ECTC2005 ECTC2005 iNEMI tin whisker modeling iNEMI tin whisker modeling

committeecommittee

Chen Xu � Cookson Electronics G. T. Galyon � IBM (presenting)

S. Lal � FCI B. Notohardjono � IBM

Asa Frye � IBM

1

Mr. Whisker Mr. Whisker

Courtesy of Peter Bush/SUNY-Buffalo

2

Agenda Agenda � Introduction � Film Physical Properties � Integrated Theory for Whisker Formation � Kirkendahl Effect and Intermetallic Formation� Finite Element Analysis for Film Stress � Film Stress Measurements � XRD � Film Stress Measurements- Flexure Beam

� Flexure Beam Observations � Flexure Beam Limitations

� Film Crystallography and Whisker Formation � Film texture: EBSD and X-Ray Analysis � Whisker Crystallography

� Wrap Up

3

Whisker Theory Whisker Theory

� Stress in Sn film� Many causes: To stress (plating process); oxide; external

forces; CTE mis-match; but

� Key cause is uneven intermetallic formation� IMC starts to form in hours; continues for days/weeks

� Root cause: copper diffusion

� Incubation time

� Substrate impacts IMC growth

� Temperature affects rate and type of IMC growth

� Causes movement of Sn within structure

� Sn film orientations enhance whisker formation� Whisker grain different orientation surrounding grains

� Methods of measuring orientation important

4

Physical PropertiesPhysical PropertiesCu,Ni,SnCu,Ni,Sn and Their and Their IntermetallicsIntermetallics

650.9309.20974.50118.7058.763.5Atomic or Molecular Weight

Gm-atoms/gm-mole

75.2534.74117.8416.306.597.11Molar Volume cc/gm-mole

8.658.908.277.288.908.90Density gms/cc

13.719.016.323.0013.016.6-17.6TCE ppm/oC(Thermal Exp. Coeff.)

0.3300.2990.3090.360.310.33-0.36Poisson�s Ratio

133.3108.385.5642214124Young�s Modulus GPA

Ni3Sn4Cu3SnCu6Sn5SnNiCuParameter

5

Introduction Introduction

� Film Stress Analyses� Need to Understand Stress States in Film Structures

� Kirkendahl Effect and Intermetallic Formation � Finite Element Analysis (FEA) of zonal film structure � Theory (FEA) needs reconciliation with measurement � XRD and Flexure Beam Data must be reconciled

� Microstructure needs to be folded into analyses � E.g. the extent of IMC at the time of stress measurement

� Whisker Grain Crystallography� A Focal Point of Presentation� EBSD versus XRD sin2Ψ� Whisker Grain Boundary Angles � Whisker Grains comparison to Adjacent Grains

6

An Integrated Theory for Whisker Formation An Integrated Theory for Whisker Formation

� Compressive stress- A necessary factor for whisker formation� Compressive stress sources

� Must be high impedance or sustaining sources, e.g. � Intermetallic formation� High humidity (oxide reactions at film surface) � Temperature cycling (differential thermal expansion)� Built in film stresses (additives/gaseous entrapment)

� Arguments to the contrary are potentially flawed� e.g., non sustaining stresses � e.g. Flexure beam observations� e.g. Bent lead-frame experiments

� Recrystallization -> formation of whisker grain� An important factor/possibly required or necessary

� Tin Self-Diffusion-not part of this presentation� Grain boundary diffusion brings tin to whisker � Driven by stress gradients/not concentration gradients

7

Kirkendahl Effect Kirkendahl Effect Intermetallic Formation & compressive stressesIntermetallic Formation & compressive stresses

� Cu6Sn5 formation at Sn/Cu Interface� Cu invades spatial region occupied by Sn & forms intermetallic (IMC)

� y gm-moles of Sn-> (y-(5x/6)) gm-moles Sn + (x/6) gm-moles Cu6Sn5� Above equation consistent with Sn/Cu interdiffusion (Kirkendahl Effect)

� i.e., Cu diffuses into Sn at a high rate � i.e., Sn diffuses into Cu at an extremely low rate

Ref. Tu and Thompson, 1993. � Molar volume for (y-(5x/6))Sn + (x/6) Cu6Sn5 > y Sn

� Therefore, IMC region has an expansionary �action� � Expansionary �action� -> compressive stress state

� Cu out-diffusion from substrate -> forms vacancies in the Cu � Vacancies -> shrinkage and tensile stresses

� Sn/Cu interdiffusion an example of Kirkendahl Effect� Well known Materials Science Phenomenon � Compressive and tensile stresses on either side of interface� Vacancy supersaturation on tensile side of interface

8

Kirkendall zone � vacancy rich copper

Copper substrate

Cu6Sn5+Sn

Sn

SnZone 1

Zone 2(compression)

Zone 3(tension)

Zone 4

The 4-zone structure for a tin/copper couple after intermetallic formation

L LKirkendall zone � vacancy rich copper

Copper substrate

Cu6Sn5+Sn

Sn

SnZone 1

Zone 2(compression)

Zone 3(tension)

Zone 4

The 4-zone structure for a tin/copper couple after intermetallic formation

L L

Oxide layer

44--Zone Structure for Sn/Cu Zone Structure for Sn/Cu

9

c o p p e r t in

c o p p e r t in

Kirkendall Vacancy regionShrinkage action Tensile Stress

Copper diffusion zone Interstitial Cu + Cu6Sn5 +Sn

Compressive Stress

Kirkendahl Effect for Tin/Copper Couple

10

Kirkendahl Effect for Sn/Cu CoupleKirkendahl Effect for Sn/Cu Couple

y

OOOOO

OOOOO

OOOOO

OOOOO

Cu Sn

Cu Sn

Cu Sn

Before Diffusion

After Diffusion

Inert markers

Original Interface

Kirkendall Shift

Original Interface

(a) Typical Text Book Presentation (b) Atypical Kirkendall Effect Schematic

y

OOOOO

OOOOO

OOOOO

OOOOO

Cu Sn

Cu Sn

Cu Sn

Before Diffusion

After Diffusion

Inert markers

Original Interface

Kirkendall Shift

Original Interface

(a) Typical Text Book Presentation (b) Atypical Kirkendall Effect Schematic

Cu Sn

Both (a) and (b) are correct: (a) is easily misinterpreted

Cu diffuses into Sn but Sn does not diffuse into CuCu diffuses into Sn but Sn does not diffuse into Cu

11

FIB Cross Section of Sn/Cu with Kirkendahl VoidingFIB Cross Section of Sn/Cu with Kirkendahl Voiding

Put one or two FIB x-sections here

Kirkendahl Voids

12

FIB Cross Section of Sn/Cu with Kirkendahl VoidingFIB Cross Section of Sn/Cu with Kirkendahl Voiding

Kirkendahl Voids

13

Sample Molar Volume Calculations Sample Molar Volume Calculations For ZoneFor Zone--2 of a Sn/Cu couple2 of a Sn/Cu couple

Y ≡ gm-moles of tin per unit volume (V) in zone-2 before interdiffusionX ≡ gm-moles of copper diffusing into volume (V) & reacting to form IMC

Y (Sn) -> (X/6) (Cu6Sn5) + (Y-(5X/6)) Sn (eq. 1) Y gm-moles of tin -> X/6 gm-moles of IMC + (Y-(5X/6)) gm-moles of tin

Y (16cc/gm-mole) -> (X/6) (118 cc/gm-mole) + (Y-(5X/6)) (16cc/gm-mole)16Y ccs -> 16Y ccs + 6.6X ccs (eq. 2)

The right side of equation 2 (above) is always >> than the left hand sideTherefore; zone-2 is �expansionary� due to IMC formation

14

Molar Volume Calculations contd. Molar Volume Calculations contd.

