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January 2009

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Subject:
PCB thickness tolerance question...Am I losing it?
From:
Kevin Glidden <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Kevin Glidden <[log in to unmask]>
Date:
Fri, 9 Jan 2009 08:42:08 -0500
Content-Type:
text/plain
Parts/Attachments:
text/plain (26 lines) 
We have a small PCB that is less than 2" x 2" square.  Material is
IPC-4101/21 FR4 with 2 oz copper.  2 layers.  Finish is Tin-Lead HASL.
 
Total thickness is specified as .040" +/- .005", and the supplier is
requesting increase to .040" +/- .006".  Is it me, or is +/- .006" excessive
for a 2 layer .040" thick PCB?  I was always taught a "rule of thumb" that
the PCB thickness tolerance is +/- 10% of the specified nominal thickness.
Is this a bad "rule of thumb"?  Even +/- .005" is 12.5%!  Is this a sign of
a problem with the supplier's process?
 
Thanks!
 
Kevin Glidden
Manufacturing Engineer
Luminescent Systems Inc.

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