The CTEs of TSOPs with Alloy 42 lead frames have been measured as low as 5.9 ppm/C; thus, they are well suited for ceramic or similar substrates but not for FR-4 without CTE-constraining features, e.g. copper-Invar-copper power and ground layers, Kevlar fibers instead of glass, etc. Ulrich Korndoerfer's TSOP, Type I, are slightly more forgiving than Type II's, since the lead stiffness for Type II's is even higher than that of Type I's. The failures I talked about on SIMMs and DIMMs were solder joint failures on TSOPs, Type I, with Alloy 42 lead frames on FR-4 substrates primarily as the result of the global expansion mismatch of the component and the substrate. As is typical for Alloy 42 leaded components these failures were in the vicinity of the lead/solder interface partially in the solder and partially at the interface. The latter failure mode results from a combination of inherently lower solder joint strength to Alloy 42 (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. (SMI), San Jose, CA, August 1994, pp. 644-655), lower solubility of the nickel in Alloy 42 into tin and thus reduced solderability as compared to copper, and the local thermal expansion mismatch between the solder (CTE=24-25 ppm/C) and the Alloy 42 (CTE=4.5 ppm/C) (see Engelmaier, W., and A. I. Attarwala, "Surface-Mount Attachment Reliability of Clip-Leaded Ceramic Chip Carriers on FR-4 Circuit Boards," IEEE Trans. Components, Hybrids, and Manufacturing Technology, Vol. CHMT-12, No. 2, June 1989, p. 284 and Clech, J-P., F. M. Langerman and J. A. Augis, "Local CTE Mismatch in SM Leaded Packages: A Potential Reliability Concern," Proc. 40th Electronic Components & Technology Conf., Las Vegas, NE, May 1990, p. 377). These three characteristics are contributing failure causes. TSOP manufacturers do not have a good record about being forthcoming with regards to the assembly/reliability problems with Alloy 42. Do you really feel comfortable with 50 cycles of -40 to +125oC? Werner Engelmaier Engelmaier Associates, Inc. Electronic Packaging, Interconnection and Reliability Consulting 23 Gunther Street Mendham, NJ 07945 USA Phone & Fax: 201-543-2747 E-Mail: [log in to unmask] *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * ***************************************************************************