For example:

11Sn -> 1 Cu6Sn5 + 6Sn in molar volumes is:

176 ccs-> 118 ccs + 96 ccs176ccs < 214 ccs (a 22% volume increase in zone-2)

For example:

12Sn-> 2 Cu6Sn5 + 2Sn

192ccs-> 236 ccs + 32 ccs192ccs < 268 ccs (a 39% volume increase in zone-2)

15

02

46

810

1214

1618

2022

2426

2830

3234

36

unrestricted% Volume Expansion in IM Region ( Zone-2)

0

10

20

30

40

50

60

70

80

90

100

% I

M (Z

one-

2) R

eact

ion

Volume Expansion in Zone-2 as a functionof IMC reaction percentage

IMC Reaction Equation: ySn-> (y-(5x/6))Sn + (x/6)Cu6Sn5% IM Reaction ≡ (x/y) X 100

Note: The reaction equation is NOT: 6Cu + 5Sn -> Cu6Sn5

16

44--zone Sn/Cu Structure zone Sn/Cu Structure -- Stress Distribution in zoneStress Distribution in zone--22

Vacancy rich copper (Kirkendall Zone)-tension

copper substrate

Cu6Sn5 (compression)

Sn (compression)

Sn

Vacancy rich copper (Kirkendall Zone)-tension

copper substrate

Cu6Sn5 (compression)

Sn (compression)

SnStress-tension at time zero

-compressive over time

Stress is the same for tin and IMC in zone-2

It cannot be that tin and IMC are at different stress levels at or near equilibrium

Oxide layer

17

CuCu66SnSn55 IMC Expansion Restrictions IMC Expansion Restrictions � Cu6Sn5 IMC has expansion potential upon formation

� Up to ~30+% per unit volume of tin� IMC formation is irregular due to Cu grain boundary diffusion

� IMC expansion is limited by stress build up � As IMC forms and expands an elastic stress build up occurs � Stress build up restricts the volume expansion of IMC � built in tensile stress can even suppress IMC formation ???

� Above statement is preliminary-see following FIB sections

� IMC expansion is aided by out-diffusion from Zone-2 � Out-diffusion decreases compressive stress level in Zone-2

� IMC formation is dependent upon film thickness � Film thickness can determine the relative effectiveness of

diffusion s.r. � i.e. Thick tin films, faster IMC formation � i.e. Thin tin films, slower IMC formation

18

Levre & Barczykowski, Wire Jour. Inter. Jan 1985-Figure 3 pp. 66-71

This data shows IMC thickness shown as weight gain is a function of film thick-�ness.

The linearity with the square root of time implies that IMC formation is a diffusion dependentreaction.

This data implies thatIMC formation is stress dependent ?

19

FIB XFIB X--sections sections -- 0.5u immersion Sn/Cu /30 days old0.5u immersion Sn/Cu /30 days old

.75 u- immersion tin

0.1 u Cu6Sn5

Copper Film Surface

Copper Substrate

20

FIB XFIB X--sections sections -- 0.5u immersion Sn/Cu /1 yr. old0.5u immersion Sn/Cu /1 yr. old

Full IMC penetration0.75 u tin film

Kirkendahl voiding under IMC

21

FIB XFIB X--sections sections -- Sn/Cu bright tin / + 40 Days Sn/Cu bright tin / + 40 Days

Cu Substrate

Bright Tin

Interface

No Intermetallic ?

10 MPa �Tensile� Stress

22

FIB XFIB X--sections sections -- Sn/Cu bright tin / + 40 DaysSn/Cu bright tin / + 40 Days

Same as Prior FIBDifferent Area

Still no IMC at Interface

10 MPa �Tensile� Stress

23

nickel tin

nickel tin

Tin diffusion zone Sn + Ni3Sn4 IMC

Compressive stress

Kirkendahl vacancy zoneShrinkage action

Tensile stress

Kirkendall Effect for Tin/Nickel/Copper Couple

24

Zone-1(compression)

Zone-2(tension)

Zone-3(compression)

Zone-4(tension)

Ni3Sn4+ Ni

Ni

Ni

Copper substrate

Tin film

Tin film

Zone-1(compression)

Zone-2(tension)

Zone-3(compression)

Zone-4(tension)

Ni3Sn4+ Ni

Ni

Ni

Copper substrate

Tin film

Tin film

The 4-zone structure for Sn/Ni/Cu after intermetallic formation

Zonal Structure for Tin with Nickel Underlay

Oxide layer

25

Sample Molar Volume Calculations Sample Molar Volume Calculations For ZoneFor Zone--3 of a 43 of a 4--layer Sn/Ni/Cu couplelayer Sn/Ni/Cu couple

� 3Ni -> Ni3Sn4� Molar Volume of Ni = 6.6 cc/gm-mole� Molar Volume of Ni3Sn4 = 75.25 cc/gm-mole � The molar volume for 3Ni < molar volume for Ni3Sn4 � 19.8ccs < 34ccs (an expansionary action)

� For Sn/Ni/Cu � The vacancy rich region is in the tin � The expansion action is in the nickel

There is no known visual confirmation of Kirkendahl vacancies for Sn/Ni/Cu structures at this time (5/05)

26

CuCu33Sn Intermetallic FormationSn Intermetallic Formation

� Cu3Sn forms from Cu6Sn5 for T > 60 oC

� At temperatures > 60 degs. C � If Cu6Sn5 -> 2Cu3Sn + 3Sn� 118 ccs/gm-mole -> 70ccs/gm-mole +48ccs/gm-mole� 118ccs->118ccs (no expansion or contraction)

� The Above Reaction provides excess tin (Sn) atoms � Excess Sn atoms permit continued outdiffusion/relaxation

� Excess tin atoms outdiffuse towards surface

� Outdiffusion reduces stress in Cu3Sn zone� Converts compressive stress to less compressive� Can convert stresses to tensile stress � Can show evidence of Kirkendall voids in Cu3Sn

27

CuCu33Sn Intermetallic Formation ? Sn Intermetallic Formation ?

Kirkendall zone � vacancy rich copperCopper substrate

Cu6Sn5+Sn

Sn

Sn

The 4-zone structure for a tin/copper couple after annealing

Cu3Sn

Zone -1

Zone-2

Zone-3

Zone-4

Kirkendall zone � vacancy rich copperCopper substrate

Cu6Sn5+Sn

Sn

Sn

The 4-zone structure for a tin/copper couple after annealing

Cu3Sn

Zone -1

Zone-2

Zone-3

Zone-4

Sn Sn

Oxide layer

28

XX--section of Cusection of Cu33Sn w Kirkendall VoidsSn w Kirkendall Voids

Picture from RPI research to be inserted.

