correlation between whisker initiation and compressive...

Kato et al. 1

Correlation between Whisker Initiation and Compressive Stress in Electrodeposited

SnCu Coating on Cu Leadframes

Takahiko Kato, Haruo Akahoshi, Takeshi Terasaki, Tomio Iwasaki, Masato Nakamura

Hitachi, Ltd., JapanTomoaki Hashimoto, Asao Nishimura

Renesas Technology Corp., Japan（E-mail: [email protected]）

Tin Whisker Workshop, Reno, NV, May 29, 2007

Kato et al. 2

OutlineOutlineSubject: whiskers formed at R.T. over long term

from SnCu coating on Cu leadframe.Part 1: Correlation between whisker formation, microstructure, and stress

- Two samples with same SnCu coating on two different Cu leadframes that demonstrate drastically different whiskerinitiation tendencies

- Whisker formation: SEM- Microstructures: FE-TEM/FE-STEM, EBSP- Stress: X-ray diffraction method - Stress gradient: calculation by FEA

Part 2: Correlation between whisker initiation sites and Sn diffusion sites

- Sn diffusion sites under compressive stress: calculation by molecular dynamics

- Whisker initiation sites: planar slice method

Kato et al. 3

Part 1Part 1

Correlation between WhiskerFormation, Microstructure,

and Stress

Kato et al. 4

The same SuCu coating on two different Cu leadframesThe same SuCu coating on two different Cu leadframes

(a) SnCu-CUCR lead

52 months No Whiskers

SEM

100 µm

(b) SnCu-CUFE lead

34 months

Whiskers: max. > 200 µm

SEM

- Electrodeposited ∗- Matted- ∼2 mass% Cu –

bal. Sn- Thickness: 10 µm

Coating

Cr: 0.3, Sn: 0.25, Zn: 0.2, Cu: bal. (mass%)CDA number: 18045SnCu-CUCR

Fe: 2.4, Zn: 0.13, P: 0.08, Cu: bal. (mass%)CDA number: C19400SuCu-CUFE

Commercial Cu leadframeSample

∗ Electrodeposition was done in a commercial environment

Kato et al. 5

Age of samples for each examination (periodic inspection)Age of samples for each examination (periodic inspection)

(Optical inspection)

SEM Observation of whiskerFE-TEM, FE-STEM analysis

SEM observation of whiskerEBSP measurement

M: months

24 M

42 M

27 M

45 M 52 M

34 M

65 M

47 M

- Two sets of samples were prepared for each examination.

- Each exam was conductedduring the same inspectionperiod.

- SnCu-CUCR sample withno whiskers is always older than SnCu-CUFEsample at eachinspection period.

SnCu-CUFE

SnCu-CUCR2005 2006 2007JFY

Made sample

X-ray stress measurement

163 days

98 days

JFY 2001 2002 2003 2004 2005 2006 2007

Made sample

SnCu-CUFE

SnCu-CUCR

Made sample

Kato et al. 6

Crystalline orientations (EBSP measurement)Crystalline orientations (EBSP measurement)

- SnCu coatings exhibit columnar structure buthave no oriented grains.

- Cu leadframes have cold-rolling texture.- Cu leadframe does not create epitaxial SnCu coating growth for either samples.

Cross-sectionof whisker

5 µm

Cross sections of leads

SnCu coating: β-Sn tetragonalstructure

Cu6Sn5 IMC:hexagonalstructure

Cu leadframe:FCC structure

65 months 47 months

(b) SnCu-CUFE lead(a) SnCu-CUCR lead

Standard triangle of stereogram

Kato et al. 7

Grain size distributionGrain size distribution

Evaluated by EBSP measurement

SnCu-CUFE (47 months)SnCu-CUCR (65 months)

1 2 3 4 50Grain diameter (µm)

0.15

0.1

0.05

0.0

0.25

0.2

Num

ber f

ract

ion

SnCucoating

Grain diameter (µm)1 2 3 4 50

0.15

0.1

0.05

0.0

0.25

0.2

Num

ber f

ract

ion

SnCucoating

1 2 3 4 50Grain diameter (µm)

0.6

0.4

0.2

0.0N

umbe

r fra

ctio

n

Cu leadframe

Grain diameter (µm)1 2 3 4 50

0.4

0.2

0.0

Num

ber f

ract

ion Cu

leadframe

0.6

- No special difference in the distribution between the two samples can be distinguished for either coating or leadframe.

