Second Generation Solder Alloys - Update

Dr. Nathan Blattau What is the Perfect Alloy? Is there one?

There will always be tradeoffs. So, there can only be a perfect alloy for a particular application. The following table addresses general consumer applications: Desired Attribute Comment Lower Melting Point Closer to 190C would be desirable Lower Modulus Reduction from 51 to 40 GPa (near SnPb) Good wetting behavior Wetting time of 0.5 sec or less Stable behavior Preferably not precipitation hardened or at least rapidly softens (so properties are consistent after assembly) Low yield strength combined Similar to SnPb – providing compliance without with low work hardening rate suffering damage in fatigue. Low Cu dissolution To prevent erosion of Cu traces Low surface tension For covering of Cu features and wicking up PTHs Solder Selection

• Considerations include SnAg?? SnAgCu SnCu ‐ PRICE! – Less silver content ‐ Insufficient performance ‐ Failure mechanisms SAC305 • Market has chosen SAC305 as the SNC default SMT pb-free solder SAC205 material. However, there is still proliferation and evolution of material sets SNCX • HASL and through hole soldering SAC105 are typically either SnCu+Ni (SNC) or SAC305 SnAgCuXX ex. Innolot

3 Solder Alloy Performance - Reliability

• Most electronic failures are thermo-mechanically related ‐ By thermally induced stresses and strains ‐ Root cause: excessive differences in coefficient of thermal expansion

Source: Syed, Ahmer. "Accumulated creep strain and energy density based thermal fatigue life prediction models for A. MacDiarmid, “Thermal Cycling SnAgCu solder joints." 2004 Proceedings. 54th Electronic Components and Technology Conference (IEEE Cat. No. Failures”, RIAC Journal, Jan., 2011. 04CH37546). Vol. 1. IEEE, 2004. Temperature cycling: The solder fatigue mechanism

SnPb Solder

• Grain growth in solder is an indicator of fatigue

Source: Werner Engelmaier, Engelmaier Associates, L.C.

TEMPERATURE CYCLING RELIABILITY OF REBALLED AND REWORKED BALL GRID ARRAY PACKAGES IN SNPB AND SAC ASSEMBLY, Lei Nie, Doctor of Philosophy (Ph.D.), 2010

Single grain solder joint (pb-free) Grain refinement solder joint (pb-free) –thermal cycling Temperature Cycling: Low Cycle Fatigue

• Impossible to directly measure the stress or strain in the solder

• Usually due to shear strain from CTE mismatch between the component and the PCB

6 Thermal Cycling - Examples

Chip Resistor Source: Tohmyoh, Hironori, et al. “Estimation and Visualization of QFN the Fatigue Life of Pb-Free Sac Solder Bump Joints Under Thermal Cycling.” Microelectronics Reliability, vol. 53, no. 2, 2013, pp. 314–320., doi:10.1016/j.microrel.2012.08.012.

SOT Alloy 42 SOT Alloy 42 Source: Sun, Peng, et al. "Underfill selection for large body (50× 50mm) lidded flip chip BGA package with ELK 40nm Pb-free bumps." Electronic Packaging Technology and High Density Packaging (ICEPT-HDP), 2012 13th International Conference on. IEEE, 2012. The Current State of Lead-Free Copper Erosion • Component suppliers ‐ SAC305 dominant, some low silver alloys (SAC205, SAC105) for BGA spheres for improved drop shock performance • Solder Paste ‐ SAC305 dominant ‐ Increasing use of High Rel Solders • Innolot, MaxRel and more complex antimony and bismuth containg alloys ‐ Increasing interest in Bismuth based solders • Wave and Rework (mostly SN100 and derivatives) ‐ Sn07Cu+Ni (SN100C) ‐ Sn07Cu+Co (SN100e) ‐ Sn07Cu+Ni+Bi (K100LD) FeSn2 How to manage wave solder alloy contaminations • HASL PCB Coating Gerjan Diepstraten & Harry Trip Cobar Europe BV Balver Zinn ‐ Sn07Cu+Ni (SN100C)

8 Solder Trends

• SAC305 is for surface mount reflow (SMT), Near eutectic is Sn3.8Ag0.7Cu

• SAC105 is being used for area array components in mobile applications to improve drop shock performance

• SNC pervasive in wave solder and HASL ‐ Increasing acceptance for SMT

• Intensive positioning for “X” alloys (SACX, SNCX) • Bismuth Tin alloys starting to be considered for temperature sensitive devices

K-W Moon et al, J. Electronic Materials, 29 (2000) 1122-1236

9 Advantages of SAC305

• It was easiest to widely adopt when the LF transition was required. • Its high strength provides better thermal cycle behavior for compliant packages (BGAs, CSPs, QFPs). • Its high yield strength enables better high cycle fatigue performance (low amplitude vibration). SAC Solder Alloy Wets Well, It Just Doesn't Spread Well, Indium Corporation, Dr. Ron Lasky • The wetting properties are sufficient for surface mount components • Its higher creep resistance enables higher operating conditions. Weaknesses of SAC305

