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Pb-Free Alloy Alternatives

Project Report: State of the

Industry

Chair: Greg Henshall, HP

Co-Chair: Stephen Tisdale, Intel

Aug 21, 2008

11

Project Team Members

16 companies; 44 individuals

Solder alloy suppliers, component suppliers, EMS providers, OEMs

2

Outline

• Background and Objectives

• Considerations in Alloy Selection

• Knowledge Assessment

– Mechanical Shock

– Thermal Fatigue

– Manufacturability

– Gaps

• Managing Alloy Change

• Summary and Conclusions

3

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.

4

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

– $16.25 - $17.50 per oz in June ‟08

(Tin ~ $0.62 per oz)

• Poor barrel fill on thick boards for

some surface finishes

• Copper dissolution

• Hot tearing and other surface

phenomena create inspection issues &

possibly unnecessary rework Ni

Cu

Solder

IMC

Fracture surface showing intermetallic layer left, no solder

5

Addressing issues using alloy alternatives

After K. Sweatman, Nihon Superior, 2008

6

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

Slide 6

7

Situational Analysis (1 of 2)

• Perceived needs of high volume consumer segment is a key

driver for use of new alloys. These needs don‟t necessarily

match those of low volume, “high reliability” segment.

• Many “high reliability” OEMs have not switched to Pb-free

technology and have rigorous requirements for evaluation and

qualification of Pb-free materials & processes

• 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

• OEMs and EMS suppliers cannot stop solder & component

suppliers from innovating and bringing new alloys to market

8

Situational Analysis (2 of 2)• New alloys:

– May provide improvements over the long run

– Are a natural part of Pb-free technology maturing

• iNEMI cannot dictate a solution

• The industry is currently very active in performing technical

investigations of various alloys. Some information is proprietary

but more and more information is becoming publicly available

• In making alloy choices the industry will consider a wider range

of alloy properties and the performance of the alloy in a wider

range of situations than it did in the first round of lead-free alloy

selection

• 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

99

(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

10

Outline





– Thermal Fatigue


– Gaps



11

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

12

Problems with high silver SAC alloys and possible solutions

Problem

Reduce or

Remove

Silver

Micro-

AlloyOther 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 X

Cost X

13

Knowledge about new alloys is evolving

• Considerable research has been performed

on the new alloys

• Some commonly investigated elements for

microalloying are nickel (Ni), bismuth (Bi),

phosphorus (P), germanium (Ge), cobalt

(Co), indium (In), and chromium (Cr), with

several already being used commercially

• Some elements selectively incorporate into

the interfacial intermetallic layer to:

– (1) control the IMC thickness

– (2) slow the growth of the IMC in service

– (3) modify its morphology

– (4) prevent disruptive phase changes

– (5) increase toughness

• Some microalloy additions go into solid

solution within the tin matrix to increase

both strength and ductility, and thus

reliability, while others control oxidation

• More knowledge is needed in

some areas

– Impacts of thermal aging

– Thermal fatigue impact of Ag,

microalloy additions

– Range of effectiveness for

some alloy additions

Ni

After K. Sweatman, Nihon Superior, 2008

14

Outline





– Thermal Fatigue


– Gaps



15

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

16

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

17

Various reasons cited for improved drop/shock behavior of

micro alloys & 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.

18

Tin-Copper Phase Diagram

Phase transformation from

hexagonal Cu6Sn5 to

monoclinic Cu6Sn5‟

with a 26% volume

change

Nickel stabilizes the

hexagonal close

packed form of the

Cu6Sn5 ensuring the

integrity of the

intermetallic layer

Possible explanation for effect of Ni -

Stabilisation of η Hexagonal Close Packed Cu6Sn5 (1 of 2)

K. Nogita & T. Nishimura,

Scripta Materialia 59, 2

(2008) 191-194.

