Evaluation of Conformal Coating for Mitigation of Tin Whisker Growth
Toshiyuki Yamada (Avionics Fukushima Co., Ltd.) Tsuyoshi Nakagawa (Nippon Avionics Co., Ltd.) Norio Nemoto (Japan Aerospace Exploration Agency) Katsuaki Suganuma (Osaka University)
October 13th, 2011
24th Microelectronics Workshop
1
Outline
1. Background 2. Purpose of Evaluation 3. Test method 4. Evaluation Results 4.1 Mitigation of Whisker growth 4.2 Shape dependence of Conformal Coating 5. Discussion 6. Summary
2
1. Background
Whiskers
Whiskers
Actual environment for space application ・Vacuum, Thermal cycle, etc. ・Prolonged storage
Whiskers growth evaluation Based on system requirements
The best acceleration examination condition of whiskers
Whiskers growth acceleration method
Whiskers
・ Conformal Coating ・ Hot solder dipping (HSD) ・ Over Plating
Whisker mitigation method
NASA
NASA
JAXA Community to Study RoHS Issues Investigation and examination for lead-free technical issues.
Establish lead-free parts control guideline.
3
2. Purpose of Evaluation 1/2
Vacuum Air
Conformal coating mitigated whisker growth in air condition.
Thin and long whisker was observed in vacuum condition.
Vacuum
Air
Thin and long whisker was observed in vacuum condition.
0
5
10
15
20
25
0 500 1000 2000
Cycle
Whis
ker
lengt
h[μ
m]
Non-coating Type A (Urethane)
Type B (Urethane) Type C (Silicone)
Whisker growth mitigation effect of Conformal coating (in Air condition)
Mitigation of Whisker growth(Vacuum)
Whisker growth mitigation effect was evaluated in vacuum condition.
2. Purpose of Evaluation 2/2
(Uncoated area) (Coated area)
Tin plated test coupon
Shape dependence of Conformal Coating
Tin plated
Coating
Coating
Chip capacitor
viewpoint Coating viewpoint
Tin Plated test coupon : Uniform thickness of conformal coating Actual device : Different thickness ( corner, steep angle, edge, etc)
Whisker growth mitigation effect of conformal coating was evaluated using ceramic chip capacitor. 4
5
Testing environment
Installation in thermal shock chamber
Sample condition (separated or board-mounted)
3. Test Method 1/2
Temperature controlled profile of test equipment Atmosphere: Air, Vacuum(10-4Pa)
0℃
-40℃
+125℃
60min 60min
20min 20min
6
Sample allocation
50 100 250 500 1000
Type A ○ ○ ◎ ○ ◎
Type B ○ ○ ◎ ○ ◎
Type C ○ ○ ◎ ○ ◎
Uncoated ○ ○ ◎ ○ ◎
Type A ○ ○ ◎ ○ ◎
Type B ○ ○ ◎ ○ ◎
Type C ○ ○ ◎ ○ ◎
Uncoated ○ ○ ◎ ○ ◎
Coatingtype
Testconditions
Vacuum
Air
Temperature cycling test (-40℃ ~ +125℃, 2hrs/cycle)
●表面観察(コンフォーマルコーティングを溶解し、ウィスカをSEMにて観察)
3. Test Method 2/2
○ Surface observations (Dissolved conformal coating in solvent and observed whiskers under SEM.) ◎ Surface and Cross-section observations (Prepared cross-section samples by cutting and grinding and observed whiskers and conformal coating under SEM.)
