Results from Whisker investigations and the corresponding

Lead free: Whisker
Results from Whisker investigations and the
corresponding conclusions
Dr. Werner A. Hügel, Dr. Verena Kirchner, Moheb Nayeri,
Dr. Lothar Henneken, and Rolf Keller
Robert Bosch GmbH, Stuttgart, Germany
Automotive Electronics
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AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of
disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Motivation
• Whiskers are known since 1946, when they were observed by H. L. Cobb
• Until today no generally valid theory is available to explain the growth of
whiskers
Models:
• Eshelby et al. Phys. Rev. 91, 775 (1953): Dislocation driven growth
• Fisher et al. Acta Metall. 2, 368 (1954): Macroscopic stress driven growth
• Furuta et al. Jap. J. of Appl. Phys. 8, 1404 (1969) Stored strain energy
• Lindborg et al. Acta Metall. 24, 181 (1976): Growth induced by diffusion and stress
• Tu et al. Phys. Rev. B 49, 2030 (1994): Stress driven growth induced by IMC
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AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of
disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Motivation
Whisker test standard will be implemented in the near future
• No further investigations necessary, most problems are solved?
• Whisker growth mechanism still unknown
• No accelerated test available
• What do we test with this test (comparability of different
results from different companies)?
• What about the comparability with qualification results and
conditions in serial production?
• What are minor and major changes?
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disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Motivation
• Impact of different
bath chemistry
• Delayed whisker
growth can be
observed
• Control of the
organic additives
is important
• What is the root
cause of the
whisker growth?
ESA STR-223, 09.1987 B.D. Dunn
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disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Observations of whiskers: Fe/Ni alloy with Cu underlayer
Base material: alloy 42
Underlayer:
4µm Cu
Plating:
5µm Sn
Post bake:
-
Storage:
ambient
conditions
for 5 months
Whiskers > 170µm
no further growth of
whiskers after 500T/C
(-60°C/+60°C)
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AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of
disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Observations of whiskers: Fe/Ni alloy
Base material Underlayer Sn plating Post bake
Alloy 42
Storage
Whisker length
-
4µm
-
T/C
80µm
Cu, 4µm
7µm
-
T/C
20µm
Cu, 4µm
7µm
-
12 months ambient
25µm
Cu, 4µm
5µm
-
5 months ambient
170µm
Cu, 4µm
10µm
-
5 months ambient
70µm
• Underlayer can reduce whisker length after T/C
• Careful usage of galvanic Cu underlayer, amplified whisker growth
possible!
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disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Observations of whiskers: Cu alloy
Base material: CuSn6
Underlayer:
-
Plating:
1-4µm Sn
Post bake:
-
Storage:
ambient
conditions
for 6 months
Whisker can carry
currents up to 60mA
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disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Observations of whiskers: Ag underlayer
Base material: Cu
Underlayer:
2µm Ag
Plating:
6µm Sn
Post bake
150°C for 1h
Storage:
6 months
ambient cond.
6 months +50°C
uncontr. hum.
Whiskers > 35µm
Ag underlayer is not an effective countermeasure against whisker growth!
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disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Observations of whiskers: 3 x reflow Tpeak260°C
No Ni in the area where whiskers are growing
Base material: Cu
Underlayer:
2µm Ni ???
Plating:
3-5µm Sn
Post bake
-
Storage:
4 months RT
1 month
50°C/85% r.h.
Whiskers > 110µm
The soldering process is not prohibiting the growth of whisker!
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AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of
disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Observations of whiskers: after solder dipping (SnPb)
SnPb Solder
Whiskers are
appearing close
to the soldered
area
Base material: Cu
Underlayer:
-
Plating:
~5µm Sn
Post bake:
1h @ 150°C
Storage:
ambient
5 months
60°C/93% r.h.
2000h
Sn plating
Whisker > 50µm
Heat impact of the solder dipping cannot prohibit the whisker growth in the
unwetted area
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disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Observations of whiskers: Cu alloy
Base material: C7025
Underlayer:
-
Plating:
5µm Sn
Post bake:
-
Storage:
ambient
conditions
for 4 months
Whisker is not just a
“longer” grain!