Kato et al. 8

Cu leadframe characteristics (TEM/EDX)Cu leadframe characteristics (TEM/EDX)

200 µm

Cr-rich particles

200 µm

Fe particles

Cr-richparticles 1. 9 x 10 20 m -3Density

Mainly 10-20 nmSize (D) Feparticles 5.0 x 10 19 m -3Density

Mainly 50-200 nm (Max. 2 µm)

Size (D)Energy (keV)

Counts

(a. u.)

0 20 40 60 80CuFe

Fe

Fe

EDX resultEx. (at%)Fe : Cu =97.7 : 2.3

(a) CUCR leadframe (b) CUFE leadframe45 months 27 months

Energy (keV)

Counts

(a. u.)

0 20 40 60 80

Cu

CuCr

Cr

CuEDX result

Ex. (at%)Cr : Cu =39. 6 : 60. 4Cr

Kato et al. 9

Microstructural characteristicsMicrostructural characteristics< SnCu coating - CUFE leadframe >

Fine-grained intermetallic compound (FGIMC) layer formed between coating and leadframe. Large-grained intermetallic compound (LGIMC) built up on the FGIMC layer with atriangle configuration (a wedge-shaped structure) along GBs in coating.

1.5 µmFGIMC layer

Sn/Cu coating

Cu leadframe

LGIMC

Grain boundaries

FE-STEM Bright Field Image EDX Mappingsame field

27 months Cu-K

Sn-L

1.5 µm

Kato et al. 10

Microstructural characteristicsMicrostructural characteristics< SnCu coating - CUCR leadframe >

same field

FGIMC layer

Cu leadframe

LGIMC

SnCu coating

LGIMC

1.5 µm

GB

FE-STEM Bright Field Image

45 months

Sn-L1.5 µm

Cu-KEDX Mapping

GB GB GBGB GB

As in the SnCu-CUFE sample, an LGIMC formed on the FGIMC layer. Note though, that LGIMC has a comb-teeth structure. GBs in the coating keep acting as LGIMC formation sites in this sample, too.

Kato et al. 11

Identification of IMCsIdentification of IMCs

pts.: number of analyzed points

SnCu-CUFE

(27 months)

（Av. of 5 pts.）

Cu Sn

at%6050403020100

Cu6Sn5

（Av. of 3 pts.）

Cu Sn

at%6050403020100

Cu6Sn5

（Av. of 5 pts.）

Cu Sn

at%6050403020100

Cu7Sn5

（Av. of 3 pts.）

Cu Sn

at%6050403020100

Cu7Sn5

LGIMC:hexagonalCu6Sn5

FGIMC:hexagonalCu7Sn5

LGIMC:hexagonalCu6Sn5

FGIMC:hexagonalCu7Sn5

00001011-1121--

0110-

Cu6Sn5 [1011]-Hexagonal

00000111-1012--

1103- -



0000

2110-2021-

0110-

EDX analysis

FGIMC Determinedphase

structureand C.C.

LGIMCSample(Age) Electron

diff. pattern EDX analysis Electrondiff. pattern

SnCu-CUCR

(45 months)


00001101-

2021-1120-

Kato et al. 12

Relationship between the distribution of Fe particles and the coRelationship between the distribution of Fe particles and the configuration of IMCsnfiguration of IMCs< SnCu Coating - CUFE Leadframe >

Fe particles are present in leadframe and FGIMC layer but not in the LGIMC and coating regions.> Interface between FGIMC layer and LGIMC is found to be the original surface position of the

Cu leadframe before electrodeposition.LGIMC growth is partially suppressed by the Fe particles in dense Fe areas like circles A to E.The top of the triangle-shaped LGIMC is always located at GBs.Superimposition of these two effects of Fe particles and GBs for IMC formation results in the triangular configuration of the LGIMC.

