Date: Fri, 8 Nov 1996 13:48:00 EST
From: Jim Marsico 516-595-5879 <[log in to unmask]>
Subject: Re: GEN: SOLDER IN LEAD BEND
To: Engelmaier <[log in to unmask]>
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Posting-date: Fri, 8 Nov 1996 14:04:00 EST
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On Nov. 8th, Werner Engelmaier wrote:

>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.

The fact of the matter is that the components are ceramic QFPs on polyimide.  
Our concern isn't with thermal excursions, though, since we restrain our 
assemblies and control the surface TCE to ~7 ppm/degrees C.  Our FEA revealed 
high stress concentrations during vibration where the lead is attached to the 
component.  The concern is that solder in the bend will shorten the lead length 
from the component body to the first bend, thus increasing the stress during 
vibration.

Any comments?

Jim Marsico
(516) 595-5879
[log in to unmask]
		
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