In all my designs I use 10% as rule for tolerance for thickness. Most likely
the common material stocked will push them to a higher or lower control. The
laminate folks can produce any thickness but not all the thickness we chose
are common. Most boards shops stock common laminates.

Thank you, 
Jim Mahoney 
Quick Turn Flex Circuits, LLC 
Applications Project Manager 
P# 603-821-6571 
F# 603-821-5723 
M# 603-305-6250 

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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kevin Glidden
Sent: Friday, January 09, 2009 8:42 AM
To: [log in to unmask]
Subject: [TN] PCB thickness tolerance question...Am I losing it?

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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