Tracer Diffusion in Whisker- Prone Tin Platings · 2006-10-10 · % Sn 118 on Whiskers (Relative to...

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Tracer Diffusion in Whisker-Prone Tin Platings

Tom Woodrow, Ph.D.Boeing Phantom Works

Seattle, WA

SMTAISeptember 27, 2006

• After 60 years of tin whisker research, little is known about diffusion in whisker-prone tin platings.

• Where does the tin in a whisker come from?• Does the tin reach the whisker through the

lattice, through the grain boundaries, or across the surface?

• Does a surface oxide layer inhibit diffusion?

• Non-radioactive isotopes can be used to track the diffusion of tin in a tin plating.

Test Coupon

• Electroplated Sn isotopes onto brass substrates• Bright and matte Sn platings

Sn120 Layer Sn118 Layer

Brass Substrate

Single Layer/Double

Layer Interface

Test Coupons (Matte Sn)

Sn120

Single Layer

Sn120/Sn118

Double Layer

Isotopic Purity of Anodes

Isotope Sn120 Anode Sn118 Anode

Sn124 0.02 0.1

Sn122 0.06 0.1

Sn120 99.6 0.5

Sn119 0.13 0.3

Sn118 0.13 97

Sn117 0.02 1.8

Sn116 0.04 0.2

Sn115 0.00 0.0

Sn114 0.00 0.0

Sn112 0.00 0.0

Relative Percentagesof Tin Isotopes

Sn Layer

2

Brass Intermetallic

Sn Layer

1

FIB Microsection (Bright Sn Plating)109 Days after Plating

Auger Analysis of FIB Microsection(Bright Sn Plating)

67 Days after Plating

Sn Plating

Sn Plating

Zn on PlatingSurface

Intermetallic Layer Brass

Nodules

Nodules Forming on FIB Microsection

(Bright Sn Plating)

18 Hours after FIB Cut Was Made

30 Minutes after FIB Cut Was Made

FIB Microsection of Nodule on Bright Sn Plating

20.0 keV

Φ 103 Sample 52 02Jun05 6/2/05

12

FOV: 20.0 µm 5.0 µm

Matte Sn Plating Surface10 Days after Plating

Whisker on Matte SnPlating Surface (No Nodule Formation)26 Days after Plating

Base of Whisker

Brass Intermetallic

Sn Layers

FIB Microsection of Matte Sn Plating158 Days after Plating

Auger Analysis of FIB Microsection(Matte Sn Plating)116 Days after Plating

Sn Plating Zn

Intermetallic Layer Brass

Whiskers Appear to Grow from Plating Surface (Matte Sn)

Ga+ Analysis Beam

(35 Degrees Off Normal)

Cs+ Sputtering Beam

(42 Degrees Off Normal)

Detector

Sample

M+, M-, MCs+ Ions

TOF-SIMS Spectrometer

Typical Depth Profile Crater

SIMS Depth Profile of a Bright Sn Double Layer3 Days after Plating

Sample 90, Double Layer

0

10

20

30

40

50

60

70

80

90

100

0 2000 4000 6000 8000 10000

Time (sec)

% o

f Com

bine

d Io

n C

ount

s

Sn118+CsSn120+CsCu63+CsZn64+CsO16+Cs

Sn120 Sn118

Sn120 Sn118

If No Diffusion Occurs

Sn118 Diffuses Up Grain Boundaries and onto the SurfaceSn118 then Diffuses from Surface Down into the Lattice

Plating Surface

Brass Substrate

Sn120

GrainSn118

Grain

Sn118 Moving Up Grain Boundaries

Sn120 Moving Down Grain Boundaries

SIMS Depth Profile of a Bright Sn Double Layer Showing Sn Isotopes Only



0

10

20

30

40

50

60

70

80

90

100

0 2000 4000 6000 8000 10000

Time (sec)

% o

f Com

bine

d Io

n C

ount

s

Sn118+CsSn120+CsA

B C

D E

Sn120 Sn118

⎟⎟⎠

⎞⎜⎜⎝

⎛−=

−−

2/1)(21

tDxerf

cccc

tos

o

Calculated a room temperature lattice diffusion coefficient of 10-15 cm2/sec which is two to three orders of magnitude larger than the self-diffusion coefficient of tin (10-17 to 10-18 cm2/sec at 25°C) as reported in the literature.