• Its high liquidus temp requires up to 260C processing (higher energy usage – not really “green”, stresses the PCB and components). This can cause higher warpage of the components and solder oxidation issues leading to a greater propensity for head in pillow defects • It is a precipitation hardened alloy so the mechanical properties change dramatically depending on processing and aging conditions. • Its marginal spreading behavior (high surface tension), Cu dissolution, and cost are not ideal. • Its high modulus results in pad cratering as a common failure mode (under dynamic strain). • The thermal cycle reliability is worse for “stiff” components such as resistors and capacitors (under severe thermal cycling). • Shock performance is much lower than SnPb. 1 Parts Standardization & Management Committee (PSMC) Spring Conference McLean, VA April 20-22, 2010 SAC105

• Improved drop and shock performance over SAC305 • Thermal cycle life is less than SAC305 • Creep rate is very high • Lower copper dissolution rates in SMT joints • Reduced intermetallic compounds and occurrences of silver tin platelets • Is greatly improved with additions of Mn or Ce (more data may prove these to be Drop Shock Reliability of Lead-Free Alloys – Effect of Micro-Additives winners as a ball alloy) Ranjit S Pandher, Brian G Lewis, Raghasudha Vangaveti, Bawa Singh Cookson Electronics, Assembly Materials Group 600 Route 440, Jersey City, NJ 07304, USA Email: [email protected] Low or No Silver Alloys (SN100C) • Less creep resistant than higher silver containing alloys • Thermal cycling reliability decreases with low/no silver content solders • Lower modulus and greater ductility improve drop shock performance

13 Low Silver Alloy Reliability

• Similar or better performance than tin-lead for BGA components

-15°C to 125°C, 60-minute dwell S. Terashima , et. al., “ Effect of Silver Content on Thermal Fatigue Life of Sn - x Ag - 0.5Cu Flip - Chip Interconnects , ” J. Electr. Mater. , 2003 . Thermal Cycling Reliability of Alternative Low-Silver Tin-based Solders Elviz George, Michael Osterman, Michael Pecht, Richard Coyle, Richard Parker, and Elizabeth Benedetto

14 Bismuth Solders • Low melting point alloy of Bismuth and Tin • Eutectic 58% Bi 42% Sn ‐ 139°C • Non Eutectic 63% Sn 37% Bi ‐ 174°C Liquidus ‐ 139°C Solidus • A lot of interest in mixed solder joints ‐ SAC solder sphere with Bismuth solder paste

‐ Critical components would have BiSn spheres THE APPLICATION OF BI- BASED SOLDERS FOR LOW TEMPERATURE REFLOW TO REDUCE COST WHILE IMPROVING SMT YIELDS IN CLIENT COMPUTING SYSTEMS Scott Mokler, Ph.D., P.E., Raiyo Aspandiar, Ph.D., Kevin Byrd, Olivia Chen, Satyajit Walwadkar, Kok Kwan Tang, Mukul Renavikar and Sandeep Sane Intel Corporation Hillsboro, OR, USA Introduction to Low Temperature Soldering - Intel [email protected]

15 Main Concerns - Bismuth

• Drop shock performance ‐ If you thought SAC was bad ‐ Worse than SAC305

• Pb contamination LOW TEMPERATURE SOLDERING USING SN-BI ALLOYS Morgana Ribas, Ph.D., Anil Kumar, Divya Kosuri, Raghu • Bismuth segregation/aging (120°C) R. Rangaraju, Pritha Choudhury, Ph.D., Suresh Telu, Ph.D., Siuli Sarkar, Ph.D. Alpha Assembly Solutions, Alpha Assembly Solutions India R&D Centre Bangalore, KA, India [email protected]

58Bi42Sn Joints contaminated with Pb (after 835 Thermal Cycles -55 to +125 °C) Suspect issue with SnPbBi ternary eutectic (Tmelt = 96 °C)

Woodrow, “The Effects of Trace Amounts of Lead on the Reliability of Six Lead-Free Solders,” IPC Proceedings of the 3rd International Conference Improving tensile and fatigue properties of Sn–58Bi/Cu on Lead-Free Components and Assemblies, San solder joints through alloying substrate Jose, CA April 23-24 (2003) QingKe Zhang, HeFei Zou, and Zhe-Feng Zhanga) Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences

16 Bismuth – Thermal Cycling Reliability

• Significant reduction in thermal cycling performance (mixed)

Low Temperature Soldering: Thermal Cycling Reliability Performance Morgana Ribas, Ph.D., Prathap Augustine, Pritha Choudhury, Ph.D., Raghu Raj Rangaraju, Anil Kumar, Siuli Sarkar, Ph.D. MacDermid Alpha Electronics Solutions Bangalore, KA, India

17 Bismuth – Thermal Cycling Reliability • Homogenous joints • Small additions of silver can improve results • Better performance than tin-lead?

