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.        *
***************************************************************************