Assuming that the near surface concentration of Sn118 is relatively constant, Curve AB has the form of

SIMS Depth Profile of a Bright Sn Double

Layer


0

10

20

30

40

50

60

70

80

90

100

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

Time (sec)

% o

f Com

bine

d Io

n C

ount

s

Sn118+CsSn120+Cs


Sn120

Sn118


0

10

20

30

40

50

60

70

80

90

100

0 2000 4000 6000 8000 10000

Time (sec)

% o

f Com

bine

d Io

n C

ount

s

Sn118+CsSn120+Cs


SIMS Depth Profile of a Second Bright Sn Double Layer187 Days after Plating


0

10

20

30

40

50

60

70

80

90

100

0 2000 4000 6000 8000 10000

Time (sec)

% o

f Com

bine

d Io

n C

ount

s

Sn118+CsSn120+Cs

Sn120

Sn118

SIMS Depth Profile of a Matte Sn Double

Layer


0

10

20

30

40

50

60

70

80

90

100

0 2000 4000 6000 8000 10000

Time (sec)

% o

f Com

bine

d Io

n C

ount

s

Sn118+CsSn120+Cs


0

10

20

30

40

50

60

70

80

90

100

0 2000 4000 6000 8000 10000

Time (sec)

% o

f Com

bine

d Io

n C

ount

s

Sn118+CsSn120+Cs



Sn120

Sn118

8330 microns

0.7%

0.7%

0.8%

0.8%

10.0%

10.4%

13.0%

15.2%

13.5%

14.6%

Sn120 Single

Layer Sn120/Sn118

Double Layer

5060 microns

Sn120 Single

Layer Sn120/Sn118

Double Layer

0.6% 6.1% 30.8% 47.4%

1 2 3 4

% Sn118 after 270 Seconds of Sputtering

• Isotope composition “frozen”on surface (by an oxide layer?)

% Sn118 on Surface of Coupon254 Days after Plating

Spot Analyses on Bright Sn (Sample 91)

Spot Analyses on Bright Sn (Sample 91)(% Sn118 vs. Sputter Time)

0.00

10.00

20.00

30.00

40.00

50.00

60.00

0 100 200 300 400 500 600

Time (sec)

%S

n118 (R

elat

ive

to T

otal

Sn)

Sample 91 Spot 1Sample 91 Spot 2Sample 91 Spot 3Sample 91 Spot 4

12

3

4

5840 microns

51.9% 55.6%

1.6% 1.9%

Sn120 Single

Layer Sn120/Sn118

Double Layer

1 2

3 4

% Sn118 on Surface of Coupon299 Days after Plating

% Sn118 after 270 Seconds of Sputtering

Spot Analyses on Matte Sn (Sample 63)

•Long range diffusion of Sn118

observed parallel to substrate•Larger quantities of Sn118 on surface vs. in lattice suggests diffusion occurred through grain boundaries

• Isotope composition “frozen”on surface (by an oxide layer?)

2.2% 3.1% 16.2% 17.3%

8730 microns

Sn120/Sn118 Double Layer

2.6% 4.4% 19.2% 19.6% 20.2%

16.2%

Sn120 Single

Layer 3.4%

Spot Analyses on Matte Sn (Sample 63)(% Sn118 vs. Sputter Time)

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

0 100 200 300 400 500 600

Time (sec)

%Sn

118 (R

elat

ive

to T

otal

Sn)

Sample 63 Spot 1Sample 63 Spot 2Sample 63 Spot 3Sample 63 Spot 4

12

3

4

Sn120 Sn120/Sn118

3A

3G

3B

3C 3F

3H 3I

3D

3E

Whiskers on Both Sides of a Matte Sn Single Layer/Double Layer InterfaceGreen Arrow Marks Whisker Base

Sn120 SIMS Ion Image

Sn120

Sn118

Whisker 3C

Whisker 3D

Ion

Cou

nt

Line Scan Data for Both Isotopes

Sn118 SIMS Ion Image

100 microns

Sn120 Sn120/Sn118

3F

3C

3G

3H3I

3D

100 microns

Sn120 Sn120/Sn118

3A

3B

3D

3E

Isotopic Composition of Whiskers on Both Sides of a Matte Sn Single Layer/Double Layer Interface