19

Electron

Diffraction Pattern

Electron

Diffraction Pattern

Transmission

Electron Micrograph

Transmission

Electron Micrograph

~9% Ni

in IMC

Possible explanation for effect of Ni -

Stabilisation of η Hexagonal Close Packed Cu6Sn5 (2 of 2)

20

Reducing Ag content and micro alloy additions can change

failure mode of SAC solder joints.

• H. Kim et al. (Intel). The majority of cracking in SAC 405 solders was

through the IMC layer (package side). Cracking in the SAC 105 joints was

more complex, with cracks going through the bulk solder near the IMC layer

and in the IMC.

Crack in IMC layer Note: Pandher et al.

(Cookson) found that when small amounts of Cr & Ni are combined in low Ag alloys (e.g. SACX), the occurrence of flat, brittle fractures (Mode 4) are reduced 80%.

Crack in solder

near IMC

SAC405

SAC105

21

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

Data of Tanakaet al., ECTC 2006

SAC305SAC305

SAC125+Ni

Dro

ps

to F

ailu

re

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.

22

Outline





– Thermal Fatigue


– Gaps



23

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.

Possible reasons the differences in life are

small include:

1. There is no significant difference in the

microstructure over the range of Ag contents

2. The component/substrate configuration

may not have subjected joints to high strain

24

Lead-Free and Mixed Assembly Solder Joint Reliability Trends- J-P Clech

IPC Printed Circuits Expo SMEMA Council APEX Designer Summit O4

Hard solder

gives better

service than

soft solder

Soft solder gives

better service than

hard solder

Analysis by Clech may provide an explanation of at least some of the

apparent conflict in reliability data – influence of strain range.

Sn-0.7Cu

SAC

25

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

26

Limited study on the impact of microalloy

additions on thermal fatigue performance

Data of Pandher and Healey, ECTC 2008

• SACX (Sn-0.3Ag-0.7Cu+Bi) shows improved performance

in temperature cycling.

– Bi addition refines grains

27

Darveaux and Reichman (ECTC 2007) used their mechanical

property data to simulate hysteresis loops & discuss

possible impact on reliability.

• Simulations of cyclic hysteresis loops for various alloys. The authors conclude

that the acceleration factor will vary by alloy.

28

Summary - Thermal fatigue resistance of new alloys is a

clear 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.

29

Outline





– Thermal Fatigue


– Gaps



30

PCA manufacturing & reliability can be affected by the 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.

31

• For non-eutectic compositions with a pasty range of greater than 10°C:

– Directional solidification resulting from temperature gradient across the solder joint

– Non-uniform microstructure

– Low melt phase accumulation at the board pad or component pad interface

– Contamination and defect accumulation at the board pad or component pad interface

• Although primarily an issue for backwards compatibility, to some degree this applies equally to SAC105/SAC105 or SAC105/SAC305 assemblies

• Some new alloys are designed to mitigate these concerns by use of microalloying additions that force eutectic behavior

– Single stage solidification with no pro-eutectic phases.

31

Low Ag Alloy/SnPb – Microstructure Formation

P.Snugovsky, et al., “Microstructure, Defects, and Reliability of Mixed Pb Free / SnPb Assemblies,”

Proceedings TMS, V 1: Materials Processing and Properties p.p. 631- 642, 2008

3232

Low Ag Alloy/SnPb – Microstructure Formation

• Pasty range comparison for solder joints formed on Cu pad:

– Pure SAC305 – 5°C

– Pure SAC105 – 12°C

– SAC405/SnPb – 30°C

– SAC105/SnPb – 40°C (40°C+ with additives)

SAC305/SnPbPure SAC305 SAC105/SnPb



3333

Low Ag Alloy/SnPb – Rework Defects

Situation exacerbated during repair due to greater directional heating

of component from rework nozzle:

– Significant formation of shrinkage voids and coalescence of impurities and

low melt eutectic at component pad

– Difficult to avoid especially if the board is thick and/or component is large

– Defect cannot be detected electrically



34

Outline





– Thermal Fatigue


– Gaps



35

Current Knowledge / Gaps (1 of 3)

