Phil,
I do not allow them for aerospace applications. Your memory is correct
and there are some other issues, but most of those are design specific.
If the non-plated mounting holes are needed for thermal, chassis ground
or other pathway issues extend the plane area on layer 1 and N
sufficiently to allow mounting hardware contact to the plane and still
allow vias in the planes to establish interconnect scenarios out of the
crush zone.
Dewey

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Bavaro
Sent: Monday, May 02, 2011 10:19 AM
To: [log in to unmask]
Subject: [TN] Mounting holes with plated barrels

I was reviewing a new design and found the mounting holes have plated
barrels.

In my past, I have always steered Design away from plating the barrels
of PWB holes used for mounting holes (and torque).

Far back in my memory, I remember experiments I had run which revealed
that the HASL solder on the mounting hole annular ring allowed solder
creep making the fasteners come loose over time.  I also seem to
remember something about not wanting to put a barrel in compression.
This is a double sided board mounted to a copper plate and the entire
assembly is tin lead plated and then fused.  On a multilayer board, the
argument is easily made to not compress the internal layers.

I am looking for something in the standards which might provide some
direction against this type of design.

The only stated reason from the mechanical designer was increased
thermal dissipation, for which my argument is that the fastener provides
much more capacity for transfer than does the barrel wall.

Am I being too conservative?

Phil



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