54 SMT Magazine • February 2015
failure
[9]
. Figure 2 depicts an acrylic conformal
coating that does not completely encapsulate
the lead wires of a quad-flat package. To address
this issue, the IPC 5-22ARR J-STD-001/Confor-
mal Coating Material and Application Indus-
try Assessment Task Group has conducted a
round-robin study to characterize the coverage
provided by commonly used conformal coating
materials with existing application processes.
The study includes 26 different material combi-
nations, and the results are expected to be used
for updating measurement and evaluation stan-
dards.
In designing safety and control electronics,
the risks of using tin must be carefully assessed.
To this end, methods for assessing failure risk
have been developed
[6]
. For example, the risk of
a tin whisker–induced short can be estimated by
identifying tin-finished surfaces, distances from
the identified surface to surfaces at a different
electrical potential, and knowledge of whisker
growth statistics. Tin whisker lengths can vary
dramatically; however, they have been found
to follow a lognormal distribution
[7]
. The den-
sity of whisker growth can also vary dramati-
cally. For assessment, software
[8]
using measured
whisker growth statistics has been published
in the open literature. With these methods, a
probability of failure can be assigned to the tin
whisker failures, and the effect of tin whisker
failure mitigation strategies that rely on separa-
tion and coating coverage can be assessed.
Thus far, the ban on the use of lead in most
electronics has not resulted in public concern
about tin whiskers. This result is likely is due
in large part to the development of industry
standards, research by industry and academic
groups, engineering functions within organiza-
tions to address this reliability threat, as well as
closed discussion related to product failures, in
general. However, this does not mean the risk
to electronics posed by tin whiskers has been
eliminated. Research into whisker mitigation
strategies and the tests for assessing whisker
risk should continue to be supported. As long
as the electronic industry continues to use tin,
tin whisker risk must be addressed. Tin whiskers
are a known problem, and forgetting to account
for their potential failure risk will cost organiza-
tions dearly.
SMT
references
1. B. Sood, M. Osterman and M. Pecht, "Tin
Whisker Analysis of Toyota's Electronic Throt-
tle Controls," Circuit World, Vol. 37, No. 3, pp.
4–9, 2011.
2. Leidecker, Panashchenko, and Brusse,
Electrical Failure of an Accelerator Pedal Posi-
tion Sensor Caused by Tin Whiskers, 5th Inter-
national Symposium on Tin Whiskers, College
Park, Maryland, 2011.
3. Tadahiro Shibutani, Michael Osterman,
and Michael Pecht, Standards for Tin Whisker
Test Methods on Lead-Free Components. IEEE
Transactions on Components and Packaging
Technologies, Vol. 32, No. 1, pp. 216–219, Mar.
2009.
4. S. Han, M. Osterman, M. Pecht, "Like-
lihood of Metal Vapor Arc by Tin Whiskers,"
IMAPS Advanced Technology Workshop on
High Reliability Microelectronics for Military
Applications, Linthicum Heights, MD, May 17–
19, 2011.
5. P. Vianco, M. Neilsen, J. Rejent, and R.
Grant, Validation of the Dynamic Recrystalliza-
tion (DRX) Mechanism for Whisker and Hillock
Growth on Thin Films, 8
th
International Sympo-
sium on Tin Whiskers, Raleigh, NC, October 2014
6. Tong Fang, Michael Osterman, Sony
Mathew, and Michael Pecht, Tin Whisker Risk
Assessment, Circuit World, Vol. 32, No. 3, pp
25–29, May 2006.
7. T. Fang, M. Osterman, and M. Pecht, Sta-
tistical Analysis of Tin Whisker Growth, Micro-
electronics Reliability, Vol. 46, Issues 5–6, pp.
846–849, May–June, 2006.
8. CalceTinWhiskerRiskCalculator, pub-
lished by CALCE, 2005.
9. Sungwon Han, Stephan Meschter, Mi-
chael Osterman and Michael Pecht, Evaluation
of Effectiveness of Conformal Coatings as Tin
Whisker Mitigation, Journal of Electronic Mate-
rials, Vol. 41, Issue 9, pp 2508–2518, July 2012.
Michael osterman is a research
scientist at university of
Maryland, center for advanced
life cycle engineering.
Feature
TIN WHISKerS reMaIN a CONCerN continues