Areas where knowledge is relatively complete

See Backup

Slide

Sufficient Knowledge

Low Ag alloys improve drop/shock resistance

Micro alloy additions significantly improve drop/shock

performance on Cu surfaces but not on Ni surfaces

Decreasing Ag content decreases elastic modulus, yield and

tensile strength of SAC

Decreasing Ag content decreases creep strength of SAC

Alloy additions can increase the creep strength of low Ag

SAC alloys

SAC alloys are not inherently brittle (though high strength

and stiffness can lead to overall brittleness of the joint)

36


High priority gap areas

Gap or Concern

High Priority

Advantages and disadvantages of specific alloys

Composition limits for microalloy additions; ranges of

effectiveness

Standard method to assess new alloys; standard data

requirements

Consistency of testing methods, including test

vehicles & assembly, test parameters, etc.

Establish the microstructural characteristics of

specific alloys

Long term reliability data for new alloys, particularly

low Ag & microalloyed

Lack of thermal cycle data for evaluating new alloys;

benchmark to Sn-Pb and SAC 305/405

37


Medium and

low priority

gap areas

Gap or Concern

Medium Priority

Assessment of new alloys for use in "mission critical,

long life" products

Impact of rework on microstructure and properties

Mixed Sn-Pb/Pb-free assembly, including rework

Impact of alloy composition on work hardening rates

& other flow properties; effect of strain rate and

temperature

Impact of alloy composition on bend/flex limits

(moderate strain rate; ICT, handling, card insertion,

etc.)

Thermal fatigue accelerations factors (not yet fully

established for SAC 305/405)

Impact of aging on microstructure and mechanical

propertiesLow Priority

Solder process margins required for new alloys used

in various product classifications

Mixing of different BGA ball alloys and paste alloys

for various component and board designs

38

Outline





– Thermal Fatigue


– Gaps



39

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).

40

Standards need updating to account for new alloys

• J-STD-006 “Requirements for Electronic Grade Solder Alloys … ”

– Alloy Team members participated in meeting at APEX „08 to address

concerns with new alloys, especially those with dopants

• J-STD-609 (component, PCA material labeling)

– Alloy Team presentation of issue made at JEDEC JC-14 meeting

Jan ‟08

– Began addressing issue with IPC/JEDEC task group at APEX „08

• 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; 1st meeting took place early April & efforts continue

41

Impact of BGA ball alloy change on component part

numbers for alloys inducing a mfg process change.

• Change in alloy is a change in fit to process (form, fit or function).

• EMSF & iNEMI statement from May 2007 calling for “Unique Part Numbers to Differentiate Ball Metallurgies on Pb-Free BGA Components”

– Mike Davisson, RoHS technical program manager for Agilent: "A change in metallurgy without the ability to track the change through MPNs will only make Pb-free conversion more difficult and could delay the process.”

– iNEMI members supporting this position include: 3M, Agilent Technologies Inc., Alcatel-Lucent, Analogic, Celestica Inc., Delphi Electronics & Safety, Hewlett-Packard Company, Huawei Technologies Co. Ltd., Intel Corporation, Jabil Circuit Inc., Microsoft Corp., Micro Systems Engineering Inc., Nihon Superior, Plexus Corp., Sanmina-SCI Corporation, Solectron Corporation and Tyco Electronics Corporation.