Trace of IMCs on EDX mapof Fe particles

1.0 µmＦｅ - K

ＡＢＣＤＥ

Cu leadframe

LGIMC

FGIMC layer

InterfacebetweenFGIMC layerand LGIMC

SnCu coating

LGIMC

GBsGBs

27 months

Kato et al. 13

< SnCu Coating - CUCR Leadframe >Distribution of CrDistribution of Cr--rich particlesrich particles

As with the Fe particles, Cr-rich particles are distributed in FGIMC and Cu leadframe but are not seen in LGIMC and coating region. > Interface between the FGIMC layer and the LGIMC is found to be the original

surface position of the Cu leadframe before electrodeposition in this case, too.Cr-rich particles have no obvious trace where LGIMC growth is suppressed, and only the GBs in the coating keep acting as LGIMC formation sites (see also page 10). > This causes the LGIMC to form the comb-teeth structure in this case.

200 nm Cr-K45 months

SnCuLGIMC

FGIMC layer

Cu leadframe

GB GB GBGB GB

Cu leadframe

SnCu coating

FE-STEM

FGIMC layer

Trace of IMCs on EDX map of Cr-rich particles

LGIMC

Kato et al. 14

Comparison of FGIMC configurationsComparison of FGIMC configurations

Distance along longitudinal direction of leads (µm)

Thi

ckne

ss o

f FG

IMC

laye

r (n

m)

Av：385 nm

0 2.5 5.0 7.00

200

400

600

800

1000

12001400 (a)

Av：859 nm

0200

400

600

800

1000

1200

1400 (b)

Cr-K

Fe-K1 µm

SnCu-CUCR

SnCu-CUFE

Thickness of FGIMC layer

Thickness of FGIMC layer

The FGIMC layer in SnCu-CUCR is more than twice as thick as that in SnCu-CUFE. > Diffusion of Sn from coating to leadframe is suppressed by Fe particles for SnCu-CUFE.

Sn diffusion is easy

Sn diffusion issuppressed byFe particles

Longitudinal direction of leads

Kato et al. 15

Comparison of LGIMC configurationsComparison of LGIMC configurations

Cu diffusion along GBs in coatingfrom Cu leadframe could be easy.

Cu diffusion along GBs in coating could be suppressed by Fe particles.

Average height of LGIMC: 4,220 nm

Cu-K1 µm

Cu diffusion could be easy

GB

GB

GB GBGB GB

LGIMC

1 µmCu-K

Cu diffusion could be suppressed byFe particles

GB GB GBGB

GBGB

LGIMC

Average height of LGIMC: 2,870 nm

Cross-sectional configuration of the comb-teeth could be given for LGIMC.

Cross-sectional triangular (wedge-shaped) configuration could be given for LGIMC.

<Assumption>

SnCu-CUCR SnCu-CUFE

Kato et al. 16

Prediction of stress induced in coatingsPrediction of stress induced in coatings

SnCu-CUFESnCu-CUCR

Cu-K1 µm

GB GB GBGB GB

LGIMC

1 µmCu-K

GB GB GBGB

LGIMC

LGIMC configuration oftriangle-shaped structure

LGIMC configuration ofcomb-teeth structure

Small compressive stresscould be induced.

Large compressive stresscould be induced.

Kato et al. 17

Residual stress measurement by XResidual stress measurement by X--ray diff.ray diff.sin2ψ method

Iso inclination methodClassification in detector scanning planeFixed ψ methodClassification in X-ray incident methodSnCu-CUFE: 98, SnCu-CUCR: 163Age of samples (days)

ψ=0 to ψ=45ºOffset angle(312) plane of β-Sn structureEvaluated peak

0.35Poisson’s ratio ν

5 mm x 4 mmStress-measured area

X: longitudinal direction of leadY: width direction of lead

43.5 G Pa

Eleven data points

Measured direction

Young’s modulus E

Data in 2 θ vs. sin2ψ diagram

Fundamental equation for stress measurementE π δ 2θ

2(1+ ν) 180 δ sin2 ψσ = cot θ0. . .

Kato et al. 18

Stress data from sinStress data from sin22ψψ--22θθ diagram diagram

Examples of sin2ψ−2θ diagram

SnCu-CUFESnCu-CUCR(Age: 98 days)(Age: 163 days)

0.0 0.2 0.4 0.6sin2ψ

144.2

144.0

143.82θ (d

eg.)

0.0 0.2 0.4 0.6sin2ψ

2θ (d

eg.)