Reference: M. Glicksman / A. Lupulescu, RPI, 2002

porosity in Cu3Sn requires outdiffusion

29

XX--section of Cusection of Cu33Sn w Kirkendall VoidsSn w Kirkendall Voids

Tin Film

Cu6Sn5

Cu3Sn

Cu Substrate

Kirkendahl voiding in Cu3SnKirkendahl voiding in Cu

Finite Element Analysis Finite Element Analysis Tin Films Tin Films

B. NotohardjonoG. Galyon (presenting)

C.XuS. Lal

31

Finite Element Analysis Finite Element Analysis Zonal Structures For Sn/CuZonal Structures For Sn/Cu

� Objective: Correlate Theory to Measurement

� Finite Element Analysis Strategy

� Break up substrate/film into zones � Assume expansions/contractions for each zone � Calculate stress states � Compare to experimental data

32

Kirkendall zone � vacancy rich copper

Copper substrate

Cu6Sn5+Sn

Sn

SnZone 1

Zone 2(compression)

Zone 3(tension)

Zone 4

The 4-zone structure for a tin/copper couple after intermetallic formation

L LKirkendall zone � vacancy rich copper

Copper substrate

Cu6Sn5+Sn

Sn

SnZone 1

Zone 2(compression)

Zone 3(tension)

Zone 4

The 4-zone structure for a tin/copper couple after intermetallic formation

L L

Oxide layer

Four Zone Structure for Sn/Cu

33

Finite Element Stress Analysis Finite Element Stress Analysis Model Parameters for Sn/CuModel Parameters for Sn/Cu

� Set zonal thickness and properties � E.g., Zone-1 at 5 microns: 100% tin� E.g., Zone-2 at 2 microns: mixture of tin and IMC� E. g.,Zone-3 at 2 microns: vacancy rich copper � E.g., Zone-4 at 10 microns: copper substrate

� Let active zones expand/contract � Zone-2 will expand: inter-diffusion/IMC formation� Zone-3 will shrink: Kirkendall vacancy formation

� Zones1/4 will react:

� Calculate stresses in reactive zones

� Compare scenario stresses to experimental results

34

0 .9 1.8

2.8

3.8

4.8

5.8 7 8 9 10

Legend: zone-2 expansion Distance from Surface-microns

-10-8-6-4-202468

Stre

ss-k

si

zone-2=0zone-2=.0001

zone-2=.001zone 2=.00125

zone 2=.002

4-Zone Structure Stress States zone-3 shrinkage = .001

zone 1 zone 2 zone 3

The flexure beamIs concave downfor all 5 scenarios

Finite Element Analysis Finite Element Analysis

35

0 .9 1.8 2.8 3.8 4.8

Legend: zone-2 expansionDistance from Film Surface (microns)

-1

0

1

2

3

4

5

6

Stre

ss-k

psi

zone-2=0zone-2=.0001zone-2=.001zone-2=.00125

zone-2=.002zone-2=.005zone-2=.01zone-2=.02

Zone-1 Stresses zone-3 shrinkage=.001An expanded presentation

for zone-1 showing how the Zone-1 stress becomes increasingly tensile with increasing zone-2 expansion

Finite Element Analysis Finite Element Analysis

36

OutOut--Diffusion of Tin: ZoneDiffusion of Tin: Zone--2 to Zone2 to Zone--11

� Tin will out-diffuse from Zone-2 to Zone-1� Diffusion driven by stress gradient / not concentration grad.� Diffusion will primarily be through grain bdries.� Bulk tin self-diffusion

� Increasingly important with increasing temperature� Very anisotropic: <001> >>> <100> or <010>� Accounts for Pedestal Structures from annealing (see Wed. pm)

� Out-diffusion-> balances Zone-1/2 stress� Zone-2 stress decreases/Zone-1 stress decreases � Diffusion stops -> total strain energy is minimized.

37

0 .9 1.8 2.8 3.8 4.8 5.8 7 8 9 10

Legend (Out-Diffusion Factor)Distance (microns) from Surface

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

6

Stre

ss (k

psi)

0 1.2 (a) 1.8(d) 2

zone-1 zone-2 zones-3,4

Starting point tin in tension

Equilibrium point tin in compression

Past equilibrium-tin in high compression

e.g. �out diffusion factor 1.2 means that the 2% expansion has been reduced 1.2% by out diffusion from zone-2 to zone-1

Finite Element Analysis Finite Element Analysis ��outdiffusionoutdiffusion

38

0 .6 1.2 1.8 2.4 3.0

Zone-2 to Zone-1Out-Diffusion Factor

0

2

4

Ene

rgy

per c

c (lb

-in x

10+

6)

out-diffusion #02% expansion:zone-2

out-diffusion factor #32% expansion zone-2

out-diffusion factor #1.82% expansion zone-2

Note: Strain energy term is total strain energy for zones 1,2,3,4 in 4-zone model

Flexure isConcave Down with R decreasingWith increasingOut-diffusion

Strain Energy Strain Energy �� Outdiffusion Outdiffusion

39

Zone-1(compression)

Zone-2(tension)

Zone-3(compression)

Zone-4(tension)

Ni3Sn4+ Ni

Ni

Ni

Copper substrate

Tin film

Tin film

Zone-1(compression)

Zone-2(tension)

Zone-3(compression)

Zone-4(tension)

Ni3Sn4+ Ni

Ni

Ni

Copper substrate

Tin film

Tin film

The 4-zone structure for Sn/Ni/Cu after intermetallic formation

Oxide layer

Sn/Ni/Cu Zonal Structure

40

Finite Element Stress Analysis Finite Element Stress Analysis Model Parameters for Sn/Ni/CuModel Parameters for Sn/Ni/Cu

� Set zonal thickness and properties � zone-1 at 3 microns: 100% tin� Zone-2 at 2 microns: vacancy rich tin � Zone-3 at 2 microns: nickel plus Ni3Sn4 intermetallic � Zone-4 at 10 microns: copper substrate

� Let active zones expand/contract � Zone-2 will shrink: Kirkendall vacancy formation � Zone-3 will expand: intermetallic formation � Zones1/4 will react:

� Calculate stresses in reactive zones

41

0 1.8 3.8 5.8 8 10distance from film surface (microns)

-200

-100

0

100

200

300

Stre

ss (p

si)

zone-3= +1.0% zone-2=-0.1%

Z1 z2 z3 z4

FEA predicts that tin stress willbe tensile at the Sn/Ni interface andcompressive at the surface for a filmthickness >2-3 us.

These calculations assume that there isno built-in stress intrinsic to the tin itself. An intrinsic tin film stress would be additive.

compressive

tensile

compressive

Finite Element AnalysisFinite Element Analysis--Sn/Ni/CuSn/Ni/Cu

42

FEA Summary FEA Summary

� FEA of Zonal Structures � Predicts Flexure Beam and XRD σ measurement in

opposition to each other

� Predicts that XRD at time zero may be tensile and over time goes compressive

� Matches up well to Sn/Ni/Cu XRD Data

� Matches up well with Lee and Lee (1998) beam data

43

Kinetics of IMC Formation/Diffusion Kinetics of IMC Formation/Diffusion

� Case 1-Fast IMC Formation/slow diffusion� XRD on thin (<~3 ums) samples

� compression at time zero decreasing w/time� XRD on thick (> 5 ums) samples

� Tensile at time zero going to compressive w/time

� Case 2-Slow IMC Formation/fast diffusion� XRD on thin (<3ums) samples

� Zero at time zero becoming compressive with time � XRD on thick (>5 ums) samples

� Zero at time zero becoming compressive with time

Note: The above based on a zero built-in tin stress.

44

What Does the Stress Data Say? What Does the Stress Data Say?

� XRD Data � C. Xu, Y. Zhang, et al: Cookson/Enthone Y2000+

� Flexure Beam Data � Lee and Lee: Seoul National University Y1998

45

XRD XRD for Sn/Ni/Cufor Sn/Ni/Cu

0.51.0

1.52.0

2.53.0

3.54.0

4.55.0

5.56.0

6.57.0

7.58,0

8.59.0

9.510

-10

-5

0

5

10

15

20

Stre

ss-M

Pa

Thickness of Tin Film for Sn/Ni/Cu Structure

For tin films <2.0 microns allXRD stress values are tensile

For tin films >2.0 microns allXRD stress values are compressive

These values are consistentwith FEA analysis of a zonalSn/Ni/Cu structure

Transition from tensile to compressive

Reference: Cookson Electronics, C. Xu, et al.

