More than likely the base material stocked is .039. That is a standard
available thickness in the laminate world. Doing the math .039 base + .0056
copper (2 X 2 oz. cu) + .001-.0014 mask gets you to .046 which is beyond the
+/-.005. That's not taking into account tolerance of material and plating
variation. Just my thought.
Dan S.
----- 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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