144.2

144.0

143.8

X direction -19 MPa X direction -29 MPa

- All data in the sin2ψ diagram reveal good linear relationships.- Residual stress is reflected in the slopes.

Kato et al. 19

Residual stress measurement resultsResidual stress measurement results

- Compressive stress of SnCu-CUFE is roughly double that of SnCu-CUCRfor both directions of X and Y.

- Drastic difference in whisker initiation tendencies between the SnCu-CUCRand SnCu-CUFE samples can be explained in terms of difference in residualstress in the coatings.

Stre

ss in

Sn/

Cu

coat

ing

(MP

a)

SnCu-CUFE(98 days)

SnCu-CUCR(163 days)

Y directionX directionY directionX direction

0

-10

-20

-30

X direction

Y directio

n

(1) Measurements for same samples were repeated twice for each direction X and Y.

(2) Error bar for each data point indicates confidence limit.

(3) X is longitudinal direction and Y is width direction of lead.

Kato et al. 20

Model of whisker initiation (intermediate summary)Model of whisker initiation (intermediate summary)

SnCu-CUCR

SnCu-CUFE

SnCu coating

Grain boundaries

Cu leadframe

Fine Cr-rich particles

Fe particles

Cu diffusion

No whiskers

Whisker initiation

Large compressive stress is induced

Fe particles prevent Cu diffusion

Cu diffusion is easyCu leadframe

Compressive stress is small

Sn diffusion is suppressed by Fe particles

Sn diffusion is easy

LGIMCFGIMC

LGIMC

FGIMC

Formation morphology of LGIMC is a wedge-shaped structure, that induces large compressive stress in coating.

Formation morphology of LGIMC is a comb-tooth structure, which diminishes compressive stress in coating.

Sn diffusion is easy due to smallness of Cr-rich particles in CUCR frame, so a thick FGIMC layer is formed.

Fe particles exist in CUFE frame. Sn diffusion is suppressed by Fe particles, so FGIMC layer is thin.

SnCu coating

Grain boundaries

Kato et al. 21

Calculation of stress distribution in coating Calculation of stress distribution in coating

- To determine the essential contribution of stress to whisker initiation

- FEA (Finite Element Analysis) to evaluate the stress distribution- Configurations of LGIMCs in the two samples were taken into account

in the FEA calculation. - To clarify the difference in stress distribution between two samples.

SnCu-CUCR SnCu-CUFEGB GB GBGB GB

Cu-K1 µm

LGIMC

LGIMC configuration ofa comb-teeth structure

1 µm Cu-K

GB GB GBGB

LGIMC

LGIMC configuration ofa wedge-shaped structure

Kato et al. 22

FEA FEA ((FFinite inite EElement lement AAnalysis)nalysis) modelmodel• A two dimensional plane strain condition

Analytic area

x

y

10 µ

m

3 µm

GB

LGIMC

2.25

µm

0.5 µm

Columnar crystal

LGIMC

GB

2 µm

3 µm

10 µ

m

x

y 1 µm

Analytic areaColumnar crystal

CUCR (excluded in model) CUFE (excluded in model)

x,y fix

x fix

y fix

x,y fix

x fix

SnCu-CUFESnCu-CUCR

y fix

Kato et al. 23

Material constantsMaterial constants

• SnCu coating: bi-linear elastic-plasticdeformation behavior

– Young’s modulus: 43.5 GPa– Poisson's ratio: 0.35– Yield stress: 30 MPa – Strain hardening coefficient: 700 MPa

(all data were estimated from bulk solder)

• LGIMC: elastic deformation behavior

– Young’s modulus: 100 GPa (speculation)– Poisson's ratio: 0.01 (speculation)

S-S curve

S-S curve

Kato et al. 24

SetSet--up conditions of stress in coatingup conditions of stress in coating

SnCu-CUCR SnCu-CUFE

GB

x

y

SnCu SnCu SnCu

SnCuSnCu

GB

x

y

SnCu SnCu SnCu

SnCuSnCu

Initial: 2.25 µm Growth: 2.86 µm Initial: 1 µm

Growth: 1.28 µm

Initial: 0.5 µmGrowth: 0.64 µm

Initial: 2 µm Growth: 2.56 µmLGIMC

Surface

LGIMC

- Initial stress in coating including LGIMC with initial configuration is zero.- Distribution of induced stress is calculated when LGIMC grows by up to25% in both x and y directions, as shown above.

