addressing opportunities and risks of the pb-free solder alloy...

Addressing Opportunities

and Risks of the Pb-Free Solder

Alloy Alternatives

June 16, 2009

Authors

Gregory Henshall,* Hewlett-Packard Robert Healey, Cookson Ranjit S. Pandher, Cookson Keith Sweatman, Nihon Superior Keith Howell, Nihon Superior Richard Coyle, Alcatel-Lucent Thilo Sack, Celestica Polina Snugovsky, Celestica Stephen Tisdale, Intel**Fay Hua, Intel Grace O’Malley, iNEMI *Chairman** Co-Chairman

Project Team Members

16 companies; 44 individualsSolder alloy suppliers, component suppliers, EMS providers, OEMs

Outline

• Background and Objectives

• Considerations in Alloy Selection

• Knowledge Assessment

– Mechanical Shock

– Thermal Fatigue

– Manufacturability

– Gaps

• Managing Alloy Change

• Future Work

• Summary and Conclusions

Near-eutectic SAC allowed industry to meet RoHS deadline of July 1, 2006

• The electronics industry adopted SAC 305 & other “near eutectic” alloys as the standard Pb-free alloys during the RoHS transition.

• These alloys were selected by industry consortia, balancing many factors.

• Major factors were the relatively low melting point and reasonable thermal fatigue reliability.

• These alloys were selected prior to understanding of impact on mechanical robustness and copper erosion.

SAC305/405 functional but not the optimal Pb-free solution

Problems with SAC305/405 include:• Poor drop/shock performance for

BGAs, especially on Ni/Au surfaces • Expense of Ag is driving the desire to

reduce Ag content – Wave solder bar main concern– $200/lb – Jan 30, 2009

(Tin ~ $5.30/lb)• Poor barrel fill on thick boards for

some surface finishes• Copper dissolution• Hot tearing and other surface

phenomena create inspection issues & possibly unnecessary rework

NiCu

Solder

IMC

Fracture surface showing intermetallic layer left, no solder

Gregorich, et al., IPC/Soldertec Global 2nd International Conference on Lead Free Electronics (2004).

The number of Pb-free alloy choices is expanding…

Wide range of alloy choices is both an opportunity and a risk.

Alloys Investigators Reference

Sn4.0Ag0.05Cu (SAC405) Pandher (Cookson), H. Kim & D. Kim (Intel), Kobayashi (Nippon Steel), Darveaux (Amkor/ASU)

ECTC 2007

Sn3.0Ag0.5Cu (SAC305) Pandher (Cookson), Syed (Amkor), Kobayashi (Nippon Steel), Darveaux (Amkor/ASU)

ECTC 2007

Sn1.0Ag0.5Cu (SAC105) Pandher (Cookson), H. Kim & D. Kim (Intel), Syed (Amkor), Kobayashi (Nippon Steel)

ECTC 2007

Sn0.3Ag0.7Cu+Bi (SACX0307) Pandher (Cookson) ECTC 2007

Sn0.3Ag0.7Cu+Bi+Ni+Cr Pandher (Cookson) ECTC 2007

SAC305+0.05Ni+0.5In Syed (Amkor) ECTC 2007

SAC255+0.5Co Syed (Amkor) ECTC 2007

SAC107+0.5Ge Syed (Amkor) ECTC 2007

SAC125+0.05-0.5Ni (LF35) Syed (Amkor), D. Kim (Intel), Kobayashi (Nippon Steel), Darveaux (Amkor/ASU)

ECTC 2007

SAC101+0.02Ni+0.05In Syed (Amkor) ECTC 2007

Sn-3.5Ag Cavasin (AMD), Darveaux (Amkor/ASU) ECTC 2007

Sn-3.5Ag+0.05-0.25La Pei & Qu (Ga. Tech) ECTC 2007

Sn-0.7Cu Darveaux (Amkor/ASU) ECTC 2007

Sn0-4Ag0.5Cu+Al+Ni Huang (Indium) ECTC 2007

Sn0.7Cu0.05Ni+Ge (SN100C) Sweatman, Miyaoka, Seki, Suenaga, Nishimura (Nihon Superior) ICS&R Toronto 2008

Situation Analysis• New alloys:

– May provide improvements over the long run

– Are a natural part of Pb-free technology maturing • But alloy proliferation brings some risks

– Many “high reliability” OEMs have not switched to Pb-free technology. Proving new alloys meet their stringent requirements would take time.

– BGA package suppliers want to minimize the number of alloys they have to deal with while still meeting customer needs

– Acceptability of alternate alloys will vary from product class to product class, and possibly from company to company

– Various alloys behave differently during soldering processes and in service. This adds complexity & risk to the supply chain.

