Kevin, why not ask your supplier  "why?"  Maybe they have extra material of 
a certain size, or need to make your order from 2 different material stock 
sizes, etc...  When you have a question about "why" from somebody, I would 
feel I owe them to get their side.

BTW-  We live by the same 10% rule, and most of our pcb documentation states 
that same 10% standard you mention.

Lenny  ERG  www.ergpower.com

----- Original Message ----- 
From: "Kevin Glidden" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, January 09, 2009 8:42 AM
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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