Kato et al. 25

FEA results for XFEA results for X--directional stress distribution directional stress distribution X-directional stressσx is normal to GBSnCu-CUCR SnCu-CUFE

Tension

Compression

-130

20

- 50- 70

-100

σx (MPa)

- 20GB

Tension

Large compressive stress fieldx

y

LGIMC

GB

Tension

Tensile stress fieldx

y

LGIMC

Compressivestress gradienttoward surface

along GB

- Two-directional compressive stress gradient toward surface and toward leadframe.- The compressive stress gradient toward surface in CUCR sample is smaller than

that in CUFE sample.

Kato et al. 26

Stress distributions along GBStress distributions along GB

- Stress gradient along GB (slope) toward surface in CUCR sample is smallerthan that in CUFE sample.

- From a relationship between stress gradient and atom flux along GB (next page), atom flux toward surface in CUFE is thought to be larger than that in CUCR.

-60

-40

-20

0

20

40

60

0 2 4 6 8Distrance from A to C(C') (µm)

σx o

n G

B

(MP

a)

CUCRCUFE

A

B

B’

Surface Leadframe

Atom fluxtoward surface

Atom flux towardtwo directions

C’

C

σ x

alon

g G

B (M

Pa)

Distance from A to C (C’) (µm)(σ x : X-directional stress normal to GB)

CUCR

B

A

C

CUFE A

B’

C’x

y

Kato et al. 27

Relationship between stress gradient and atom fluxRelationship between stress gradient and atom flux

µ = µ0 − σn Ω (2)

: Atom flux: Coefficient of GB diffusion : Boltzmann constant: Absolute temperature: Reference value of chemical potential : Normal stress to the GB: Atomic volume: Direction along GB

JBDBKTµ0σnΩs

DB δµ

kT δsJ B = along GB (1)

(1) A. Needleman and J.R. Rice, Acta Metallurgica, 28 (1980), 1315.(2) C. Herring, J. Applied Physics, 21 (1950), 437.

Columnar crystal

Com

pres

sive

str

ess

Com

pres

sive

str

ess

GB

Crystal growth Whisker

Stress gradientnormal to GB

s

Atom flux along GB

Grain boundary (GB) diffusion

Kato et al. 28

Conclusions of FEAConclusions of FEA

(1) Gradient along GB of compressive stress normal to GB in SnCu-CUCR sample is smaller than thatin SnCu-CUFE sample.

(2) Difference between whisker initiation tendenciesof two samples is attributable to difference inamount of atom flux toward surface along GB, which is due to compressive stress gradient.

Kato et al. 29

Part 2Part 2

Correlation between Whisker Initiation Sites and

Sn Diffusion Sites

Kato et al. 30

Simulation of atom diffusion by molecular dynamicsSimulation of atom diffusion by molecular dynamics

- Periodic boundary conditions applied in x, y, and z directions- Diffusion coefficient for y direction calculated from Einstein’s relation　　　　　

D = lim D (t), D(t) = ＜ [yi(t +t 0)－ yi(t 0)]2 ＞/2t　

yi(t +t 0)－ yi(t 0): displacement of atom i in y direction＜＞: average over diffusing atoms

t → ∞

- Atomic diffusion simulated by solving Newton’s equation of motion: mid2ri/dt2=Fi

- To clarify grain boundary diffusion as compared with bulk diffusion- Stress dependence of GB-diffusion coefficient by applying stress σx

１nm

Grain BGrain A

Stress σx

Coincident grain boundary Σ 5 (110)

Stress σx

A bi-crystal model (2x104 atoms) Bulk model (1x104 atoms)

x

y

x

y

Single crystal

Cu atoms(1at%Cu-Sn)

１nm

Kato et al. 31

Simulation results of interface and bulk diffusionSimulation results of interface and bulk diffusion

Diffusion coefficient calculated from molecular dynamics

1.E-55

1.E-50

1.E-45

1.E-40

1.E-35

1.E-30

1.E-25

1.E-20

1.E-15

0 0.5 1 1.5 2 2.5 3 3.5

Diff

usio

n co

effic

ient

（m

2 /s）

1/T (103/K)

Appliedcompressivestress σx

200 MPa50 MPa0 MPa

200 MPa50 MPa0 MPa

Bulk

20ºC100ºC

200ºC

GB: Σ 5 (110)

- Grain boundary diffusion coefficients are much larger than bulk- diffusion coefficients under same temperature and stress conditions.

