tin whisker mitigation: research into mechanisms and strategies. … · 2019. 9. 16. ·...
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Tin whisker mitigation: research into mechanisms and strategies. Part 2,Tin whisker mitigation: research into mechanisms and strategies. Part 2,Post-plating mitigation methodsPost-plating mitigation methods
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Ashworth, Mark A., G.D. Wilcox, Helen E. Pearson, and D. Haspel. 2017. “Tin Whisker Mitigation: Researchinto Mechanisms and Strategies. Part 2, Post-plating Mitigation Methods”. figshare.https://hdl.handle.net/2134/23687.
Tin Whisker Mitigation – Research into Mechanisms & Strategies
M.A. Ashworth, G.D. Wilcox, H. Pearson and D. Haspel
Tin Whisker Mitigation Methodologies Loughborough University
24th November 2016
PART 1: EFFECT OF PLATING METHODOLOGIES PART 2: POST-PLATING MITIGATION METHODS
Tin Whisker Mitigation - Research into Mechanisms & Strategies
Part 2: Outline of presentation
• Introduction • Novel conformal coatings • Electrochemical oxidation • Atomic layer deposition • Summary
Introduction • Although whisker growth may be reduced by choice of
electroplating bath composition and optimisation of deposition parameters, such methods cannot be relied upon to fully mitigate whisker growth
• Given the unpredictable nature of whisker growth, even the ‘best’ electroplated tin coatings should be considered ‘whisker-resistant’ rather than ‘whisker-proof’
• Additional control measures, in the form of surface coatings or treatments, are required to further suppress the growth of whiskers
1. NOVEL CONFORMAL COATINGS
Post-plating mitigation methods
Tin whiskers and conformal coatings • Conformal coatings are applied to protect
printed circuit boards and associated electronic components
• Currently, tin whisker mitigation is attempted with conformal coatings that have not been designed to prevent whisker growth
• WHISKERMIT 2 research programme aimed to develop novel conformal coatings specifically designed to mitigate whisker growth
Research Aims and Objectives
• Engender polymers with physical barriers to whisker growth through the inclusion of nano-fillers in the conformal coating polymer formulation.
• Apply concept to commercial conformal coatings that are currently used for environmental protection in electronic components
• Evaluate the microstructure, mechanical properties and whisker resistance of the modified coating formulations
Evaluation of whisker mitigation
Only view/count whiskers that are growing out of the coating
• Whisker growth has been investigated using brass coupons electroplated with 2 µm of bright tin at 10 mA cm-2
• Modified coatings based on a HumiSeal acrylic formulation
• All conformal coatings applied by spraying • Three sets of trials currently under evaluation
(batches 1, 2 and 3) • Samples stored in an environmental chamber at
55°C/85% humidity • Whisker growth evaluated at periodic intervals
using a stereo microscope
Evaluation of whisker growth: Batch 1
N.b. 418 days analysis conducted using x64 magnification; all others used 50x
3% nanofiller modified acrylic coatings vs. unmodified 2x coating (29-7-15), Samples stored throughout at 55°C/85% humidity
0
10
20
30
40
50
60
0 50 100 150 200 250 300 350 400 450 500
Whi
sker
den
sity
(cm
-2)
Days after coating
Whisker penetration reduced by ~ 2/3 for
3% nanofiller modified samples
Unmodified
100µm
3% nanofiller 100µm
Evaluation of whisker growth: Batch 2
0
20
40
60
80
100
120
140
0 50 100 150 200 250 300 350 400
Pene
trat
ing
whi
sker
den
sity
(cm
-2)
Days after coating
3% nanofiller modified acrylic coatings vs. unmodified 2x coating (9-10-15), Samples stored at 55°C/85% humidity and RT
Whisker penetration
reduced by ~ 2/3 for 3% nanofiller modified samples
Unmodified
3% nanofiller
0
50
100
150
200
250
300
350
400
Unmodified 3% 5% 7%
Whi
sker
den
sity
(cm
-2)
Evaluation of whisker growth: Batch 3
0
100
200
300
400
#27 #45 #63 #43 #59 #75 #21 #37 #69 #31 #39 #47
Whi
sker
den
sity
(cm
-2)
Sample #
0
10
20
30
40
50
60
70
80
#1 #2 #72 #5 #6 #35 #9 #53 #61 #13 #23 #71
Aver
age
coat
ing
thic
knes
s (µm
)
Sample #
top of sample middle of sample base of sample
Unmodified
3%
5%
7%
5% modified samples are typically
thicker than unmodified 7% modified
samples are comparable or thinner than
the unmodified
Modified acrylic coatings vs. unmodified 2x coating (11-5-16), Samples stored at 55°C/85% humidity
Unmodified
3% nanofiller
5% nanofiller 7% nanofiller