46

Lee and Lee�s (1998)Flexure Beam Stress DataLee and Lee�s (1998)Flexure Beam Stress Data

0 5 10 15 20 25 30 35 40 45 50 55 60

Aging Time (Days)

-10

-5

0

5

10

15

Stre

ss M

Pa

Complex Stoney�s Equation σ= ET2 (1/R � 1/Ro)

6 6(1-γ)t)

The stress at time zero is tensile and within 2 days is compressive.

We do not know what the radii of curvatures were�we can assume that the beam went from concave up to concave down as the stress went from tensile to compressive. But it�spossible the radius was concavedown thruout the test cycle.

tensile

compressive

Tin on Phos-Bronze Substrate

47

Published Stress Data SummaryPublished Stress Data Summary

� Compressive Stress: � A Necessary Factor for Whisker Formation � Arguments to Contrary are Flawed

� See Zonal Structure / FEA / XRD / Flexure Beam Sections � Stoney�s equation generally invalid for analysis of tin films

� Consensus essentially established

� Compressive Stress Sources � IMC Formation / Kirkendahl Effect: established in

presentation � Oxide Reactions� Thermal cycling� Built in Film Stresses, e.g. Cu alloying (not discussed)

Stress Measurement Stress Measurement by XRDby XRD

Chen Xu-CooksonG. T. Galyon-IBM (presenting)

S. Lal-FCI B. Notohardjono-IBM

49

AgendaAgenda--XRD Stress Analysis XRD Stress Analysis

� Stress Classifications

� Stress Measurement Overview-Sin2θ

� Sn/Ni/Cu Data Set

� Sn/Cu Data Set

50

Types of Residual StressesTypes of Residual Stresses

� Type II� Structural Microstress� Short range: Over the distance of one grain.

� Type III� Microstress:� Short range: several atomic distances within the grain.

� Type I� Macroscopic Stress� Long-range: over several grains

XRD and Mechanical

XRD

XRD (synchrotron)

51

Type 2: Structural MicrostressType 2: Structural Microstress

� The average stress for an individual grain

� Occurs between different phases

� Determined by XRD with x-ray beam � typically a micro-focused Synchrotron X-Ray

52

Why using XRD for the residual stress measurement?

DetectorX-ray

Non-destructive and spatial resolvedMonitoring the stress evolutionStress measurement on real partMeasuring the stress of individual layer

53

XRD Spectra Shifts & Stress TypeXRD Spectra Shifts & Stress Type

71 72 73 74

0

100

200

300

400

500

stress-free tensile stress compressive stress

Diff

ract

ion

Inte

nsity

2θ70 71 72 73 74 75

0

100

200

300

400

500

low microstress high microstress

Diff

ract

ion

Inte

nsity

2θ

Type 1 and 2 Stress Type 3 Microstress

54

Type 1: Macroscopic StressType 1: Macroscopic Stress

� Tensor: direction and magnitude

� Normal and shear stress

yx

z

Normal

Shear

σσσσx, σ, σ, σ, σy, σ, σ, σ, σz

σσσσxy, σ, σ, σ, σxz, σ, σ, σ, σyz

55

Stress Measurement Based on Mechanical MethodsStress Measurement Based on Mechanical Methods

Compressive Tensile

Basis Metal Plated Film

Note: Above True for 2-zone structures / not true for 3 or 4 Zone Structures

56

Stress Measurement using XRDStress Measurement using XRD

Z

X

ψ

εx(ψ) = υ+1E

σx sin2ψ - υE

(σx + σy)

57

Stress Measurement using XRDStress Measurement using XRD

εx(ψ) = υ+1E

σx sin2ψ - υE

(σx + σy)St

rain

sin2ψψψψ

Tensile Stress

Compressive Stress

58

Shear Stress?Shear Stress?

0.0 0.1 0.2 0.3 0.4 0.5 0.6

-0.0016

-0.0014

-0.0012

-0.0010

-0.0008

-0.0006

-0.0004

-0.0002

0.0000

0.0002

sin2ψ

Stra

in

+ψ -ψ

z

xψψ

59

Biaxial Stress StateBiaxial Stress State

0.0 0.1 0.2 0.3 0.4 0.5-0.0020

-0.0018

-0.0016

-0.0014

-0.0012

-0.0010

-0.0008

-0.0006

-0.0004

-0.0002

0.0000

0.0002

sin2ψ

σx= 851 MPa σy= 842 MPa

Stra

in

x

y

Stress Measurement of Stress Measurement of Sn/NiSn/Ni

Chen Xu-CooksonG. T. Galyon-IBM (presenting)

61

Experimental SetupExperimental Setup

� D8 Discover with GADDS by Bruker.

� Cr-radiation and 0.5mm beam.

� Diffraction peak (312) at 2θ=143.8o.

� The strains were measured at 19 different ψ angles from �45o to 45o.

62

Stress MeasurementStress Measurement

-0.0008

-0.0007

-0.0006

0.5µ Sn/Ni/Cu

Stra

in

2µ Sn/Ni/Cu1µ Sn/Ni/Cu

0.0 0.1 0.2 0.3 0.4 0.5

-0.0008

-0.0007

-0.0006

3µ Sn/Ni/Cu

Stra

in

sin2(α)0.0 0.1 0.2 0.3 0.4 0.5

-0.0008

-0.0007

-0.0006

10µ Sn/Ni/Cu

sin2(α)

63

Stress MeasurementStress Measurement

0 2 4 6 8 10

-5

0

5

10

15bright Sn/Ni/Cu alloy, aged at RT for 40 days

Compressive Stress

Tensile Stress

Stre

ss in

Sn

Film

(MP

a)

Sn Film Thickness (microns)Reference Cookson Electronics: C. Xu, et al.

64

Stress Evolution in Sn FilmStress Evolution in Sn FilmPlated Directly Over CuPlated Directly Over Cu

Stress in MPa

As PlatedBright -3±1SB Sn -1±1

As Plated 4 MonthsBright -3±1 -10±1SB Sn -1±1 -7±1

65

Com

pres

sive

Ten s

ile

Aging Time

XRD Stress Data Over TimeXRD Stress Data Over Time--Schematic Schematic

Sn/Ni/Cu XRD Data Ref. C. Xu

Sn/Cu XRD Data Ref. C. Xu

This Region

Expected but

Minimal Data to

Date

66

XRD Stress Analysis Summary XRD Stress Analysis Summary

� XRD on Sn/Ni/Cu � Well behaved data set

� Stresses are biaxial

� Results are in synch with Zonal Structure Theory � Show predicted tensile/compression transition

� Stress over Time analysis� Tensile Stresses Increase with time to about 15 MPa

� XRD on Sn/Cu � Historical Data Set

� Stresses are biaxial � Compressive Stresses increase with time to 15-20 MPas

� Current data set very preliminary � Work in Progress � Latest Results will be discussed in Conference

Flexure Beam Flexure Beam

S. Lal-FCI (Presenting) G. Galyon-IBMC. Xu-Cookson

B. Notohardjono-IBM

68

Flexure Beam Apparatus Flexure Beam Apparatus

R = L2/2δ

Schematic ref. Lee and Lee, 1998.

