMTAI, p.348-358, Orlando, FL, Oct. 14-18, 2012

High Ag result in long TCT life

All BGA assembled with SAC305 paste

Polymer Core Solder Ball

84

43

Polymer Core Solder Ball

(PCSB)

Hiroya Ishida, Kiyoto Matsushita (Sekisui Chemical), “Characteristics of Ceramic BGA using Polymer Core Solder Balls”, ECTC, p.404-410, May 27-30, 2014

550µ670µ

(95Sn5Ag) → SAC(on solder. Form (Cu,Ni)6Sn5 at soldering)

Diffusion barrier

(divinyl benzeneCTE 40ppm)

(20µ)

85

Mottled nickel doping on the solder surface is used to realize a finer IMC, (Cu,Ni)6Sn5, at the electrode interface.

Smaller standoff

Hiroya Ishida, Kiyoto Matsushita (Sekisui Chemical), “Characteristics of Ceramic BGA using Polymer Core Solder Balls”, ECTC, p.404-410, May 27-30, 2014

Bending Test

TCT

TCT

TCT

TCT

86

1. Higher standoff2. The Young’s modulus of the

polymer core (4.7 GPa) is smaller than that of the solder (53.9 Gpa), the polymer core disperses the stress more and also weakens the stress at the edge of bump.

2X

3X

44

87

Part 7Reliability - Fragility

88

Ref:Vijay Wakharkar& Ashay Dani, “Microelectronic Packaging Materials Microelectronic Packaging Materials Development & Integration Development & Integration Challenges for Lead Free Challenges for Lead Free”, Lead-free workshop, TMS, San Antonio, TX, March 12, 2006.

45

89

How Hard is SAC Alloys

M. Date, T. Shoji, M. Fujiyoshi, and K. Sato (Hitachi), “Pb-free Solder Ball with Higher Impact Reliability”, Intel Pb-free Technology Forum, 18th – 20th July 2005, Penang, Malaysia

90

Ref:Vijay Wakharkar& Ashay Dani, “Microelectronic Packaging Materials Microelectronic Packaging Materials Development & Integration Development & Integration Challenges for Lead Free Challenges for Lead Free”, Lead-free workshop, TMS, San Antonio, TX, March 12, 2006.

46

Effect of Component Finish

91

Ref: Minna Arra, Todd Castello, Dongkai Shangguan, Eero Ristolainen, " CHARACTERIZATION OF MECHANICAL PERFORMANCE OF SN/AG/CU SOLDER JOINTS WITH DIFFERENT COMPONENT LEAD COATINGS", SMTAI, p.728-734, Chicago, IL, September, 2003.

Better wetting (heel fillet height) results in higher pull strength, but not better drop performance.

Wetting dictated by finish chemistries.

92Robert Darveaux, Corey Reichman, Nokibul Islam (Amkor Technology, Inc.), “Interface Failure in Lead Free Solder Joints”, 56th

ECTC Proceedings, P. 906-917, San Diego, CA, May 30-June 2, 2006

1. More reflow heat cause more brittle failure on Cu than on NiAu

2. Propensity toward brittle failure: ImSn, ENIG > OSP > NiAu

CuCu/ImSn

Cu/ImSn

Cu

ENIG

NiAu NiAu

47

93Fubin Song and S. W. Ricky Lee, “Investigation of IMC Thickness Effect on the Lead-free Solder Ball Attachment Strength: Comparison between Ball Shear Test and Cold Bump Pull Test Results”, 56th ECTC Proceedings, P. 1196-1203, San Diego, CA, May 30-June 2, 2006

The brittle failure on the ENIG was induced by weak joint between the IMC and the Ni layers and the brittleness of IMC itself. The brittle failure on the OSP pad was induced mainly by weak joint of Cu6Sn5 and Cu3Sn IMC phases.

Cu6Sn5Cu3Sn

Cu

SAC

Ni3Sn4

Ni

SAC

94

Fubin Song and S. W. Ricky Lee, “Investigation of IMC Thickness Effect on the Lead-free Solder Ball Attachment Strength: Comparison between Ball Shear Test and Cold Bump Pull Test Results”, 56th ECTC Proceedings, P. 1196-1203, San Diego, CA, May 30-June 2, 2006

IMC growth rate:

SA > SAC

OSP > ENIG

(150C)

OSP

ENIG

IMC Growth Rate – OSP vs ENIG

SA

SAC

SA

SAC

48

Effect of Isothermal Aging

95Ref: Tz-Cheng Chiu, Kejun Zeng, Roger Stierman and Darvin Edwards, Kazuaki Ano, " Effect of Thermal Aging on Board Level Drop Reliability for Pb-free BGA Packages", 54th ECTC, P.1256-1262, June 1-4, 2004, Las Vegas, Nevada.

