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July 1997

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Tue, 15 Jul 1997 21:54:12 -0400 (EDT)
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Hi Charles,
The reasons for problems with the reliability of solderjoibts of TSOPs with
Alloy 42 lead/leadframes are threefold: (1) and most important, the Alloy 42
leadframe has a CTE of ~5 ppm/oC whereas FR-4 is ~17+-2ppm/oC, giving you a
large global thermal expansion mismatch; (2) solder joints to Alloy 42 are
(even under the best of circumstances) weaker than those to copper, and in
some cases (perhaps one supplier of Alloy 42 in the Far East (South Korea?))
some Alloy 42 has been found to be virtually non-solderable, a problem HP
(see Engelmaier, W., and B. Fuentes, "Alloy 42: A Material to be Avoided for
Surface Mount Solder Component Leads and Lead Frames," Proc. Surface Mount
International Conf., San Jose, CA, August-September 1994, pp. 644-655; also
in Proc. Int. Electronics Packaging Conf. (IEPS), Atlanta, September 1994,
pp. 503-516; also in Proc. 19th Ann. Electronics Manufacturing Sem., Naval
Air Warfare Center, China Lake, CA, February 1995, pp. 5-20; also in Proc.
NEPCON West '95, Anaheim, CA, February-March 1995, pp. 385-395; also in
Soldering & Surface Mount Technology, No. 21, October 1995, pp. 20-25) and
others have run into; (3) there also exists a local expansion mismatch
between Alloy 42 and the solder (CTE~25ppm/oC), which because of the short
foot length of the leads is less important. 
Thus, you need to underfill the TSOPs totally. It is however far better to
use TSOPs with copper leadframes.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 973-543-2747
E-mail: [log in to unmask]

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