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AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of
disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Overview of observed whisker
Base material Underlayer Sn plating Post bake
CuSn6
C7025
Tammac
C194
Cu alloy
CuSn0.15
Storage
Whisker
-
4µm
-
6 months ambient
130µm
-
4µm
-
12 months ambient
200µm
-
5µm
-
4 months ambient
70µm
-
10µm
-
6 months ambient
15µm
10µm
Ni, 1µm
5µm
-
6 months ambient &
500 TC
Ni, 1µm
5µm
-
4 months ambient
no
-
5µm
-
500 TC
6µm
Ni, 1µm
5µm
-
500 TC
11µm
-
8µm
-
5 months ambient
no
-
8µm
150°C, 1h
no
Ag, 2µm
6µm
150°C, 1h
5 months ambient
6 months ambient &
6 months HT
35µm
Ag, 2µm
4µm
-
2 months ambient
15µm
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AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of
disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Whisker length / µm
Effectiveness of Ni underlayer, storage at 20°C and 50%r.h.
much longer whisker
no saturation
saturation
Time / ww
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disposal such as copying and passing on to third parties.
CR/APT1
Results from Whisker investigations and the
corresponding conclusions
Growth of whisker:
• growth of whisker has no uniform behavior
• Linear growth (worst case) as well as square root like growth
(diffusion) has to be assumed
• a complete saturation of the whisker growth could not be observed
within 2 years (100ww)
• Samples without countermeasures are showing different distribution
of length and growth rate for whiskers
• risk assessment is for each plating system unique
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disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Estimation for
ONE device:
Fine Pitch Device (defect rate over 15 years)
10µm Sn on Cu lead frame without further
countermeasures like postbake or Ni-underlayer
Device 1
square root
law
80-Pin Plastic
QFP
Defect rate
144-Pin Plastic
LQFP (Fine
Pitch)
< 480 ppm
Device 1
square root
law
Defect rate
< 3600 ppm
Device 1
time
proportional
< 98,9%
Device 1
time
proportional
100%
Device 2
square root
law
100%
100%
Device 2
square root
law
Device 2
time
proportional
100%
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Device 2
time
proportional
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disposal such as copying and passing on to third parties.
100%
Results from Whisker investigations and the
corresponding conclusions
Whisker length / µm
Plating thickness and supplier of the galvanic chemistry
stored at 20°C and 50%r.h. for 21 weeks
CR/APT1
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AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of
disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Whisker evaluation for lead free plating:
Base material:
Plating thickness:
Plating system:
alloy 42
5µm, 10µm, 15µm
lead free
Whisker definition: > 100µm
Tests performed:
Result:
1000TC (-85°C / +85°C)
10.000h (RT)
4000h (60°C / 93% r.h.)
NO whisker found, evaluation and qualification passed
ready for release?
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AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of
disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Facts and further consideration:
• Any mechanism which causes whisker growth is following the laws of
physics e.g. diffusion and relaxation
• It is not possible to apply the test procedures for standard device
qualification as a whisker acceleration test
• Process control of the galvanic chemistry becomes more important for
lead free production
• Until a real test for measuring the propensity of whisker growth and a
real knowledge of the growth mechanism behind the growth is
available we need test durations of 4000 hours and a well
implemented process control during the plating
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AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of
disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Parameters influencing whisker growth:
• Alloy of lead frame or base material (SnZn, SnCu, …)
• Plating conditions (current density, bath temperature, belt speed, …)
• Plating chemistry (bath supplier, composition, …)
• Control of the bath parameters (metals, additives, …)
• Bath parameters defined by the bath supplier have to be continuously
controlled at the supplier (SPC, OCAP, …)
• Build-up of retained samples and implementation of a whisker monitoring
at the supplier
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AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of
disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Tests for the future?
Measurement according to Warren(1)
• Measurement of the
inhomogeneous
broadened line widths
due to the micro stress
in the grains
• The slope shows the
stress distribution over
grains
4µm Sn without post bake
(1) B. E. Warren, X-ray diffraction, Dover
Publications, Inc. New York, 1990
4µm Sn on 2µm Ni
4µm Sn with post bake
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AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of
disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Conclusion:
• Qualification of the bath and the lead free samples is very important
• Bath has to be qualified for the whole required parameter range
• Whisker test has to be performed with the corner samples under
conditions equal to the serial production
• Base material with highest propensity of whisker growth has to be used
as a reference for a qualification test
• Pass criteria for a whisker test are that no whisker growth is observed on
the samples for storage at constant temperature and 30µm after 500 T/C
from -55°C / + 85°C
• Long term study (over application life time) of whisker growth for
countermeasures like postbake and Ni underlayer is necessary
Automotive Electronics
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AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of
disposal such as copying and passing on to third parties.
Results from Whisker investigations and the
corresponding conclusions
Thank you for your attention
Contact:
werner.huegel@de.bosch.com
Whiskers
are they
a problem?
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AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of
disposal such as copying and passing on to third parties.