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May 2011

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Subject:
From:
"Brooks, Bill" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Brooks, Bill
Date:
Mon, 2 May 2011 10:56:40 -0700
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Hi Phil, 
The mounting holes should not be plated as a solution to provide an
electrical path to connect them from the external or internal layers to
other layers. It's unreliable.  The barrel plating in the mounting hole
is likely to crack... because of the compression of the screw and
tensions in the board during thermal excursions and vibration. That
doesn't mean that it will, but it is likely to happen. 

Can they be plated? Sure. Can they be relied upon for electrical
connections? In my opinion, NO.  (Also the Mil-Std's forbade the
practice for the same reasons) 
 
Thermal... there are other more reliable ways to get the heat out and
thru the board... ask the designer about them.
The HASL solder in the holes can change the size of the hole a great
deal... not desirable but can be compensated for in the design by making
the hole larger. Do I recommend it? No... Is it okay to do it if the
designer wants it? Sure. Just meet the hole diameter finished size
requirement on the drawing. 

Phil you are wise to continue to steer designers away from this
practice... but ultimately the design is the designer's. I would
question it, however he/she may have some valid reason for it.  You can
also suggest that the hole be non-plated and they connect the planes
thru vias that are not being compressed under the screw in the near
proximity to the hole. 

Best regards,

Bill Brooks | Datron World Communications, Inc.
PCB Designer/Engineer | Office: 760-602-7004| Fax: 760-597-3777 |
[log in to unmask]
3055 Enterprise Court, Vista, CA 92081 | www.dtwc.com

Performance You Require. Value You ExpectTM

-----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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