69

Flexure Beam Stress AnalysisFlexure Beam Stress Analysis

� Stoney�s eq. σ= ET2/6(1-γ)tR� σ is film stress � E is Young�s modulus for substrate material � γ is Poisson�s ratio for substrate material � t = film thickness� T= substrate thickness� R = curvature radius of flexure beam

� Stoney�s eq. only valid for t/T ratios ≤ 0.01(1%)

� Std. Flexure Beam thicknesses 50-200 microns� An 0.5u film has t/T = 0.01 (1%) for a 50u beam � A 2.0u film has t/T= 0.01 (1%) for a 200u beam

70

Classic Flexure Beam Two zone structure all action in top zone

Stoney's solution

Exact solution

Reference: T. W. Clyne

71

Stoney�s versus Exact Solutions Stoney�s versus Exact Solutions Ref. T.W. Ref. T.W. ClyneClyne

1

0.1

0.01

0.0010001 .001 ,01 .1 1

Exact solution Stress at film/substrate interface

Exact solution Stress at film surface

Stoney�s solution Stress

(compressive)

Film / Substrate Thickness Ratio (t/T)

ε (film strain) = constant

72

Stoney�s equation contd. Stoney�s equation contd.

� Not accurate for thick films (t/T> 0.01)

� Predicts decreasing σ with increasing film thickness � For constant film strain ε� Where strain is distributed uniformly thru whole film

� i.e. a 2-zone structure

� Real World Flexure Beams� 0.1 <<(t/T) << 0.8� Stoney�s stress is 5X-100X < maximum exact stress � 2-Zone structure -> linear Stoney�s σ versus film thk

� Must be corroborated by microstructural examination � i.e., there must be no intermetallic formation

73

Complex Stoney�s equation Complex Stoney�s equation

� Etching off the tin�based on change in radius � σ= ET2/6(1-γ)t(1/R � 1/Ro)

� R is the radius after etching off tin � Ro is the radius before etching off tin

� Etchant Removes tin with varying stress levels� Leaves any intermetallic intact

� Result is an averaging of stresses thruout tin

� Etching of the intermetallic� Also removes copper layer � Need etchant that removes only intermetallic

� No known suitable etchant for Sn/Cu systems

74

Lee and Lee�s (1998)Flexure Beam Stress DataLee and Lee�s (1998)Flexure Beam Stress Data

0 5 10 15 20 25 30 35 40 45 50 55 60

Aging Time (Days)

-10

-5

0

5

10

15

Stre

ss M

Pa

Complex Stoney�s Equation σ= ET2 (1/R � 1/Ro)

6 6(1-γ)t)

The stress at time zero is tensile and within 2 days is compressive.

We do not know what the radii of curvatures were�we can assume that the beam went from concave up to concave down as the stress went from tensile to compressive. But it�spossible the radius was concavedown thruout the test cycle.

tensile

compressive

Tin on phos-bronze substrate

75

Flexure Beam Analysis Flexure Beam Analysis �� AnalysisAnalysis Shortcomings Shortcomings

� Intermetallic Reaction at Interface � Kirkendahl Effect must be accounted for � Intermetallic Reaction at must be accounted for

� Oxide Reactions at surface � surface expansion effect must be accounted for

� Alloying additions to tin (e.g. copper)� Intermetallic Reaction must be accounted for

� Zonal structure must be accounted for

76

Flexure beam with Concave Up curvature (i.e. �smiley faced)Simple Stoney�s analysis shows tensile stress in tin film

Zone-1 tin

Zone-2 imc

Zone-3Kirkendall vacs.

Zone-4Cu substrate

Note: Zone-1 (tin) may be compressive if the curvature is due to shrinkage in either zones 2/3.

Neutral axis

44--zone structure: Flexure Beam Analysis zone structure: Flexure Beam Analysis

77

Zone 1-tin film

Zone 2-IMC+Sn

Zone 3-Cu K.Z

Zone 4-Cu

Neutral Axis

Flexure Beam with Concave Down Curvature (i.e. �Grumpy Faced� ) Simple Stoney�s analysis shows compressive stress in tin film

Note: Zone-1 may be tensile even though Stoney�s eq. says it is in compression. i.e., If Zone-1 only �reacts� to the expansionary action of underlying zones it

will be in tension even though the curvature is concave down

44--Zone Structure: Flexure Beam Analysis Zone Structure: Flexure Beam Analysis

78

Time zero: substrate only R=0

T1: In situ (deposition) : R = negative IMC formation negligibleKirkendall vacancy region dominates tin film in compressionStoney�s eq: tin in tension

T2: Shortly after deposition: R= positive ++Significant IMC formation Negligible Out-diffusion tin film in tensionStoney�s eq: tin in compression

T3: Long Term: R = positive +Significant IMC formation Significant Out-Diffusion tin film in compressionStoney�s eq: tin in compression

44--Zone Structure Flexure Beam Scenario Zone Structure Flexure Beam Scenario

79

Flexure Beam Analysis Flexure Beam Analysis -- SummarySummary

� Sn on Cu � Flexure Beam stress analysis is misleading� Fails to account for layered (zonal) structure� Can indicate wrong stress polarity very easily

� Sn on Fe� IMC formed at interface/zonal structure similar to Sn/Cu

� Sn on alloy 42 � Stoney�s eq. should work ok for thin (t/T≤ 1%)

� For thick films Stoney�s solution is a very rough average � No IMC formation at interface / A 2-zone structure � No flexure beam Sn/A42 data in published record to date

80

Flexure Beam Data (Schematic Format) Flexure Beam Data (Schematic Format)

Concave Up “Tensile”

Time

Concave Down “Compressive”

Tin Etched Off at Time Zero

Tin Etched off at Time t

Ref. S. Lal � FCI Corporation

81

The BentThe Bent--Lead Frame ArgumentLead Frame Argument

Tin film (in compression)

Tin film(in tension)

Lead-frame

�since whiskers grow on both sides of a bent lead-frame: stress is not a factor�These author�s disagree

82

The BentThe Bent--Lead Frame Argument Lead Frame Argument -- contd. contd.

These author�s maintain that: � The bending force is not maintained

� Stresses never exceed the yield point � Relaxation commences immediately

� R.T. is equivalent to about 0.6 Tmelting point � Relaxation is very quick and probably complete (i.e. to

zero)� Residual stress in tin films from bending-> zero

� Residual stresses due to intermetallic formation� Highly compressive on both sides of bend � A 4-zone structure is established on both sides� Both sides have essentially equivalent stress build-up

83

The CThe C--Clamp Argument Clamp Argument The c-clamp argument is the same as the bent lead-frame argument�i.e. whiskers are seen on both the tensile and compressive sides of the fixture. Therefore, whiskers have nothingto do with compressive or tensile nature of stresses.

These author�s disagree:The applied stressesare uniaxial and immediately relax after the application of the �constant extension�. Inter-metalic formation then comes the dominant stress source for both sides of the C-clamp fixture.

