Second Generation Solder Alloys - Update

Dr. Nathan Blattau

What is the Perfect Alloy? Is there one?

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

with low work hardening rate

Similar to SnPb – providing compliance without

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

There will always be tradeoffs. So, there can only be a perfect alloy for a particular application. The following table addresses general consumer applications:

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Solder Selection

• Considerations include‐ PRICE! – Less silver content‐ Insufficient performance‐ Failure mechanisms

• Market has chosen SAC305 as the default SMT pb-free solder material. However, there is still proliferation and evolution of material sets

• HASL and through hole soldering are typically either SnCu+Ni (SNC) or SAC305

SAC305

SAC205

SAC105

SNC

SnAg

SNCX

SnCuSnAgCu??

SnAgCuXX

ex. Innolot

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

A. MacDiarmid, “Thermal Cycling Failures”, RIAC Journal, Jan., 2011.

Source: Syed, Ahmer. "Accumulated creep strain and energy density based thermal fatigue life prediction models for SnAgCu solder joints." 2004 Proceedings. 54th Electronic Components and Technology Conference (IEEE Cat. No. 04CH37546). Vol. 1. IEEE, 2004.

Temperature cycling: The solder fatigue mechanism

• Grain growth in solder is an indicator of fatigue

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

TEMPERATURE CYCLING RELIABILITY OF REBALLED AND REWORKED BALL GRIDARRAY 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

SnPb Solder

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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

Thermal Cycling - Examples

SOT Alloy 42

Chip Resistor QFN

SOT Alloy 42Source: 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.

Source: Tohmyoh, Hironori, et al. “Estimation and Visualization of 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.

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The Current State of Lead-Free• 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)

• HASL PCB Coating‐ Sn07Cu+Ni (SN100C)

How to manage wave solder alloy contaminationsGerjan Diepstraten & Harry Trip Cobar Europe BV Balver Zinn

FeSn2

Copper Erosion

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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

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).

• The wetting properties are sufficient for surface mount components

• Its higher creep resistance enables higher operating conditions.

SAC Solder Alloy Wets Well, It Just Doesn't Spread Well, Indium Corporation, Dr. Ron Lasky

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 winners as a ball alloy)

Drop Shock Reliability of Lead-Free Alloys – Effect of Micro-AdditivesRanjit S Pandher, Brian G Lewis, Raghasudha Vangaveti, Bawa Singh

Cookson Electronics, Assembly Materials Group600 Route 440, Jersey City, NJ 07304, USAEmail: rpandher@cooksonelectronics.com

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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

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Low Silver Alloy Reliability

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

Thermal Cycling Reliability of Alternative Low-Silver Tin-based SoldersElviz George, Michael Osterman, Michael Pecht, Richard Coyle, Richard Parker, and Elizabeth Benedetto

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 .

-15°C to 125°C, 60-minute dwell

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

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Introduction to Low Temperature Soldering - Intel

THE APPLICATION OF BI-BASED SOLDERS FOR LOW TEMPERATUREREFLOW TO REDUCE COST WHILE IMPROVING SMT YIELDS IN CLIENTCOMPUTING SYSTEMSScott Mokler, Ph.D., P.E., Raiyo Aspandiar, Ph.D., Kevin Byrd, Olivia Chen, Satyajit Walwadkar, KokKwan Tang, Mukul Renavikarand Sandeep SaneIntel CorporationHillsboro, OR, USAscott.mokler@intel.com

Main Concerns - Bismuth• Drop shock performance

‐ If you thought SAC was bad‐ Worse than SAC305

• Pb contamination

• Bismuth segregation/aging (120°C)

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LOW TEMPERATURE SOLDERING USING SN-BI ALLOYSMorgana Ribas, Ph.D., Anil Kumar, Divya Kosuri, Raghu

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, Indiasiuli.sarkar@alphaassembly.com

Improving tensile and fatigue properties of Sn–58Bi/Cusolder joints through alloying substrate

QingKe Zhang, HeFei Zou, and Zhe-Feng Zhanga)Shenyang National Laboratory for Materials Science, Institute of Metal Research,

Chinese Academy of Sciences

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 on Lead-Free Components and Assemblies, San Jose, CA April 23-24 (2003)

Bismuth – Thermal Cycling Reliability

• Significant reduction in thermal cycling performance (mixed)

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Low Temperature Soldering: Thermal Cycling Reliability PerformanceMorgana 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

Bismuth – Thermal Cycling Reliability

• Homogenous joints

• Small additions of silver can improve results

• Better performance than tin-lead?

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Improved Reliability and Mechanical Performance of Sn58Bi Solder AlloysGuang Ren and Maurice N. CollinsStokes Laboratories, Bernal Institute, University of Limerick, IrelandCorrespondence: maurice.collins@ul/.ie (M.C.), guang.ren@ul/.ie (G.R.)

1206 resistors, 0-100°C Cycling

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

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Reliability Study of Low Silver Alloy Solder PastesJennifer Nguyen, David Geiger and Murad Kurwa

Flextronics International847 Gibraltar DriveMilpitas, CA, USA

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’‐ Changes stress states

‐ Increase distance required to move

‐ Increases energy required to move vacancies, break atomic bonds

• Innolot (MaxRel, 90SiC, etc)Sn-Ag3.7Cu0.65Bi3.0Sb1.43Ni0.15

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Innolot Solder

• Added antimony (Sb) and bismuth (Bi) for solid solution hardening ‐ Balances change in melt temperature

(Sb increases, Bi decreases)

• Added nickel (Ni) for dispersion hardening (intermetallic formation)

• End Result is a Six (6) Part Alloy:‐ Sn3.8Ag0.7Cu3.0Bi1.4Sb0.15Ni

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Innolot Solder Reliability

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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, USAsandete@auburn.edu

Main Advantage of Innolot

• Creep Resistance• Tin lead solder stress relaxes at very

low stresses• Stress relaxation is damage• Typically, it is not recommended to

use a solder alloy change to solve a thermal cycling fatigue issue

• However, there are certain situations that significant improvement can be achieved by switching to a high reliability solder‐ Large ceramic components with Kovar or

Alloy 42 leads‐ Harder solders resist deformation and

force the component leads to bend and flex (tin lead solder is too soft)

‐ Might drive failures into the braze or the actual lead itself

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Visible creep ratcheting of SnPb solder

Conformal coating pulling tin-lead solder out of joints

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

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Coating, Potting and Underfill

• Concerns – CTE and Tg

( )

+

−=

2211

12

11

EAEA

TF

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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.

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

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Live Q&A

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