125C aging

Kirkendall voiding developed quickly at 125C

3D 10D

20D 40D

96Luhua Xu and John H.L. Pang (Nanyang Technological University), “Effect of Intermetallic and Kirkendall Voids Growth on BoardLevel Drop Reliability for SnAgCu Lead-free BGA Solder Joint”, 56th ECTC Proceedings, P. 275-282, San Diego, CA, May 30-June 2, 2006

Fragility Reliability –OSP vs ENIG

Drop lifetime decreases with increasing TC aging.

Before TC aging, OSP has longer drop lifetime than ENIG.

After TC aging, OSP degrades rapidly & has shorter lifetime than ENIG

Effect of Thermal Cycling

49

97

Drop Test: Sn63 > SAC3051500G, 0.5 ms pulse, face-down, 30 drops

Gordon Gray (Tessera), “Lead-free soldering for CSP”, IPC/JEDEC 5th International Conference on Lead Free Electronic Components and Assemblies, San Jose, CA, March 18-19, 2004

Effect of Intermetallic Morphology

Effect of Dopant on IMC Growth

98

50

99

• High Shock Resistance, Good TCT Performance (Low Ag)

– Example (SACM0510 – Sn0.5Ag1Cu+Mn)-Sphere

Medium Melting Alloys

“The Second Generation Shock Resistant And Thermally Reliable Low Ag SAC Solder Doped With Mn”, SMTA International, Fort Worth, TX, Oct. 13-17, 2013

Thermal Cycling Test Results

Bump AlloyAssembly Condition

Characteristic Life 63.2% (η)

Characteristic Life 50%

Weibull Slope (β)

ValueRatio

(Mn/105)Value

Ratio (Mn/105)

SAC0510MFlux

24271.61

23171.62

7.26

SAC105 1510 1431 6.68

SAC0510MSAC305 paste

23611.37

22581.45

8.03SAC105 1726 1559 4.36 100

Bump Alloy

Characteristic Life 63.2% (η)

ValueRatio

(alloy/105)SAC105 1468 1

SAC105M 2034 1.39SAC305 1905 1.30

TCT (-40°C/125°C) performance of BGA assemblies

-55C/125C

Liu etc, SMTAI, p.920-934, October 4-8, 2009, San Diego, CA.


51

TCT Results

101

Mn105

Significant recrystallization

Minute recrystallization

Earlier work Mn stabilize microstructure

0510M

Liu etc, SMTAI, p.920-934, October 4-8, 2009, San Diego, CA.


Effect of Additive on Grain Size and IMC Type

102

102

102

52

K. Okamoto, K. Nomora, S. Doi, T. Akamatsu, S. Sakuyama, and K. Uenishi (Fujitsu Laboratories, Osaka Univ.), “Effect of Sb and Zn addition on Impact Resistance Improvement of Sn-Bi Solder Joints”, IMAPS 2014, Sept. 30-Oct. 3, 2013, Orlando, FL, USA.

100 balls on electrolytic NiAu, with Au 0.05µ, and Ni 2-5µ; 0.45mm pitch.BGA assembled with 180C peak temp, using pastes1. Sn-57Bi-0.5Sb-0.5Zn2. Sn-57Bi-0.5Sb3. Sn-57Bi-1Ag4. Sn-58Bi

103

Sn-57Bi-1Ag

A: brittle crystalline BiSnB: brittle CuSn IMC

103


Sb refined grain size, and significantly increased drop no.

But, this Sb benefit vanished after thermal aging.

104

104

104

53


Bi

Sn

Sub-micron sized SnSb IMC particles precipitated at grain boundaries of near Sn phase, refined grain size. Zn was depleted from solder by forming Cu-Zn IMC at interface.

IMC layer of Sn-Bi-Sb-Zn is lower in hardness than that of Sn-Bi-Sb, thus serve as an effective stress relaxation and suppressed crack formation at drop test

105

105

105


IMC of SnBiSbZn changed from Cu6Sn5 to Cu5Zn8, plus w/o Bi-rich layer, the joint is more crack resistant after high temp aging.

106

106

106

54

Part 8

Reliability –

Rigidity & Ductility

107

Composition of Solder Alloys Evaluated & Test Condition

108Jie Geng, Hongwen Zhang, Francis Mutuku, Ning-Cheng Lee, “Novel Solder Alloy with Wide Service Temperature Capability for Automotive Applications”, APEX, San Diego, CA, Feb 27-Mar 1, 2018

55

Solder joint RT shear strength measured after thermal aging

109

Jie Geng, Hongwen Zhang, Francis Mutuku, Ning-Cheng Lee, “Novel Solder Alloy with Wide Service Temperature Capability for Automotive Applications”, APEX, San Diego, CA, Feb 27-Mar 1, 2018

Solder joint RT shear strength after harsh

TCT (-40°C/175°C) & TST (-55/155°C) up to

3000 cycles.