84

Oxide Reactions Oxide Reactions

� Oxygen penetrates tin surface � x Sn -> SnO2 + (x-1) Sn where x is an integer >1� SnO2 tends to hydrate -> SnO2 + nH20� Molar volume of SnO2 + nH2O >> Sn� Therefore, oxide reactions result in expansion

� Expansion ranges 20+% by volume depending on n

� Any oxide reaction zone (layer) will be in compression

Reference: Barsoum, et al.-2003�

85

Compressive Stresses/Thermal CyclingCompressive Stresses/Thermal Cycling

� Thermal cycling creates stress � Sn thermal expansion coefficient 21X10-6ppm� A42 thermal expansion coefficient 10X10-6ppm� Cu alloy thermal expansion coefficient 18X10-6ppm� Positive thermal ∆ (+85oC) -> compressive stress in tin� Negative thermal ∆ (-55oC) -> tensile stress in tin

� Thermal cycling over time � More stress exposure compared to isothermal aging � Diffusion related relaxation countered by �fast� cycling

86

High Impedance Stress Sources High Impedance Stress Sources -- A summaryA summary

� Intermetallic formation/oxide reactions/thermal cycling� All the above provide a constant driving force for stress

� Constant driving force counters diffusion driven relaxation� Maintains stress level despite diffusion driven relaxation � Intermetallic formation at interface creates high compression � Oxide reaction at surface creates high compression � Thermal cycling maintains constant compression thruout film

� Stress states are generally biaxial with σz = 0� Based on �free surface� assumption

� Stresses are the same in all x/y directions σx = σy� Based on fact that grain boundaries cannot support ∆σ� Confirmed by XRD measurement � ref. C. Xu

� Micro-strains are different in x/y directions.. εx ≠εy

87

Compressive Stress ScorecardCompressive Stress Scorecard

Pro S. Madra

grain-boundary/grain growth model ConK. Tsuji

Pro R. Schetty

Pro Hutchinson, et al.

Tin-indium samples Pro M. Barsoum, et al.

grain-boundary/grain growth model ConP. Bush

Pro Boettinger, et al.

Pro Osenbach, et al.

Tin-manganese samples ConK. Chen, et al.

Pro K.N. Tu, et al.

Pro W. Choi, et al.

Pro P. Oberndorff, et al.

Pro C. Xu

Pro S. Lal

Pro Galyon/Palmer

CommentConPro Author/s

88

Compressive Stress ScorecardCompressive Stress Scorecard--ContdContd..

1987 (internal stresses/not mechanical)ProB.D. Dunn

1998 Pro Lee & Lee

1984ProEndicott & Kisner

1983 Pro Kawanaka, et al.

1974, Zinc whiskers Pro U. Lindborg

1969Pro Furuta & Hamamura

1963Pro Pitt & Henning

1961Pro V.K. Glazunova, N.T. Kuryavtsev

1961Pro V.K. Glazunova

1958Pro W.C. Ellis, D. Gibbons, R.g. Treuting

1955Pro R.R. Hasiguti

1954, Tin on steel Pro Fisher,Darken,Carroll

CommentConPro Author/s

89

Compressive StressCompressive StressDriving Force for Whisker FormationDriving Force for Whisker Formation

� Internal compressive stress� Agreed to by great majority of published authors� These authors agree

� Applied (external) mechanical stress � Not frequently addressed by published authors � Established by Fisher, Pitt, Glazunova� Some negative results (Dunn, private communications) � These authors agree: mech. Stress can induce whiskers

� Role of intermetallic at substrate interface � Majority believe IMC compresses film� These authors agree

Tin Crystal Structure Tin Crystal Structure and Whisker and Whisker Formation Formation

Asa Frye-IBM (presenting) G.T. Galyon-IBM

91

Overview Overview

� EBSD VS. XRD

� EBSD will be compared to standard XRD Texture Data

� There will be no correlation XRD/EBSD

� EBSD Data will show weak�not strong textures

� EBSD will show film orientations are near random

� EBSD will show texture a function of film stress

92

AgendaAgenda--Elastic Properties & Crystalline StructureElastic Properties & Crystalline Structure

� Tin crystallography� Body centered tetragonal

� Biaxial stress state

� Stress versus strain relationships� Matrix representations of vectors and tensors� Compliance tensors

� Sij where εi = Sij σj or� Cij where σj = Cij εi

� Principal stresses σ1 and σ2

� Stiffness Definition ( σ1+σ2)/ε

93

<011> <111>

<001>

<110>C (or z)

Tin Body Centered Tetragonal Structure

Lattice direction vectors

94

Stereographic Projection Stereographic Projection

Folded Inverted Pole Figure

or FIPF

95

Body centered tetragonal (hkl) planesBody centered tetragonal (hkl) planes

(221) = (hkl) (521) = (hkl)

(111)=(hkl) (222)=(hkl)

For Body Centered Tetragonal: Plane normal vector is NOT <hkl> as in Cubic Structures

96

XRD SinXRD Sin22θθ Film TextureFilm Texture--Random TextureRandom Texture

(200

)(1

01)

(220

)(2

11)

(301

)

(112

)(4

00) (321

)

(420

)(4

11)

(312

)

(431

)

(103

)(3

32)

(440

)

x10 3̂

2.0

4.0

6.0

8.0

10.0

Inte

nsity

(Cou

nts)

(1) 00-004-0673> Tin - Sn

30 40 50 60 70 80 90Two-Theta (deg)

[P00-004-0673.dif] Tin - Sn

Simulation of RandomlyOriented Tin

97

XRD SinXRD Sin22θθ Film TextureFilm Texture--(101) Texture(101) Texture

(200

)(1

01)

(211

)

(112

)

(312

)

0

2500

5000

7500

Inte

nsity

(Cou

nts)

(1) 00-004-0673> Tin - Sn

20 30 40 50 60 70 80 90Two-Theta (deg)

[w122101.rd] A- Alloy 42/ bright

Substrate

Sample #20041505 ABright Tin on Alloy 42

98

XRDXRD SinSin22θθ Texture Assessments Texture Assessments ��contd. contd.

� Intensity of Peaks

� function of number of grains with (hkl)

� function of (hkl) reflection �strength� (structure factor)

� higher order reflections generally �weak�

� e.g. (110)>(220)>(440)

� most high order reflections not �seen� at all

� rule: if h+k+l = odd integer, Ipeak = 0

� XRD doesn�t see (111), (100), (120) peaks at all

99

Electron Back Scatter Diffraction (EBSD) Electron Back Scatter Diffraction (EBSD) Film Texture AnalysisFilm Texture Analysis

� Electron Beam used � submicron spot size

� able to interrogate individual grains

� beam stepped in 2 micron increments

� Kikuchi spectra lines define each grain orientation

� spectra dependent on surface orientation to beam

� early equipment required surface preparation techniques

� recent advances permit analysis w/o surface preparation

� EBSD Texture Determinations

� Each grain plotted on Folded Inverted Pole Figure

100

EBSD Pattern Formation MechanismEBSD Pattern Formation Mechanism

2θθθθB

~ 70°

SEM electron source

Phosphor screen/CCD

sample

� electrons in sample satisfy conditionfor Bragg diffraction: λ = 2dsinθθθθB

� for 20keV electrons, θθθθB ∼ 1°� ∴∴∴∴ diffraction cones have apex angle

∼ 180°; get a pair of parallel lines foreach diffracting crystal plane

101

EBSD Scan EBSD Scan ��Tin on Brass (Tin on Brass (CuZnCuZn))

FIPF from Hutchison,et al., 2004. MaterialsScience Forum, Vols. 467-470.

The data shows a textureAt about <223>-<334>

102

EBSD Scan EBSD Scan ��Bright Tin on CDA194Bright Tin on CDA194

<223> <320>

<103> <102>

This data shows EBSDstepping data for 4000individual grains plottedas a density map.

XRD showed this sampleto be a very strong (321) texture.

EBSD shows the same filmto be a very �weak� <223> or (552) texture with a secondary texture at <001>that XRD never saw at all.

weak≡ <30% of grains within 30o XRD Texture

103

EBSD ScanEBSD Scan--Bright Tin on Alloy 42Bright Tin on Alloy 42

<334>Or (552)

XRD Texture Point

Note: Same tin chemistryAs shown on previous slide

104

EBSD Scan EBSD Scan ��Bright Tin on CDA194Bright Tin on CDA194�Built �Built inTensileinTensile Stress�Stress�

<116>

<115>The �tensile� stresswas determined by a flexure beam usingStoney�s equation.

