(This is my second attempt at sending this.  I'm still trying to 
figure out how to get mail out of here.  Sorry if it's a repeat 
question for some, but I didn't get any responses.)

Does anyone have information on the affect on reliability in a thermal 
cycle environment of gull wing leaded devices that are not completely 
on the pad (up to 25% side overhang).  For Class 3 hardware, J-STD-001 
allows up to 25% overhang, a value that was determined by pull test to 
not degrade reliability significantly.  I have seen one paper that 
suggested greater than 50% decrease in thermal cycle fatigue life with 
leads 25% off the pad (actual testing, no analysis performed).  
Reliability prediction models show a decrease in fatigue life with 
decreasing solder area under the lead, however, when microsectioning 
leads I found very little difference in solder area between a 25% and 
0% overhang.  The only significant difference was the absence of a 
fillet on the overhanging side.
Thanks.

Ed Knowles
Lockheed Martin Astronautics
[log in to unmask]
phone 303-977-3560

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