- Grain boundary diffusion could be a more dominant factor in whisker initiation than bulk diffusion.

- Whiskers could preferentially initiate immediately on the GBs of the coating.

Kato et al. 32

Confirmation of whisker initiation sites Confirmation of whisker initiation sites -- planar slice method planar slice method --

1

Direction 1side elevation view

4Direction 4side elevation view

side elevation view

2

3

Direction 3side elevation view

Whiskers from front view (tilt=0º)

2 µm

A

B

2 µm Direction 2(tilt=45º)

A B


A

B


AB


A

B

SEMLocation of whisker roots

Direction 2

SEM observation of whiskers from various angles in SnCu-CUFE sample with an age of 47 months

Kato et al. 33

Planar slice methodPlanar slice method- To confirm correlation between whisker root and coating microstructure,sample was sliced horizontally using a planer slice method.

Direction of SEM observation Location of whisker roots

LGIMC10nmFGIMC

Cu leadframe

SnCu coating

Surface

Whiskers

GB

Location of whisker roots

A

B

SEM

Whisker AWhisker B

Whiskers in a front of view (tilt=0º)

Kato et al. 34

Planar slice methodPlanar slice method

Step 1: Protecting whiskers with resin coating

10nm

Thinned resin

Horizontal section of whisker

10nm

Resin

Whisker

Whisker AWhisker B

A

B

Backscattered electron image in a front of view


Thinned transparent resin coating

Direction of BEI observation(BEI: backscattered electron image)

Protect whiskers by plastering resin on surface

Cutting whiskers by mechanical reduction of resin

Kato et al. 35

Standard triangle of stereogram

Step 2: Thinning of SnCu coating

1 µm

EBSP IPF (orientation) map: normal direction

SnCugrain

IMC


1 µmBackscattered electron image

IMC

SnCugrain

- Whisker roots always located at intersections of GBs.- Whisker initiation sites must be grains immediately on top of GBs.

Direction of BEI observation and EBSP measurement

∼ 1 µm

10nm

Thinning of SnCu coating surface



Kato et al. 36

Direction of SEM observation

Step 3: Removal of SnCu coatingby chemicaldissolution

1 µm


LGIMCs with a shape of pyramid A front view of the shape of LGIMCs in SEM image

Cu leadframe



Coating was completely removed.

LGIMC

Whisker roots always located above peaks and ridge lines of LGIMC-pyramids.

Kato et al. 37

Discussion of Part 2 Discussion of Part 2 (1) Whisker initiation sites correlate with Sn diffusion sites (i.e.,

GBs), which is explained by the finding that whisker roots arealways located on top of the intersections of GBs (a result ofplanar slice method) and that dominant Sn diffusion sites are GBs when compressive stress is applied from the normal direction (a result of molecular dynamics calculation).

(2) Whisker initiation sites are located above the peaks of LGIMC-pyramids (a result of planar slice method). So, taking into account the results of FEA and X-ray stress measurement, it is reasonable to assume that LGIMC-pyramids could induce a large compressive stress field above the pyramids, resulting in the enhanced Sn diffusion at GBs immediately on top of the pyramids.

(3) It can be concluded that whiskers could initiate from the grain located immediately on top of the GB due to the compressive stress field normal to the GB, which is induced by the LGIMC-pyramid.

Kato et al. 38

SummarySummary(1) A considerable difference between the whisker initiation

tendencies of SnCu-CUCR and SnCu-CUFE samples is explainable through the correlation between whisker formation, microstructure, and stress.This correlation was supported by evaluation of the stress gradient in the SnCu coating deduced by FEA.

(2) Whisker initiation sites in the SnCu-CUFE sample are preferentially located on top of the intersections of grain boundaries in the SnCu coating and above the peaks of the LGIMC-pyramids. This fact corresponds to findings for dominant tin diffusion sites (GBs) calculated by molecular dynamics.

correlation between whisker initiation and compressive...

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