Type A,B:Urethane , Type C:Silicone
7
4. Evaluation Results 4.1 Mitigation of Whisker growth 4.2 Shape dependence of Conformal Coating
8
Occurrence of whiskers under vacuum conditions
SEM photos taken after dissolving conformal coating (vacuum, 50 cycles)
Type A
Type B Type C
Uncoated
4.1 Mitigation of Whisker growth 1/6
9
4.1 Mitigation of Whisker growth 2/6
Occurrence of whiskers under air conditions
SEM photos taken after dissolving conformal coating (air, 50 cycles)
Type A
Type B Type C
Uncoated
10
Type A, 250 cycles
Coating interface Coating interface
SEM photos of whisker cross-section in conformal coating
Vacuum Air
4.1 Mitigation of Whisker growth 3/6
conformal coating in groove conformal coating in groove
11
Type B, 250 cycles Vacuum Air
Coating interface Coating interface
SEM photos of whisker cross-section in conformal coating
4.1 Mitigation of Whisker growth 4/6
detachment detachment
conformal coating in groove conformal coating in groove
Type A Type A
12
Type C, 250 cycles
Coating interface Coating interface
Vacuum Air
SEM photos of whisker cross-section in conformal coating
4.1 Mitigation of Whisker growth 5/6
conformal coating in groove
conformal coating in groove
13
Whisker length (µm)
0
5
10
15
20
25
30
35
0 200 400 600 800 1000 1200
No. of test cycles
vacuum TypeA vacuum TypeB vacuum TypeC vacuum Uncoated
air TypeA air TypeB air TypeC air Uncoated
0
200
400
600
800
1000
1200
1400
0 200 400 600 800 1000 1200No. of test cycles
Whisker density(Whiskers/mm2)
vacuum TypeA vacuum TypeB vacuum TypeC vacuum Uncoated
air TypeA air TypeB air TypeC air Uncoated
4.1 Mitigation of Whisker growth 6/6
Conformal coating mitigates whisker growth in vacuum and air condition.
Whisker length measurement under conformal coating
14
4. Evaluation Results 4.1 Mitigation of Whisker growth 4.2 Shape Dependence of Conformal Coating
Chip capacitor electrode edge whisker observations
Initial Air, 250 cycles Air, 1000 cycles Vacuum, 250cycles Vacuum,1000cycle
Type A
Type B
Type C
Conformal Coating thickness of corner and steep angle electrode was decreased as the thermal cycles increases in both vacuum and air condition.
4.2 Shape dependence of Conformal Coating 1/2
Whisker was penetrated in thin coating and exposed area. 15
16
Cross-section observations of whisker on chip capacitor electrode edge (Type B, Air, 2000 cycles)
1
Enlargement of 1
Coating interface Penetrated Whisker 2
Enlargement of 2
Cross -section
4.2 Shape dependence of Conformal Coating 2/2
17
5. Discussion ・Whiskers growths during thermal cycling ・Temperature dependence of Conformal Coatings
18
5.Discussion
・Conformal coating mitigated whisker growth in vacuum and air conditions.
・Conformal coating was observed in whisker groove. ・Conformal coating thickness was decreased after thermal
cycle test.
Whisker growth mechanism
Conformal coating materials’ characteristics at low and high temperature
Whisker growth mitigation mechanism of conformal coating
19
In-situ testing method
low-vacuum FE-SEM
Sample stage
Whiskers growths during thermal cycling 1/2
《Test temperature profile》
0℃
-40℃
+125℃
30min 30min
10min 10min
① ②
Temperature cycle in FE-SEM
① Cooling ramp (High to low temperature)
② Heating ramp (Low to high temperature)
Temperature cycle test was performed using FE-SEM sample stage heating. During the temperature cycle test, surface of chip capacitor was observed by FE-SEM.
20
In-situ testing result
Whiskers growths during thermal cycling 2/2
②Heating
Close grain boundary Open grain boundary
①Cooling
Tin grain boundary was moved during temperature cycling in cooling and heating ramp.
Cooling : Tin grain boundary spread Heating : Tin grain moved closer to other grain
cycling
Whisker
Whisker example
CTE mismatch of Tin and base material
It’s believed that whiskers grow during heating.
21
Material properties in high-temperature environments
《Improvements to nano-indenter》
Heater Sample tray
Heated
Data logger
Copper jig
Sample gripped in jig
Temperature measurement and
monitoring
Nano-indenter
RT measurement Time (hrs)
125℃
85℃
RT measurement
Heated to 85℃
Temperature maintained for 5 minutes after reaching 85℃
Temperature maintained for 5 minutes after reaching 85℃
Temperature maintained for 5 minutes after reaching 125℃
Measurement at 85℃
Measurement at 85℃
Measurement at 125℃
《Measurement profile》
Temperature dependence of Conformal Coatings 1/3
Testing equipment
Conformal coating characteristics (hardness and Young’s modulus) were measured at high and low temperature.
