Post on 10-Jul-2020
BOEING is a trademark of Boeing Management Company.Copyright © 2006 Boeing. All rights reserved.
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)
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
3 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+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
Sample 90, 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
66 Days after Plating
Sn120
Sn118
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+Cs
3 Days after Plating
SIMS Depth Profile of a Second Bright Sn Double Layer187 Days after Plating
Sample 89, 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
Sn120
Sn118
SIMS Depth Profile of a Matte Sn Double
Layer
Sample 52, 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
Sample 52, 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
117 Days after Plating
10 Days after Plating
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
Distance Between Analysis Points (microns)
%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 17B
Sample 52 Whisker 18
Sample 52 Whisker 19A
Sample 52 Whisker 19B
Matte Sn, Sample 52 – 117 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 20 40 60 80 100
Distance Between Analysis Points (microns)
%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
Sample 89 Whisker 21
Sample 89 Whisker 22
Bright Sn, Sample 89
Whisker Base
Whisker Tip
Isotopic Composition is Constant from Base to Tip of Whisker
(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