• The market will ultimately decide which alloy or alloys “win”• The best way to handle alloy proliferation in the near term is to provide

industry guidance on how to manage through the issues/concerns, not to define alloys that are acceptable or unacceptable

(1) Help manage the supply chain complexity created by alloy choices (2) Address reliability concerns (3) Highlight the opportunities & risks created by the new Pb-free alloy

alternatives.

Specific goals in Phase 1 include:• Assess existing knowledge and identify critical gaps related to new

Pb-free alloys. Provide technical information to the industry that will make selection and management of alloys easier.

• Raise awareness of this information through publication and presentation of findings.

• Work with industry standards bodies (e.g. IPC, JEDEC) to address standards that require updating to account for new alloys.

• Use findings to drive follow-on work, if required, in Phase 2.

Pb-Free Alloy Alternatives Project Objectives

Outline





– Thermal Fatigue


– Gaps


• Future Work


Initial assessment of SAC alloys was limited

Near-eutectic SAC alloys (SAC405, SAC387, SAC305, etc.)

Assessed On• Melting point• Wetting rate• Spread• Reflow soldering• Joint microstructure• Thermal cycling of reflowed test

assembly (0-100 C, 10 minute dwells)

• Thermal shock of reflowed test assembly (-40-125 C, 5 minute dwells)

• Cross-sectioning after thermal cycling

Not Assessed On• Aggressiveness towards copper

(copper dissolution)

• Aggressiveness towards stainless steel (solder pot erosion)

• Reliability in shock loading (e.g. drop test)

• Wave soldering

• Selective soldering

• Hand soldering

• Rework

Problems with high silver SAC alloys and possible solutions

ProblemReduce or Remove

Silver

Micro-Alloy Other Solutions

High Flow Stress X X

Brittle Joint Failure X X

Low Impact Strength X X

Shrinkage Defects X XMove Closer to Eutectic Composition

Copper Erosion X XCost X

Outline





– Thermal Fatigue


– Gaps


• Future Work


Mechanical Shock Reliability

Studies consistently show that low Ag alloys perform better in drop/shock than high Ag alloys.

D. Kim, et al. (Intel)

LF35 = Sn1.2Ag0.5Cu+Ni

A. Syed, et al. (Amkor)

SAC 305

1% AgAlloys

Micro alloying additions improve drop/shock performance in most studies.

• Improved performance from dopants also seen in data of H. Kim et al. (Intel) on previous slide (LF35 vs SAC 105).

• Only Syed et al. (Amkor) did not observe a clear improvement in drop/shock performance in low Ag alloys with dopants compared to similar non-doped alloys –depending on surface finish.

Data of Pandher et al. (Cookson)

SACX =Sn0.3Ag0.7Cu0 +Bi

0.1% Ni

No Ni

0.03% Cr

No Cr

15

Improved drop/shock behavior of microalloys & low Ag alloys

• Pandher, et al. (Cookson).

– Micro alloying additions slow inter diffusion, thus reducing IMC thickness or propensity for void formation.

– Small amounts of Ni can decrease Cu3Sn growth, improving reliability.

– Low Ag stated to decrease strength & modulus, transferring less stress to the solder/substrate interface.

• H. Kim et al. (Intel), and D. Kim et al. (Intel).

– Low modulus and low yield strength improve performance of solder joints with low Ag alloys.

– Optimization of these properties requires increasing the amount of primary Sn relative to the Ag3Sn and Cu6Sn5 phases in the alloy.

Drop/shock performance depends on pad finish (Ni/Au vs Cu)

Data of Tanakaet al., ECTC 2006

SAC305SAC305

SAC125+Ni

Dro

ps to

Fai

lure

SAC125+Ni

• Studies by Syed et al. (Amkor) showed that SAC 125 + Ni does not produce a significant drop/shock performance improvement over SAC 305 for Ni/Au package finish. However, this alloy is the best performer for Cu-OSP package finish.

– PCB finish was Cu-OSP in both cases.• Other literature data indicate this dependence of drop performance on pad finish.

Outline





– Thermal Fatigue


– Gaps


• Future Work


Accelerated thermal cycle performance not sensitive to Ag content in the range 2.1% to 3.8% - data of Kang et al.

Average failure life (N50) estimated from ATC failure data of SAC BGA solder joints.Data of Kang et al., ECTC, p. 661 (2004).

• No systematic change in life as afunction of Ag content

• Differences in life are small and may even be within experimentalerror.

Large changes in Ag content can have significant impact on ATC reliability – Data of Terashima et al.

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

• Thermal fatigue reliability appears to be dependent on process and microstructure and those dependencies have yet to be characterized fully and understood

– Terashima surmised that higher Ag content inhibited microstructural coarsening and prolonged fatigue life. The microstructural coarsening hypothesis is consistent with results of Kang.

