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July 2010

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Subject:
From:
Darren Hitchcock <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Darren Hitchcock <[log in to unmask]>
Date:
Thu, 8 Jul 2010 11:44:42 -0500
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To my knowledge there is no default tolerance for board thickness in 
IPC specifications.  The previous post sending you to IPC-T-50 won't 
help because it defines terms and definitions, but does not contain any 
specifications.  The other post about IPC-4101 applies to bare 
laminate, but not directly to the finished PCB.  IPC-4101 is used by the 
board fabricators to specify to the raw material suppliers what they 
want to purchase for laminate, but does not directly apply to the 
finished PCB.  If you have a multilayer PCB, then it won't help you with 
the tolerance on thickness for prepreg dielectrics, only 
cores/laminates.  The copper thickness specifications only specify 
minimums, so you won't know the average or the maximum values of 
those through IPC specifications.

So you were correct in turning to IPC-A-600 and IPC-6012 for the 
finished PCB requirements.  Look at Section 3.4 on Printed Board 
Dimensional Requirements.  "All dimensional characteristics such 
as...thickness...shall be as specified in the procurement 
documentation."  So it must be listed on the fabrication drawing.

I hope this helps.

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