Yes, I fully agree.
However in this case, eventually I was told that the working heritage design has been to plate all the mounting holes, including those in multilayer boards.
As others have already mentioned, the design ultimately belongs to the designer and in this case, the Mechanical Engineer made this policy change many years ago. They add adhesive to all of the fasteners to prevent loosening.
Trying to plead the case for the sliver resulting from a cracked barrel to annular ring seems to be a mute point except in the case of a field return unit.
If the opportunity ever comes up, I will ask for one of these multilayer board mounting holes be cross sectioned with the hope of showing a crushed barrel and/or internal interlayer connection damage. Electrically there will never be a failure as they do drop stitched vias connected to the mounting hole on multilayer boards.
Phil
-----Original Message-----
From: Smith, Rick [mailto:[log in to unmask]]
Sent: Thursday, May 05, 2011 8:31 AM
To: [log in to unmask]; Bavaro, Phillip @ PRG - TW
Subject: RE: Mounting holes with plated barrels
Do not rely on the plated barrel but it should be stitched profusely with vias all around the plated mounting hole.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Bavaro
Sent: Monday, May 02, 2011 1:19 PM
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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