– However, Terashima reported better reliability in the high Ag alloy, whereas Kang reported coarser structures and better reliability in the low Ag alloy.

• Performance relative to eutectic Sn-Pb not reported

Recent data support findings of Terashima et al.

Data of Henshall et al., APEX 2009

0/100°C10 min. dwells

• BGAs with SAC105 have lower performance than high Ag alloys (Sn-3.5Ag, Sn-3Ag-0.5Cu)

• BGAs with Sn-3.5Ag and SAC305 perform similarly

Summary Thermal fatigue resistance of new alloys is a gap issue

• ATC evaluations have focused on the “high-reliability alloys,” SAC 405 and 305 and little data exist for lower Ag alloys

• Only a small number of studies address the impact of alloy composition on life during accelerated thermal cycle testing (ATC)

– Data sometimes conflicting • The performance of low Ag alloys relative

to eutectic Sn-Pb is not clear, especially under field use conditions

• The impact of microalloy additions is largely unknown

• Structure-property relationships have not been defined

• The impact of significant alloy changes on the acceleration factor that relates field life to accelerated test life is unknown

Overall, the impact of ball alloy composition on thermal fatigue life

in the field is difficult to judge at this time.

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Outline





– Thermal Fatigue


– Gaps


• Future Work


Manufacturability

Different alloys have different liquidus (melting) temperatures –can affect solder process

• Sn-Ag-Cu eutectic temperature: 217°C• Most common alloys today

– Sn-3.0Ag-0.5Cu: 221°C– Sn-4.0Ag-0.5Cu: 221°C

• New alloy examples– Sn-1.0Ag-0.5Cu: 227°C– Sn-0.7Cu: 228°C

• SAC 305/405• Alternative SAC alloys• SAC + dopants

Choice of Ball Alloy

Incomplete solder joint formation for a 1% Ag ball alloy assembled at the low end of typical Pb-free reflow process window.(Images courtesy of Hewlett-Packard Co.)

Unmelted solder ball

Unacceptable solder joints

CSP Package

CSP Package

PCB

PCB

• Low Ag BGA ball alloy may have an impact on PCA manufacturing due to high melting point (>225°C)– Note: Because of their behavior in the

molten state and their solidification behavior some alternative alloys require less superheat than others.

• The change to low Ag alloys in BGA balls may require a change to PCA manufacturing processes.– Assembly and repair facilities can have

unexpected yield losses due to low Ag alloys if they are not aware of their presence.

• Improperly soldered low Ag joints are a significant reliability risk because they may pass electrical test but still have unacceptable solder joints and unmelted regions.

25

Outline





– Thermal Fatigue


– Gaps


• Future Work


26

Current Knowledge / Gaps (1 of 3)

Areas where knowledge is relatively complete

27

Current Knowledge / Gaps (2 of 3)High priority gap areas

28

Current Knowledge / Gaps (3 of 3)

Medium and low priority gap areas

29

Outline





– Thermal Fatigue


– Gaps


• Future Work


30

Management of alloy choice presents challenges for large OEMs

• Product portfolios can be broad– Reliability in one product line can

impact sales in others.• Supply chains are very complicated

– Business models range from no-touch to in-house design.

• Assurance of supply concern if only one patented material with limited licensees.

• Management of multiple alloys, even at a single factory site, is complicated.– Product support and repair

becomes more complex.• Part number change for BGA/CSP

components switching from high Ag to low Ag ball alloy (mfg. process impact).

31

Standards need updating to account for new alloys• J-STD-006 “Requirements for Electronic Grade Solder Alloys … ”

– Alloy Team members participated with the Committee to address concerns with new alloys, especially those with dopants

• J-STD-609 (component, PCA material labeling)– New ‘e-code’ for low Ag alloys– Add examples for new alloys and a decision flow chart to minimize

confusion– Ballot draft sent by IPC; voting ended June 5

• Part numbers for BGAs; customer notifications– Alloy Team presentation of issue made at JEDEC JC-14 meeting

Jan ’08– JC-14.4 chairman has established a task group including Alloy Team

members; efforts continue

32

Defined information requirements needed to increase acceptance and reduce risks of using new alloys

• The lack of defined information requirements for alloy acceptance:– Creates uncertainty (fear) in the industry regarding new alloys – Slows the adoption of improved materials

• The acceptability of any alloy may vary from product class to product class, and possibly from company to company. However, the methodology and data requirements may be largely same.