110


56

Dye-and-Pry results for samples after 2343 cycles at TCT (-40°C/175°C) treatment

111

Only very little bonding (light colored) remained for 305 & Innolot

276 remain bonded mostly


SEM microstructure of cross-sectioned solder joints after reflowed with peak255 profile, with blocky Ag3Sn

plates or rods indicated by arrow sign

112

Fine, uniform microstructureJie Geng, Hongwen Zhang, Francis Mutuku, Ning-Cheng Lee, “Novel Solder Alloy with Wide Service Temperature Capability for Automotive Applications”, APEX, San Diego, CA, Feb 27-Mar 1, 2018

57

Shear test curves for solder joints of Cu die (3mm x 3mm) on Alloy42 substrate

113

At high temperature, 276 more ductile than both SAC305 & InnolotInnolot very brittle.

brittle brittleductile

ductile

276276

Innolot

Innolot

305305


Fractured die back surface of as-reflowed SAC305 joints

114

SAC305 & 276 showed ductile texture

SAC305 Innolot

276

Brittle, ceramic-like texture


58

Conclusion

• 90.6Sn3.2Ag0.7Cu5.5Sb (Indalloy276) melting range 223 to 232°C• For less stressed aging condition, at 125C aging, three alloys were comparable. At

175C aging, 276 up to 2.5X stronger than Innolot & SAC305

• At harsh condition, TST -55°C/155°C for 3000 cycles, die shear strength of 276 was 8X of that of Innolot and SAC305. For TCT (-40°C/175°C) for 3000 cycles, die shear strength of 276 was 11X to 20X of Innolot and SAC305

• For SMT BGA solder joints at harsh condition TCT (-55°C/125°C and -40°C/150°C), for 1% failure, 276 was 2X to 3X higher in cycling life than Innolot,

• Both 276 and Innolot are alloys based on SnAgCu, but reinforced with precipitate hardening and solution hardening. 276 exhibited a finer microstructure with less particles dispersed, while Innolot exhibited more particles with some blocky Ag3Sn plates or rods. 276 is rigid and ductile, while Innolot is rigid but brittle

• Under the harsh test condition where ΔT was high, the dimension mismatch between parts and substrate became very significant due to CTE mismatch. This significant dimension mismatch would cause a brittle joint to crack quickly, as seen on Innolot. The challenge was more tolerable for a ductile joint, as shown by 276.

• Overall, to achieve high reliability under a wide service temperature environment, a balanced ductility and rigidity for solder alloy is critical for success.

115Jie Geng, Hongwen Zhang, Francis Mutuku, Ning-Cheng Lee, “Novel Solder Alloy with Wide Service Temperature Capability for Automotive Applications”, APEX, San Diego, CA, Feb 27-Mar 1, 2018

116

Part 9Reliability -

Electromigration

59

117

Influence of Current, Polarity & Metallization on Solder Joint

Microstructure

118Ref: Bernd Ebersberger, Robert Bauer, Lars Alexa, " Reliability of Lead-Free SnAg Solder Bumps: Influence of Electromigration and Temperature", 55th ECTC, P.1407-1415, May 31-June 3, 2005

150C/13000h 150C/200mA/10000h

NiNi

Cu Cu

Current accelerate IMC formation

60

119Ref: Bernd Ebersberger, Robert Bauer, Lars Alexa, " Reliability of Lead-Free SnAg Solder Bumps: Influence of Electromigration and Temperature", 55th ECTC, P.1407-1415, May 31-June 3, 2005

Ni

CuCu

Ni

Electron flow from Cu to Ni cause faster IMC growth than Ni to Cu

120

Effect of Current Density & Metallization on IMC Thickness

61

121

Cu/SAC387/Cu

At cathode side, IMC layer diminishing rate & void formation rate increase with increasing current density & time

Cu Cu

anode

cathode

eRef: Shengquan Ou and K. N. Tu, " A Study of Electromigration in Sn3.5Ag and Sn3.8Ag0.7Cu Solder Lines", ", 55th ECTC, P.1445-1450, May 31-June 3, 2005

Increasing current density

1X 2X 4X

122

Luhua Xu and John H.L. Pang (Nanyang Technological University), “Combined Thermal and Electromigration Exposure Effect on SnAgCu BGA Solder Joint Reliability”, 56th ECTC Proceedings, P. 1154-1159, San Diego, CA, May 30-June 2, 2006

eFor ENIG/SAC387/ENIG:

IMC thickness increase with time under high current density

Anode side grow faster than cathode side. Both are faster than isothermal.

