On Mar 6, 2010, at 12:17 PM, Marechiare wrote:

> course on Physics might help you to get rid of the need in any
> assumptions about other people knowledge

My impression is that PCB materials, component lead materials, and  
solder alloys are all chosen to minimize the differential thermal  
expansion effects you allude to.  Not to mention the relatively low  
temperature extremes that most equipment is subject to.
I could look up numbers, I supposed, but I'm not sure I have any  
knowledge about how to come up with a qualitative evaluation of what  
sort of differences would be "dangerous."

The NASA document referenced seems to be more worried about stress  
induced on the component leads by differential expansion of the  
component itself and the PCB material.  It looks like they want some  
length of bendable lead in the path between component and solder  
connection.  For instance, there is no mention of clinching being  
needed for axially mounted parts, and Section 8.4.5 states explicitly  
"The lead may
be terminated by clinch, straight-through, or lap configuration."  Lot  
of diagrams with "SR" (strain relief) detail labels pointing at a  
section of lead that is not soldered.

I dunno.  The tradeoffs seem pretty ambiguous.  Demonstrably, such  
clinching is NOT done in most consumer products.  I don't see how to  
tell whether that's because the tradeoffs have been carefully  
analyzed, or just due to the difficulty and expense of doing it...

BillW

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