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!