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April 1998

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Fri, 24 Apr 1998 20:36:21 EDT
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In a message dated 98-04-24 18:05:55 EDT, you write:
<< We use both, depending on the test specification, so I am curious, how do
 the damage and/or failure mechanisms differ between thermal cycling and
 temperature cycling?
  >>

Heidi,
Air-to-air thermal cycling (or  single chamber cycling) and liquid-to-liquid
"thermal shock" can both lead to solder joint fatigue. For supporting
evidence, see (for example) detailed failure mode analysis in the following
papers:
1) Darveaux, R., Banerji, K., Mawer, A. and Dody, G., "Reliability of plastic
ball grid array assemblies", Chap. 13, Ball Grid Array Technology, ed. J. H.
Lau, McGraw-Hill, 1995, pp. 379-442.
2) Frear, D. R., "Microstructural evolution during thermomechanical fatigue of
62Sn-36Pb-2Ag and 60Sn-40Pb solder joints", Proceedings, 40th Electronic
Components and Technology Conference, Las Vegas, NV, May 11-13, 1990, Vol. 1,
pp. 518-524.
3)Frear, D., Grivas, D. and Morris, J. W., "Parameters affecting thermal
fatigue behavior of 60Sn-40Pb solder joints", Journal of Electronic Materials,
Vol. 18, No. 6, 1989, pp. 671-680.
Liquid-to-liquid transfer from -55C to 125C lead to solder joint fatigue
failures similar to failures in air-to-air thermal cycling.

When interfaces and metallizations are the weakest links (for either
mechanical or chemical / cleanliness reasons), they may also fail under both
types of test.
Mechanical cycling is also used and provides useful information. When boards
are subject to isothermal cyclic twist, for some devices you get solder joint
fatigue failures, for others the leads may break. By raising the test
temperature, lead failures can be eliminated and solder joints will fail.

So, there is no single answer to your question. Whatever the test, the best
way to find out what mechanism is being accelerated is to conduct a thorough
failure mode analysis. With experience on your company's products, and by
looking at your company's accelerated test database, you probably can tell
what test conditions are right for the requirements of your application.

Regards,
Jean-Paul
________________________________________________
Jean-Paul Clech
EPSI Inc., P. O. Box 1522, Montclair, NJ 07042
SMT / BGA / Flip-Chip / CSP reliability specialists
tel: (973)746-3796, fax: (973)655-0815
home page: http://members.aol.com/Epsiinc1/index.html
(last updated on April 23, 1998)

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