The texture is �weak�. About 30% of the grains would be within 30o of the texture focus between <115> and <116>

XRD Texture Point

105

EBSD Scan EBSD Scan ��Bright Tin/Nickel/CDA194Bright Tin/Nickel/CDA194�Tensile� Stress�Tensile� Stress

<001> <105> <100>

<110>

<116>

<115>

The �tensile� stress wasdetermined by XRD.

Texture is �weak� XRD Texture Point

XRD Texture Point

106

46% of the detected grainsare within 30o of <110>.

Probable overlap of <334>and <110> orientations.

EBSD Scan EBSD Scan ��Matte Tin on PhosMatte Tin on Phos--Bronze Compressive StressBronze Compressive Stress

107

5 u tin/2u Ni/Cu Tensile stress

(112)/(101) / <116>Sn/Ni/Cu

10 u tinTensile stress

TBD / <116>Bright tinon C194

Ibid NA / NA (341) / (001)(321) / (552)Bright tin on C194

Ibid(541) / <032>(341) / <342>(321) / (552)Bright tinon C1020

10 u tin compressively stressed

(321) / NA(013) / (001)(110) / (552)Bright Tin on A42

Comments TeriaryTexture

XRD/EBSD

Secondary Texture

XRD/EBSD

Primary Texture

XRD/EBSD

Sample

Table - XRD versus EBSD Texture Analysis

108

EBSD Texture Determinations EBSD Texture Determinations -- SummarySummary

� Data Set To Date is �preliminary� � sample base needs to be expanded� all conclusions considered to be preliminary

� Sn on Cu Texture� EBSD Data: No correlation with �times random� XRD data

� i.e. Published XRD Texture Data is largely �not valid� � EBSD: films show weak textures; essentially random � EBSD: compressively stressed tin (552) or <223> - <334>� EBSD: tensile stressed tin <115> to <116>� EBSD: Matte tin on phos-bronze <110>� EBSD: Sn/Cu textures to date on line connecting <001> - <110>

� Sn/Ni/Cu texture� EBSD: No correlation with “times random” XRD data � EBSD: �straddles� <001>-<110> & <001>-<100> line

� EBSD: Sn/Ni/Cu texture very near to <001> (~5-10 degrees)

109

EBSD Whisker/Whisker Grain Orientations EBSD Whisker/Whisker Grain Orientations

� Historical Data � Whisker Grains are �different� than �texture�

� These author�s agree

� EBSD Analysis Strategy � Determine whisker grain orientations

� Determine immediately adjacent grain orientations

� Map above into Folded Inverted Pole Figure (FIPF)

� Analyze results relative to crystal mech. properties � e.g. stiffness of the whisker grain plane parallel to surface � The Integrated Theory�predicts whisker grain stiffness will be

less than immediately adjacent grains

110

SchematicSchematic--Morphology of Whisker Grains Morphology of Whisker Grains

Cu6Sn5+Sn

Sn

Copper substrate

Columnar grain boundary

Whisker grain boundary

Whisker root grain boundary

Whisker

Whisker grain

111

FIBFIB--Whisker Grain Morphology Whisker Grain Morphology

Courtesy of Nick Vo - Freescale

Whisker Grain

Intermetallic Cu Substrate

Adjacent Grain

112

σz = 0

σx

σy

Stiffness= σ1+σ2/ε

ε

(001)

Biaxial Stress States (001 PLANE)Biaxial Stress States (001 PLANE)

113

ε1 = ε2 = ε

σ1= ε (s12 - s22) / (s12)2 - s11s22

σ2 = ε (s12 - s11) / (s12)2-s11s22

Stiffness ≡ (σ1+σ2) / 2ε

Note: by convention subscripts 1,2 refer to x and y directions and subscript 3 to z i.e. subscripts 1,2 refer to <100>/<010> and subscript 3 refers to <001>

Stress/Strain equations Stress/Strain equations

114

s11 s12 s13 0 0 0s12 s11 s13 0 0 0 s13 s13 s13 0 0 0

0 0 0 s44 0 00 0 0 0 s44 00 0 0 0 0 s66

Sij =

Where εi = sij σj

General ElasticityGeneral Elasticity--tetragonal symmetrytetragonal symmetry

115

Biaxial Stress States (hkl) for Biaxial Stress States (hkl) for b.c.tb.c.t. .

(001) (hkl)

σ1

σ2

σ1

σ2

σ1 = σ2 σ1≠σ2

Planar stress states for b.c.t. planes for an imposed unit strain ε in the plane of the film

116

<001>

<110>

<100><105> <104> <103> <102> <101> <201> <301><401><501>

<115>

<114> <225> <112> <223> <111> <221>

<510> <410> <310>

<210>

<320>

<430>

Stereo Projection Quadrant for Tin BCT c/a=.545

66 64 60 56 36

26

direction< >

Young's Modulus

(Gpa) <001> 67.6 <100> 23.6<110> 64.4

Figure 16- Lee and Lee, 1998 Young's Modulus (Gpa) versus uniaxial stress direction

Folded Inverted Pole FigureFolded Inverted Pole Figure--Young�s ModulusYoung�s Modulus

117

<001>

<110>

<100><105> <104> <103> <102> <101> <201> <301><401><501>

<115>

<114> <225> <112> <223> <111> <221>

<510> <410> <310>

<210>

<320>

<430>

Stereo Projection Quadrant for Tin BCT c/a=.545

0.08 0.10 0 .20 0 .28

0.08

direction < >

strain (z)

<001> 0.069<100> 0.079<110> 0.294

Figure 17- Lee and Lee, 1998 Elastic Strain in z-direction for 8MPa biaxial compression in xy plane

Folded Inverted Pole FigureFolded Inverted Pole Figure-- εεz z plotplot

118

<001>

<110>

<100><105> <104> <103> <102> <101> <201> <301><401><501> 47o

<115>

<114> 45o <225> <112> <223> <111> <221>

<510> <410> <310>

<210>

<420>

<320>

<430>

(7.8)

(8.43)

(9.64)

(11.0)

(8.32)

(8.20)(8.12)

(7.97)

(7.86)

(10.54)(12.95)

Stiffness values are “Bolded and Blue ” Direction vectors are < >

Stiffest plane

Least Stiff Plane

Folded Inverted Pole Figure �Stiffness Plot

119

Whisker Grain Orientations Whisker Grain Orientations -- Hutchinson, et al., 2004 Hutchinson, et al., 2004

EBSD data for tin on brass.

Data similar to EBSDresults from NIST (private communication) and from the author�s work.

Choi, Tu, 2003Lee & Lee, 1998

Lee & Lee, 1998

Lee & Lee, 1998

Lee & Lee, 1998

Frye & Galyon, 2005

Frye & Galyon, 2005

120

Whisker Grain Orientations Whisker Grain Orientations --SummarySummary

� Whisker Grain Orientations � Possible exclusion at or near <001> ??

� <001> or (001) plane = the stiffest plane� Reference: Madra, 2004.

� Not exclusively low stiffness planes � i.e., not necessarily near to (110)

� Evenly distributed over �most� of FIPF space

� Data sets to date: � Hutchinson, et al. � Frye & Galyon� Kil-Won Moon (NIST-private communication)

121

Whisker Grain and Adjacent GrainsWhisker Grain and Adjacent GrainsRelative OrientationsRelative Orientations

� EBSD Analysis Strategy� Probe whisker orientation � Probe immediately adjacent grains � Plot results on FIPF

� FIPF= (Folded Inverted Pole Fig.)