22
0
1
10
100
1000
25℃1st
85℃1st
125℃ 85℃2nd
25℃2nd
-20℃
Har
dnes
s(M
Pa)
TypeA TypeC TypeB
1
10
100
1000
10000
25℃1st
85℃1st
125℃ 85℃2nd
25℃2nd
-20℃
You
ng’s
mod
ulus
(MP
a)TypeA TypeC TypeB
Room temperature - high-temperature – low-temperature comparison (hardness, Young’s modulus)
Changes in material properties at high-temperature and low-temperature
Type A (Urethane)
Type C (Silicone)
Type A (Urethane)
Type C (Silicone)
Temperature dependence of Conformal Coatings 2/3
Type B (Urethane) Type B
(Urethane)
Type B and Type C became soft at high-temperature, Type A exhibited no significant change.
23
0.1
1
10
100
1000
0 500cycle
Har
dnes
s(M
pa)
TypeA 25℃ TypeA 85℃ TypeA 125℃
TypeB 25℃ TypeB 85℃ TypeB 125℃
TypeC 25℃ TypeC 85℃ TypeC 125℃
1
10
100
1000
10000
0 500cycleYo
ung’
s m
odul
us(M
pa)
TypeA 25℃ TypeA 85℃ TypeA 125℃
TypeB 25℃ TypeB 85℃ TypeB 125℃
TypeC 25℃ TypeC 85℃ TypeC 125℃
Material properties in high-temperature environments
Hardness at high-temperature was increased after the 500cycle thermal cycles test.
Hardness at room temperature was not change after the 500cycle thermal cycles test.
Type A
Type B
Type C
Type A
Type B
Type C
Temperature dependence of Conformal Coatings 3/3
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
0 200 400 600 800 1000 1200
vacuum TypeC① vacuum TypeC② vacuum Uncoated
air TypeC① air TypeC② air Uncoated
(um)
(cycles)
24
Conformal Coating Area Dependence
Uncoated
・Whisker growth mitigation effect of conformal coating was degraded after thermal cycles as long whiskers were observed at exposed electrode.
・It would become more important to control coating thickness, especially of the corner of the electrode, for whisker growth mitigation.
Point①
Point②
Coated: Point①
Exposed: Point②
Figure: Type C coating after 500cycles in vacuum
25
6. Summary 1/2 (1) Mitigation of Whisker growth in Vacuum condition ・Mitigation of whisker growth in both air and vacuum conditions, rather than whisker growth penetrating conformal coatings, was verified. ・Properties of the types of conformal coatings are as follows. (Properties of soft materials: Type A, Type C) Whiskers penetrate conformal coating. Entrapment of urethane was observed at whisker root. (Properties of hard materials: Type B) Whiskers lift conformal coating and make gaps between them. Tin was observed spreading horizontally up to 250 cycles.
(2) Shape dependence of Conformal Coating ・In the temperature cycling test, shrinkage of the conformal coating exposed the tin on the corners of the electrodes, resulting in whisker growth. Care is required for the coating thickness.
26
6. Summary 2/2
(3) Conformal Coating Mitigation of Tin Whisker Growth ・It’s believed that tin whiskers grow at heating ramp. ・Conformal coating materials’ hardness and Young’s
modulus have temperature dependence. These characteristics are decreased at high temperature.
・Whisker growth mitigation effect of conformal coating is
important for high temperature characteristics, because whiskers grow at high temperature.
27
Acknowledgements
High-Reliability Engineering & Components Corporation.
KOBELCO RESARCH INSTITUTE,INC.
<http://www.hirec.co.jp/eng/index.html>
<http://www.kobelcokaken.co.jp/en/index_e.html>
Thanks to Kobelco Research Institute for material evaluation of a conformal coating using nano-indentation data.
HIREC Corporation is in charge of the secretariat of JAXA RoHS community. Thanks to HIREC Corporation for advice regarding the evaluation of conformal coating for mitigation of Tin whisker growth.
28
Thank you for your attention!