ENIG/SAC387/ENIG Ni

Ni

Ni

Ni

Sn → Ni

Ni → Sn

isothermal

Anode

Cathode

0 hr

47 hr

86 hr

cathode

anode

e

62

Effect of Solder Composition

123

Design of Cu Pillars & Solder Bumps

124

Ni Cu

Cu Cu

63

125

The EM resistance ranking (145C, with Ni or thick Cu): High Pb < Sn63 < SnAg < Cu Pillar & μ-bump

Ahmer Syed, Karthikeyan Dhandapani, Robert Moody, Lou Nicholls, and Mike Kelly – Amkor Technology, “Cu Pillar and μ-Bump Electromigration Reliability and Comparison with High Pb, SnPb, and SnAg Bumps“, ECTC, p.332-339, Lake Buena Vista, Florida, May 31-June 3, 2011

all of these bumps were tested using Cu+SOP finish on the substrate

Cu Pillar can result in 3 to 4X better performance than SnAg bump

Cu-SnAg-Cu μ-bump

5500 hours of testing has been completed for Cu-SnAg-Cu μ-bump for all 5 conditions without any failure

At 145C, Pb migrate faster than Sn

So, Is High-Pb Bad on TCT?

126

J. D. Wu, P. J. Zheng, C.W. Lee, S.C. Hung, and J.J. Lee (ASE),” A Study in Flip-Chip UBiWBump Reliability with Effects of SnPb Solder Composition”, IEEE 03CH37400. 41st A nnual International Reliability Physics Symposium, p.132-139, Dallas, Texas, 2003

EM resistance: solder bump on thin film Al/NiV/Cu UBM: Pb95 > Pb90 > Sn63 bonded on ENIG pad (150-160C).Thin Cu could not deplete Sn in high Pb. At high temp, although Pb migrate faster, remaining Sn retarded Pb migration. (Sn migrate slower, higher mp, also less SnPb grain boundary)

TTF:Time to first failure

MTTF:Mean time to failure

High temp but with thin Cu

64

EM Overview• EM mechanisms

– Current density >103 A/cm2 prone to have EM

– Grain boundary between Sn & Pb easy migration path.

– In RDL, EM thru grain boundary of Al & Cu

– Cu & Ni diffuse through C-axis of Sn grain fast

– Ag ≥ 1% in SnAg retard migration due to IMCs at cyclic twinning boundaries

• EM resistance of solder joints

– (< 100C): Sn < Pb

– (> 100C): Pb < Sn, but presence of Sn retards Pb migration

– (thick Cu pad): high Pb < Sn63 < LF < Cu pillar

– (thin Cu pad): Sn63 < high Pb

– solder (< Al) < Cu < Ni (< IMC ?)

– large solder volume < small solder volume (easier IMC conversion)

– Ni to Ni & Cu to Cu < Ni to Cu (exhibit CuSn IMC protection)

– Cu/solder/Cu < Cu/solder/Ni < Cu pillar/solder/Ni < Ni/solder/Ni < Ni/thin Cu/solder/Ni < Cu/IMC/Cu, Ni/IMC/Ni (Cu pillar under Ni desired for all Ni)

• EM resistance of RDL

– larger RDL in via diam, larger pad opening, thicker Cu UBM, larger taper trace, avoid solder on RDL in via reduce EM

127Ning-Cheng Lee, EM course, 2014

128

Part 10Reliability –Tin Whisker

65

129Swaminath Prasad, Flynn Carson, G.S. Kim, J.S. Lee, Y.C. Park, Y.S. Kim & K.S. Min, S.S. Lu, Liu Hui, & Xu Hai, S.H. Khor & C.L. Tan, “Plating Chemical Evaluations and Reliability of Pb-Free Leadframe Packages”, Pan Pacific: February 13, 2001.

134

130

Tin Whisker Suppression• Growth due to compressive stress• Augmented by

– Cu-Sn IMC formation at grain boundary

– Small grain size• Alleviated by

– Ni plating on Cu– Large grain size– Thin solder layer– Surface oxide– Alloying with Bi, In– Baking treatment (150°C/1 hr)

Chen Xu, Yun Zhang, Chonglun Fan and Joseph A. Abys, "Driving Force for Sn Whisker Formation: Compressive Stress - Pathways for its generation and Remedies for its Elimination and Minimization", QuickStart Conference, Libertyville, IL, Sept. 25, 2004

66

131

Is Your Joint OK?

Date post:	13-Jul-2020
Category:	Documents
Upload:	others
View:	38 times
Download:	3 times

Achieving High Reliability for Lead-Free Solder Joints ...€¦ · Ref: Joel Flumerfelt and Tom...

Documents