� Questions � Evaluate Grain boundary angles (low or high) � Evaluate Stiffness relationships

122

15.0µm

from alloy C1020

� Whisker growing from nodule� Four of five �adjacent grains at (001) orientation� One adjacent grain (250)� Whisker grain (circled) near (450) orientation� All adjacent grains are �stiffer / especially the (001) grains

~ (450)

~ (250)1

2

3

4

5

6

FIPF for bright tin on C1020 substrate-JEITA Sample

123

15µm

� Whisker orientation is circled in FIPF; orientation ofthree neighboring grains also shown

�Adjacent grains are �less stiff� /adjacent grain boundary angles are �intermediate (10-30 degrees)

from alloy C1020

FIPF for bright tin on C1020 substrate-JEITA Sample

124

� Whisker grown on 194 alloy (Stoney�s stress=tensile)� Whisker orientation is circled in FIPF�Adjacent grains are stiffer /gr. Boundary angles are �large� (>30 degrees)

1.0µm

from alloy E194

FIPF for bright tin on C194 substrate-Built in Tensile Stress

125

1.0µm

from alloy E194

� Whisker grown on 20040958 E194 alloy (Stoney�s stress=tensile)� Whisker orientation is circled in FIPF�Adjacent grains are much stiffer/gr. Boundry angles are �large�

FIPF for bright tin on C194 substrate-Built in Tensile Stress

126

1.0µm

� Whisker grown on 20040958 E194 alloy (Stoney�s stress=tensile)� Whisker orientation is red in FIPF�Adjacent grains are stiffer / gr. Boundary angles are �small�

from alloy E194

FIPF for bright tin on C194 substrate-Built in Tensile Stress

127

� Filamentary whisker � FIPF shows four grains at whisker base� Orientation of whisker (circled)�Adjacent grains slightly less stiff �Gr. Bdry. Angles are �small�

FIPF for matte tin on phosphor-bronze substrate -

128

� Whisker grown on matte tin/phos-bronze (stress prob. Compressive) � Whisker orientation is red in FIPF�Adjacent grains are stiffer / gr. Bdry. Angles are �small�

10.0µm

FIPF for matte tin on phosphor-bronze substrate -

129

10.0µm

� Whisker grown on matte tin/phos. Bronze (stress prob. compressive) � Whisker orientation is red in FIPF�Adjacent grains are �less stiff� / gr. Bdry. Angles are �small�

FIPF for matte tin on phosphor-bronze substrate -

130

� Filamentary whisker (red)� FIPF shows seven grains at whisker base�Adjacent grains are a mix of stiffer and less stiff -Gr. Bdry. Angles are �small�

FIPF for matte tin on phosphor-bronze substrate -

131

Adjacent Grain Analysis Adjacent Grain Analysis -- SummarySummary

� Grain Boundary Angles � range from 10-80 degrees

� Stiffness Relationship � None; some stiffer, some not � Large gr. bdry. Angles -> less stiff whisker grains � Lower gr. bdry. Angles -> no stiffness relationship

� Orientation of adjacent grains � Similar orientations (i.e. clustered) � not reflective of randomness

New Data Set

New Data Set

New Data Set

Wrap UpWrap Up

G. Galyon-IBM (presenting)Chen Xu-Cookson

S. Lal-FCIB. Notohardjono-IBM

133

Wrap Up Wrap Up ��Compressive StressesCompressive Stresses� Compressive Stress:

� A Necessary Factor for Whisker Formation � Arguments to Contrary are Flawed

� See Zonal Structure / FEA / XRD / Flexure Beam Sections � Stoney�s equation generally invalid for analysis of tin films

� Consensus essentially established

� Compressive Stress Sources � IMC Formation / Kirkendahl Effect: established in presentation � Oxide Reactions� Thermal cycling� Built in Film Stresses, e.g. Cu alloying (not discussed)

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Wrap Up Wrap Up �� Whisker Crystallography Whisker Crystallography � Tin Film and Whisker Crystallography

� Tin Films essentially random with weak textures � EBSD Data not correlatable to XRD � EBSD enhances ability to look at individual grain orientations

� Whisker grains randomly oriented with exclusion zone ~ <001>

� Adjacent Grain Analysis: � Gr. Bdr. Angles range from small to large (10o to 80+o)� Adjacent grains: stiffer for large angle gr. Boundaries � Adjacent grains: similar stiffness for small angle boundaries.

� Whisker Grain Morphology� See FIB x-sections / VEE (oblique) grain boundaries

� Established on over 90% of all known FIB X-sections � A critical element in �end-game� models

135

Wrap UpWrap Up-- Recrystallization Recrystallization

� For Very Bright Tin� Recrystallization is a necessary event � Whisker formation has been observed in-situ � Whiskers form from nodules � Nodules are surface growths / i.e., a recrystallization event

� For Matte Tin � Recrystallization possible but not established�� Morphology (vee boundaries) not similar to plated grains� Recrystallization should imply whisker grains less stiff � EBSD Data shows relative relative stiffness is random

� Some correlation to grain boundary angles

� Investigation ongoing

136

Wrap UpWrap Up--FEA and New Insights FEA and New Insights

� Zonal FEA Analysis � Sn/Ni/Cu � Good Match to Sn/Ni/Cu XRD Stress Data � FEA Scenario consistent with Kirkendahl Effect

� i.e., Vacancy Rich region in 1st 2 microns of tin film � i.e., tensile stress in vacancy rich region � i.e., conversion to compressive stress for films > 2 microns

� Zonal FEA Analysis � Sn/Cu � Some stress data consistent with FEA scenario

� i.e., time zero tensile stress in tin film � i.e., conversion to compressive stresses over time

� Work is ongoing � Built in film stresses fluctuate from tensile to compressive� XRD versus flexure beam comparison data base: U/I U/I ≡ Under Investigation

137

Wrap UpWrap Up--Conclusion Conclusion

� Zonal Structure Analyses-Appropriate model

� Stress Measurement inTin Films

� Requires combination of XRD/Flexure/Microstructure

� Requires a corresponding FEA zonal structure scenario

� Recrystallization and Whisker Grains

� Established to a degree-needs future work on matte tins

� Oxide Reactions-To be addressed / future work

� Recognized as a long-term source of compression

� Thermal Cycling-To be addressed / future work

� Recognized as a long-term source of compression

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Sermon Chart Sermon Chart

� Zonal Structure together with FEA Analysis is a new approach � Resolves some apparent anomalies

� E.g., Sn/Ni/Cu XRD stress data (tensile to compressive as thks. increases) � E.g., Sn/Cu XRD stress data (tensile to compressive over time)� And there are other anomalies not discussed in this presentation

� Consistent with basic physical laws (e.g. Kirkendahl effect)

� XRD + Flexure Beam + Microstructural Analysis +FEA � All 4 are synergistic and provide truer picture of stress state � Future work should follow similar analysis strategy

� EBSD is a new tool which gives true analysis of film texture � Data shows new insight into physical metallurgy of tin films � Researchers are encouraged to incorporate and reproduce data

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The Challenge The Challenge

� Go and do Likewise: � Check out the Integrated Theory Hypotheses � Check out the reported data

� E.g. Sn/Ni/Cu Stress States � Tin Films with concave up flexure beams / i.e. �tensile�

� Synchronize Flexure Beam / XRD / Microstructure � Check intermetallic formation as a function of �stress�

� Let�s share your inputs on a real time basis � Open invitation from the iNEMI modeling group � Chairman: G. T. Galyon (galyon@us.ibm.com)