I can not help you with the history of the changes allowing solder in the
upper bend on gull wing leads in J-STD-001B (paragraph 9.2.6.1, Figure 9-4,
Note 1) and MIL-STD-2000A (paragraph 4.23.7.5).  
However, technically this change makes sense. Gullwing leads, particularly in
fine pitch, have ample lead compliancy to accommodate the small thermal
expansion mismatches between a PLCC and FR-4 substrate. Thus, the small loss
of lead compliancy due to the solder (solder has a low modulus of elasticity
that is even lower at elevated temperatures) in the shoulder bend has for no
practical consequence. However, to my knowledge nobody has ever
experimentally verified this--this test would be very time consuming.
The above would not be true, if the component is a CLCC.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945
Phone & Fax: 201-543-2747
E-mail: [log in to unmask]

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