Very significant reduction in whisker
penetration
3% modified samples are thinner than unmodified
Batch 2: mechanical property data
Yield stress (MPa) % elongation at failure Failure stress (MPa) Young’s modulus (MPa)
Samples tested approximately 1 month after fabrication
Modulus and yield strength are significantly improved for the modified formulations whilst elongation to failure is only slightly reduced
2
3
4
5
100
120
140
160
180
200
220
240
300
320
340
360
380
400
420
440
460
480
500
2
3
4
5
6
2. ELECTROCHEMICAL OXIDATION
Post-plating mitigation methods
The role of the surface oxide in whisker growth In 1994, Tu proposed his “cracked oxide theory” 1
whisker growth occurs at certain weak spots on the surface where the oxide has been broken
In the absence of an oxide no whisker growth would occur
Later adding 2 …
“if the surface oxide is very thick, it will physically block the growth of any hillocks and whiskers”
1 Physical Review B 49, 2030, 1994
2 Proceedings of the IEEE Electronic Components and Technology Conference, 2002 p1194–1200
Can we mitigate whisker growth by increasing the thickness of the surface oxide? Electrochemical oxidation in borate buffer and potassium
carbonate/bicarbonate solutions used to develop thicker oxides
Although the oxide film thickness is increased, other characteristics of the Sn coating (e.g. grain size, grain orientation and residual plating stresses) should be unaffected
XPS analysis to evaluate the thickness and composition of the oxide layers formed
Whisker growth evaluated using SEM and optical microscopy
Electrochemical oxidation • Electrochemical oxidation can be used to develop oxide
films that are much greater in thickness than a native Sn oxide
XPS sputter depth profiles comparing the thickness of the oxide films 1 day after Sn deposition and electrochemical oxidation (at 1.2 V vs. SCE)
Comparable thicknesses of oxide are
developed for borate buffer and
potassium bicarbonate-carbonate
baths Increased oxide
thickness
0
10
20
30
40
50
60
70
80
0 100 200 300 400 500 600
Oxy
gen
conc
entr
atio
n (a
t%)
Sputter time (s)
Native
Borate buffer (40 mC cm-2)
Potassium bicarb-carb. (40 mC cm-2)
Effect of electrochemical oxide on whisker growth
2 µm SnCu deposits on Cu electroplated at 10 mAcm-2
Whisker growth is considerably reduced for samples given a post electroplating oxidation treatment at 1.2 V vs SCE
Comparable reduction in whisker growth achieved for borate buffer and potassium bicarbonate-carbonate solutions
Still effective at mitigating whisker growth after ~ 3 years
0
500
1000
1500
2000
0 200 400 600 800 1000 1200
Whi
sker
den
sity
(cm
-2)
Time (days)
Native air formed oxide
Borate buffer solution
Potassium bicarbonate-carbonate solution
Whisker growth evaluated over ~ 3 years
Electrochemical oxidation at 1.2 V vs SCE
Order of magnitude
reduction in whisker growth
Whisker development with time
• Study underway to compare the growth of whiskers, and the development of the oxide film, on native and electrochemically oxidised deposits as a function of time
• 2 µm SnCu deposits on Cu
• Electrochemical oxidation at 2 V vs. SCE in potassium bicarbonate/carbonate solution
Native oxide
Electrochemical oxide
0
500
1000
1500
2000
2500
0 10 20 30 40 50
Whi
sker
Den
sity
(cm
-2)
Days after electroplating
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
Native 1.2V 0.12C 1.6V 0.12C 2V 0.12C 1.2V 0.24C 1.6V 0.24C 2V 0.24C
Whi
sker
Den
sity
(cm
-2)
0.12 C charge passed 0.24 C charge passed
Whisker growth evaluated after ~ 9 months
Electrochemical oxidation at higher potentials
05
101520253035404550
0 100 200 300 400 500 600
Sn (o
xide
) Con
cent
ratio
n (a
t%)
Sputter time (s)
1.2V
1.6V 2V
XPS depth profiles for Sn(oxide)
Electrochemical oxidation of SnCu deposits on Cu in potassium bicarbonate-carbonate electrolyte
Oxide film thickness is increased at higher potentials
However, there is no clear improvement in whisker mitigation at higher oxidation potentials compared with 1.2 V
Whisker mitigation: Sn deposits on brass • Electrochemical oxidation
of 2 µm pure Sn deposits on brass in borate buffer solution
• Whisker growth is greatly reduced for electrochemically oxidised Sn deposits on brass
• Whisker mitigation is more effective at the higher oxidation potential
• Still an effective barrier to whisker growth after almost 4 years 5915 83 61 39
0
50
100
150
200
250
300
350
400
450
500
Native 1.2V 1.6V 2.0V
Whi
sker
den
sity
(cm
-2)
