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May 1999

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Subject:
Re: Re[2]: [TN] Electromigration of Tin Lead?
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 27 May 1999 08:22:20 EDT
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Hi Ken,
The answer you got from Ingemar Hernefjord is pretty good, except he does not
tell us under what conditions they observed the dentritic growth. I would
guess it was under some kind of accelerated temperature/humidity testing.
The problem with CAF is that it does not occur under normal operating
conditions, except when physical water (condensation, drops) makes contact.
Accelerated temperature/humidity testing is typically donef0r200 hours+ at
85C/85%RH or worse; and even under those conditions there is little
consistency. There also is no established acceleration model; in fact, some
workers in the field have postulated a threshold. However, occcasional
failures in the field that fit the same symptoms have been observed, but
failure mode analysis is essetially always inconclusive. Most product runs
under conditions, i.e. heat dissipation, somewhat controlled environments,
that this style of problem should not occur. For additional information you
may want to look at a White Paper written for the Surface Mount Council:
Engelmaier, W., and L. Turbini, "Design for Reliability in Advanced
Electronic Packaging," Proc. Surface Mount International Conf., San Jose, CA,
August 1995, pp. 844-879, and IPC-D-279, Design Guidelines for Reliable
Surface Mount Technology Printed Board Assemblies, Appendic C.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com

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