• HP is developing a systematic methodology for alloy assessment (SMTAI, Aug ’08)– Data needed to make assessment of alloy acceptability– Common/standard test methods, parameters, test vehicles– Controls for comparison with historical and currently accepted alloys– Allows direct comparisons between different alloys (“apples to apples”)

33

Start with HP approach and modify as needed for broader industry

• Start with approach published by HP at SMTAI 2008

• Team agrees the focus will be on tests and methods, not pass/fail criteria– Pass/fail criteria are product

dependent; tests and methods may not be

• Tests to be identified for evaluating:– Material properties– Reliability– Impact to manufacturing processes

• iNEMI Alloy Alternatives project plans to use this as a starting point and push for industry acceptance & standardization

34

Outline





– Thermal Fatigue


– Gaps


• Future Work


35

Future Work• New project forming: “Characterization of Pb-Free Alloy

Alternatives”– Open to all iNEMI members– Anticipate formal project start August 2009– Two major focus areas: thermal fatigue and alloy test requirements

• Thermal fatigue testing– Test planning nearing completion– Multiple alloys (~15) with Sn-Pb and SAC405/305 controls– Multiple thermal cycle profiles for determination of acceleration

behavior

• Alloy test requirements– Begin with HP approach and modify for broader industry– Engage with IPC and the Solder Products Value Council (SPVC)

36

Outline





– Thermal Fatigue


– Gaps


• Future Work


37

Summary and Conclusions (1 of 3)

The knowledge assessment efforts of the iNEMI Alloy Alternatives team have been described. This multi-company, multi-sector team has assessed the recent literature regarding new Pb-free solder alloys alternatives and come to the following conclusions.

1. Considerable progress has been made in understanding the fundamental relationships between alloying elements and properties for the SAC family of new Pb free solders. Additional work is needed to fully characterize the complex microstructures and their influence on physical and mechanical properties.

38

Summary and Conclusions (2 of 3)2. Areas where the performance of new alloys is reasonably

well established have been identified. Some of these include: (i) impact of Ag content and microalloy additions on mechanical shock reliability; (ii) impact of Ag content on elastic stiffness, plastic flow and creep behavior of SAC alloys.

3. Areas where more knowledge is needed in order to properly assess the benefits and potential risks of new alloys also have been identified. Some of these include: (i) thermal fatigue performance, including the impact of microalloy additions and development of acceleration models; (ii) the impact of alloy composition on the full range of solder processes; (iii) impact of thermal aging on microstructure and properties; (iv) impact of composition on bend/flex limits related to PCA manufacturing, test, board handling, etc.

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Summary and Conclusions (3 of 3)

4. Standardized data requirements for assessment of new alloys are needed so that each company can compare alloy performance with product requirements over the full range of relevant properties. The iNEMI Alloy Alternatives team is currently considering the HP approach as a starting point for such standardization.

5. The iNEMI Alloy Alternatives team is actively engaged with relevant standards bodies to create or update industry standards related to new Pb-free solder alloys.

6. Future work will focus on thermal fatigue resistance of Pb-free alloys and standardizing test data requirements.

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www.inemi.orgEmail contacts:Grace O’Malley

[email protected]

Addressing Opportunities and Risks of the Pb-Free Solder Alloy Alternatives �Slide Number 2Slide Number 3OutlineNear-eutectic SAC allowed industry to meet RoHS deadline of July 1, 2006SAC305/405 functional but not the �optimal Pb-free solutionThe number of Pb-free alloy choices is expanding…Situation AnalysisSlide Number 9OutlineInitial assessment of SAC alloys was limitedProblems with high silver SAC alloys and possible solutionsOutlineMechanical Shock Reliability��Studies consistently show that low Ag alloys perform better in drop/shock than high Ag alloys.Micro alloying additions improve drop/shock performance in most studies.Improved drop/shock behavior of �microalloys & low Ag alloysDrop/shock performance depends on pad finish (Ni/Au vs Cu)OutlineAccelerated thermal cycle performance not sensitive to Ag content in the range 2.1% to 3.8% - data of Kang et al.Large changes in Ag content can have significant impact on ATC reliability – Data of Terashima et al.Recent data support findings of Terashima et al.Summary �Thermal fatigue resistance of new alloys is a gap issue OutlineManufacturability��Different alloys have different liquidus (melting) temperatures – can affect solder processChoice of Ball AlloyOutlineCurrent Knowledge / Gaps (1 of 3)Current Knowledge / Gaps (2 of 3)Current Knowledge / Gaps (3 of 3)OutlineManagement of alloy choice presents challenges for large OEMsStandards need updating to account for new alloysDefined information requirements needed to increase acceptance and reduce risks of using new alloysStart with HP approach and modify as needed for broader industryOutlineFuture WorkOutlineSummary and Conclusions (1 of 3)Summary and Conclusions (2 of 3)Summary and Conclusions (3 of 3)Slide Number 41

addressing opportunities and risks of the pb-free solder alloy...

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