(Before Sputtering and After Sputtering)

0

5

10

15

20

25

30

35

40

45

50

-250 -200 -150 -100 -50 0 50 100

Distance of Whisker Base from Interface (microns)

% S

n118 on

Whi

sker

s (R

elat

ive

to T

otal

Sn)

Pre-Sputter (Whisker Base)

Post-Sputter (Whisker Base)

3I

3H

3G

3B

3E

3D3B

3E3D

Sn120 Single Layer

Sn120/Sn118

Double Layer

3A

3F

3C

3F

Larger quantities of Sn118

in whisker vs. in lattice suggests diffusion occurred through the grain boundaries

Isotopic Composition of a Whisker Centered on a Matte Sn Double Layer

Line Scan 1

Line Scan 2

100 microns

Sn120

Sn118

Ion

Cou

nt

Whisker 1 Tip

Sn120

Sn118

Ion

Cou

nt

Whisker 1 Base Line Scan 2

Line Scan 1

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100

Distance Between Analysis Points (microns)

%Sn

118

on W

hisk

ers

(Rel

ativ

e to

Tot

al S

n)

Sample 62 (Compositionwhen Double Layer isCompletely Mixed)Sample 62 Whisker 1

Sample 62 Whisker 5A

Sample 62 Whisker 5B

Sample 62 Whisker 5C

Matte Sn, Sample 62 – 44 Days after

Plating

Whisker Base

Whisker Tip

Isotopic Composition is Constant from Base to Tip of Whisker

(Matte Sn Double Layer; Double Layer is not Mixed Yet)

0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400


%Sn

118 o

n W

hisk

ers

(Rel

ativ

e to

Tot

al S

n)

Sample 52 (Compositionwhen Double Layer isCompletely Mixed)Sample 52 Whisker 17A


Sample 52 Whisker 18

Sample 52 Whisker 19A


Matte Sn, Sample 52 – 117 Days after Plating

Whisker Base

Whisker Tip


(Matte Sn Double Layer; Double Layer is not Mixed Yet)

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100


%Sn

118 o

n W

hisk

ers

(Rel

ativ

e to

Tot

al S

n)

Sample 89 (Compositionwhen Double Layer isCompletely Mixed)Sample 89 Whisker 7



Bright Sn, Sample 89

Whisker Base

Whisker Tip


(Bright Sn Double Layer)

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250 300Time after Plating (days)

% S

n118 o

n W

hisk

ers

(Rel

ativ

e to

Tot

al S

n)

Whisker 1 BaseWhisker 1 TipWhisker 5B BaseWhisker 5B TipWhisker 5C BaseWhisker 5C TipWhisker 3G BaseWhisker 3G TipWhisker 3H BaseWhisker 3H TipWhisker 3I BaseWhisker 3F BaseWhisker 3F Tip

Pre-Sputter

Post-Sputter

Whiskers on Sn120 Single

Layer

Whiskers on Sn120/Sn118

Double Layer

Isotopic Composition of Whiskers on Matte Snbefore and after Sputtering (Sample 62)

• The isotopic composition of the interior of a whisker on matte Sn can be different from the exterior composition

Step 1. Sn in grain boundaries feeding whisker has not reached an equilibrium isotopic composition when whisker forms (i.e., it is Sn120 rich)

Step 2. Sn on surface of whisker is in constant flux and eventually reaches an equilibrium composition

Whisker

Oxide Layer

Sn Plating

• Whiskers grew from grains on the surface of the matte Sn and from recrystallized nodules on the bright Sn

• Interdiffusion of the isotope layers perpendicular to the substrate was fast (grain size was unchanged during mixing)

• Long range diffusion of the isotopes parallel to the substrate did occur

• Diffusion was through the grain boundaries• Isotopic compositions were frozen on the

surfaces of the platings (by an oxide layer?)• The isotopic composition of the interior of a

whisker on matte Sn can be different from the exterior composition

Conclusions

Tracer Diffusion in Whisker- Prone Tin Platings · 2006-10-10 · % Sn 118 on Whiskers (Relative to...

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