Whisker growth evaluated after ~ 44 months
Whisker mitigation mechanisms For Sn deposits on Cu, IMC formation appears unaffected by electrochemical oxidation
Whisker mitigation must simply be derived from the increased thickness of the oxide layer
No reduction in the driving force for tin whisker growth (intermetallic formation)
Cu6Sn5 intermetallic
2 µm
Cu6Sn5 intermetallic
2 µm
Electrochemical oxidation at 1.2V borate buffer
2 µm
No ZnO formation
Reduced intermetallic
formation 2 µm
Pronounced ZnO
formation
Large intermetallics
For Sn deposits on brass, electrochemical oxidation reduces whisker growth by preventing the formation of Zn oxide at the deposit surface
Reduced IMC formation may also influence whisker formation
Reduced driving force for whisker growth
Native air formed oxide
SnCu
dep
osits
on
Cu
Sn d
epos
its o
n br
ass
Electrochemical oxidation at 1.2V borate buffer Native air formed oxide
3. ATOMIC LAYER DEPOSITION Post-plating mitigation methods
ALD as a whisker mitigation strategy • Atomic layer deposition (ALD) has been
investigated as a potential whisker mitigation strategy
• ESA funded collaborative research programme between Loughborough University, Picosun and Oy Poltronic Ab
• Initial studies to investigate the effect of coating process variables on whisker mitigation
Experimental approach
• Copper coupons electroplated with 2 µm Sn-Cu to promote whisker growth
• Electroplated coupons shipped to Picosun for ALD coating
• A range of pre-treatments and process conditions investigated
• Uncoated control samples included in each batch of ALD processed samples
• Whisker growth evaluated over ~12 month period using optical and scanning electron microscopy
ALD Coating Trials Batch Pre-treatment Coating
Material Coating
Thickness
1 Pre-treatment
trials ‘ALD 1’ ‘Standard’ 2
3
4 - 5x ‘standard’
5 - ‘ALD 2’
‘Standard’
6 - 5x ‘standard’
7 - ‘ALD 3’
‘Standard’
8 - 5x ‘standard’
Batch 5
Batch 6
Batch 8
Whisker growth test coupons
Analysis of whisker growth
For the majority of the process batches
investigated, Filament whisker growth is
considerably reduced on the ALD coated samples
compared with control samples
Control sample
Batch 6 (‘ALD 2’, 5x standard thickness)
10µm 10µm
10µm 10µm
ALD coated
Batch 4 (‘ALD 1’, 5x standard thickness)
Control sample ALD coated
18 46 0 353 1 357 2 345 1 370 103 346 329 336 0
50
100
150
200
250
300
350
400
450
B1 B2 B3 B3-C B4 B4-C B5 B5-C B6 B6-C B7 B7-C B8 B8-C
Aver
age
num
ber o
f whi
sker
s cou
nted
(20
fram
es p
er sa
mpl
e)
Batch No.
Effect of ALD coating on filament whisker growth
Batch 3
Batch 4
Batch 5
Batch 6
Batch 7 Batch 8
Batch 2
Batch 1
Suffix ‘C’ refers to control samples
For batches 3, 4, 5 and 6 the growth of filament whiskers is almost fully
prevented by ALD coating (‘ALD 1 and 2’)
Batches 1 and 2 showed increased whisker growth but had a longer delay between electroplating and ALD coating
Reduced whisker mitigation for batches 7 and 8 (‘ALD 3’)
Whisker growth prior to coating? • SEM analysis indicates that the whiskers on the ALD
processed samples are typically encased within a
shell of oxide, i.e. they were already present at the
time of coating
• No examples where whisker growth has definitely initiated post ALD coating
1 µm
1 µm
Batch 4 Example of whisker growth on
Batch 2 (‘ALD 1’)
1 µm
Examples of whisker growth on Batch 4 (‘ALD 1’, 5x thickness)
Batch 2
Exposed Sn
whisker
Whiskers are encased in the
ALD coating
1 µm
Exposed tin whisker
Cracked ALD coating
Example of whisker growth on Batch 7 (‘ALD 3’)
Batch 7
SEM analysis of batch 8 (‘ALD 3’) Control sample ALD coated
5 µm 10 µm
10 µm 10 µm
ALD coated
Coating failure may be attributed to the combined effect of coating material, lack of pre-treatment and the application of too thick a layer
2 µm
1 µm
However, SEM analysis suggests whiskers were present prior to deposition of the ALD coating
Exposed tin whisker
Oxide shell
Filament whisker density for batch 8 was comparable to
that of the uncoated samples
Summary • Conformal coating studies
Novel conformal coatings are under development. Whisker growth studies show a significant improvement in whisker mitigation compared with unmodified coatings
• Electrochemical oxidation treatment
Electrochemical oxidation has been shown to provide long term whisker mitigation for both SnCu deposits on Cu and Sn deposits on brass
• Atomic layer deposition
Significant reductions in whisker growth have been achieved as a result of ALD processing
Any questions or comments?