Improved Reliability and Mechanical Performance of Sn58Bi Solder Alloys Guang Ren and Maurice N. Collins Stokes Laboratories, Bernal Institute, University of Limerick, Ireland Correspondence: maurice.collins@ul/.ie (M.C.), guang.ren@ul/.ie (G.R.) 1206 resistors, 0-100°C Cycling

18 Bismuth – Thermal Cycling Reliability • Large differentiation in performance for homogenous joints ‐ Low silver alloys perform the worst ‐ SAC305 and SnBiAg performed the best • SAC solder balls with Bismuth solder paste is the worst

Reliability Study of Low Silver Alloy Solder Pastes Jennifer Nguyen, David Geiger and Murad Kurwa Flextronics International 847 Gibraltar Drive Milpitas, CA, USA

19 High Reliability Solders

• Creep is the movement of dislocations (specifically climb or out of plane movement) • Dislocations can be ‘pinned’ or blocked by the presence of ‘impurities’ • Innolot (MaxRel, 90SiC, etc) ‐ Changes stress states Sn-Ag3.7Cu0.65Bi3.0Sb1.43Ni0.15 ‐ Increase distance required to move ‐ Increases energy required to move vacancies, break atomic bonds

20 Innolot Solder

• Added antimony (Sb) and bismuth • End Result is a Six (6) Part Alloy: (Bi) for solid solution hardening ‐ Sn3.8Ag0.7Cu3.0Bi1.4Sb0.15Ni ‐ Balances change in melt temperature (Sb increases, Bi decreases) • Added nickel (Ni) for dispersion hardening (intermetallic formation)

21 Innolot Solder Reliability

Dr R. Ratchev, Presentation in LIVE Project Seminar, Berlin, October 2008

COMPONENT LEVEL RELIABILITY FOR HIGH TEMPERATURE POWER COMPUTING WITH SAC305 AND ALTERNATIVE HIGH RELIABILITY SOLDERS Thomas Sanders, Sivasubramanian Thirugnanasambandam and John Evans, Ph.D. Auburn University, Department of Industrial & Systems Engineering Auburn, AL, USA [email protected]

22 Main Advantage of Innolot • However, there are certain situations that significant improvement can be achieved by switching to a high • Creep Resistance reliability solder • Tin lead solder stress relaxes at very ‐ Large ceramic components with Kovar or low stresses Alloy 42 leads ‐ Harder solders resist deformation and • Stress relaxation is damage force the component leads to bend and flex (tin lead solder is too soft) • Typically, it is not recommended to ‐ Might drive failures into the braze or the use a solder alloy change to solve a actual lead itself thermal cycling fatigue issue

Conformal coating pulling tin-lead solder out of joints

Visible creep ratcheting of SnPb solder

23 Solder Joint Reliability – Mitigation and Other Issues

• Regardless of the solder alloy the reliability can be improved or reduced • The use of underfills, potting compounds and thick conformal coatings can greatly influence the failure behavior under thermal cycling ‐ Any time a material goes through its glass transition temperature problems tend to occur ‐ Conformal coating should not bridge between the PCB and the component ‐ Underfills designed for enhancing shock robustness do not tend to enhance thermal cycling robustness Coating, Potting and Underfill

• Concerns – CTE and Tg

T( − ) F = 2 1  1 1   +   A1E1 A2E2 

25 BGA/CSP Mirroring

• Avoided in earlier designs (challenges with rework and X-ray inspection) ‐ Increasingly required due to higher densities and higher speed memory • Reduces lifetime by 1.5X to 5X, but the error in the predictions increases due to the complex stress state

Based on Darveaux Model

Ye, Yuming, et al. "Assessment on reliability of BGA package double-sided assembled." High Density Packaging and Microsystem Integration, 2007. HDP'07. International Symposium on. IEEE, 2007.

26 Next Generation Solder Challenges

• Harder solder will impart more stress into the component and the leads ‐ Reduced shock performance which may need to be mitigated with corner staking or underfill ‐ Low cycle fatigue of the component leads under severe thermal cycling conditions • This a very high stress situation in which SnPb solder fails very quickly • Could crack braze or component, typically the SnPb joint would fail first • Manufacturing concerns ‐ Totally different process windows for these complex solder alloys ‐ Solder quality issues can quickly negate any increase in fatigue performance gained with the alloy ‐ Much lower process temperatures for Bismuth Tin • Mixed alloy systems are being promoted for complex temperature sensitive devices (intel). ‐ SAC components soldered down with Bismuth Tin solder paste ‐ Doesn’t appear to be much of an issue as long as the joint is homogenous (Resistors, leaded devices) ‐ Significant risk for devices with SAC solder balls

27 Live Q&A

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