42

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”)

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

43

HP alternate alloy data requirements -

overviewNon-technical• Complete alloy composition*• Microalloy benefit range *• Trade names*• Test labs*• Patent status• Submitter‟s licensing status• Cost of one kg • Target application(s)• Target implementation date• Advantages over SAC305

Reliability• Thermal cycling*• Mechanical shock*• Vibration*• Four-point bend*• Physical samples *• Alloy cross-compatibility with SAC305 *

Material properties• Solidus temperature*• Liquidus temperature*• DSC or DMA curve*• Electrical conductivity• Thermal conductivity• Density• Expansion vs temperature, TCE• Young‟s modulus (dynamic)• Yield strength (0.2%)• UTS• Elongation• Stress strain curves• Poisson‟s ratio• Hardness• Literature search• Contamination reports

Manufacturing• HP DOE for wave pot temperatures*• HP DOE for reflow profiles*• Laminate damage assessment*

• Pastes must meet HP Solder Paste

Specification *• Wetting balance

* required

44

Standard test methods and controls

• In order to properly compare data from various sources and for

multiple alloys, standard test methods are required.

– Details are important

• Generally follow IPC-9701A; exceptions provided (mostly to simplify)

• Test vehicle design

– Components (ball alloy, component type, size, etc.)

– PCBs (thickness, layer count, materials, finish, etc.)

– Board assembly (solder alloy/paste, reflow profile, etc.)

– Controls (SAC305 and eutectic Sn-Pb)

• Test details

– Thermal cycle profile, # parts, preconditioning, failure criterion, etc.

• Reporting requirements

– Tabulated failure data, Weibull curves

• Controls: Sn-Pb, SAC305

Example for Accelerated Thermal Cycling (ATC)

45

Outline





– Thermal Fatigue


– Gaps



46

Summary and Conclusions (3 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.

47


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.

48


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.

49

Backup Slides

50

η‟- η Cu6Sn5,

Heated/cooled at 1°C/min *

Ni-stabilized η (Cu,Ni)6Sn5,

Heated/cooled at 1°C/min**

*Fig4 (b) from G. Ghosh and M. Asta: Journal of Materials Research, 20(2005) 3102-3117.

Ni Stabilisation of η Hexagonal Close Packed Cu6Sn5

η‟-

Cu6Sn5

η

Cu6Sn5

438±18 J/mol 186°C

** (Cu,Ni)6Sn5 taken from Sn0.7Cu0.05Ni alloy,

DSC by Nihon Superior Co. Ltd.

No phase transformation

Transformation from

hexagonal to monoclinic

Hexagonal form retained

at room temperature

„K. Nogita and T. Nishimura, Scripta Materialia 59, 2 (2008) 191-194.‟

Supplement to Slide 17-3

1. Ni segregates to the eutectic Cu6Sn5 phase

2. Ni is relatively homogeneously distributed in the eutectic Cu6Sn5

Synchrotron micro-XRF element mapping

of Cu6Sn5 phase formed in Sn-0.7Cu-0.05Ni

T. Ventura, C.M. Gourlay, K. Nogita, T. Nishimura, M. Rappaz, A.K. Dahle,

Journal of Electronic Materials, 37, 1 (2008) 32-39.



52

OSP Substrate

After 2 reflow cycles and

500 hour @ 125°C

Sn-0.7Cu-0.05Ni

SAC305

Cracking of IMC due to

phase change on cooling

Crack-free IMC

stabilized by Ni



53

SAC alloys, both high and low Ag, are NOT “brittle”

• Ductility measured by Kim et al. for all SAC alloys is high

• Huang et al. observed elongations to failure in the range of 12% to 20% for a wide

variety of SAC & SAC+Al/Ni alloys.

• SAC solders are elastically stiff (high modulus) and strong (high YS, UTS).

• Solder joints may exhibit brittle fracture due to high stress imposed in IMC layers,

PCB laminate, etc. Fracture does not take place in the solder.

D. Kim et al., ECTC 2007.

Also note: Modulus decreases as Ag concentration decreases.(Also observed by Huang et al. (Indium)).

• ASM Materials Engineering Dictionary defines “brittle” as: “Permitting little or no

plastic (permanent) deformation prior to fracture.” Elongation to failure ~ 0%.

54

www.inemi.orgEmail contacts:

Jim McElroy

[email protected]

Bob Pfahl

[email protected]

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