Assuming your transmission lines are on the external layers, the only
place you *really* need a tighter tolerance is between the external
layer and its reference plane layer. This should be unaffected by an
overall 10% tolerance. The overall thickness tolerance allows the
fabricator some leeway with the variables of lamination.


Denis Lefebvre, CID+
Sr. PCB Designer
Finisar Corporation
(408)542-3832

-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of
Babcock John
Sent: Tuesday, June 26, 2007 8:21 AM
To: [log in to unmask]
Subject: [DC] Board Thickness Tolerance

Fellow Designers,

 

Long time lurker, first time poster.  I've had a few posts over the
years, just not very many.

 

Ok here goes.

 

What is a good overall thickness tolerance to put on your dwg for a
0.125" thick board?  This board is impedance controlled and should stay
pretty close to the 0.125" to meet our required impedance values.

 

I am kind of in a unique situation where we deal directly with an
assembly house and that assembly house farms out the board fab to
whoever gives them the best quote.  So I never know which board house
will get the board.  Hence I never know the capabilities of the board
house where the board gets sent to be made.

 

With that being said, XYZ board house wants to loosen up the tolerance
to +/- 0.012" on the overall board thickness.  This would throw our
impedance calculations off.

 

Just kind of curious what everyone would expect as a good tolerance on a
0.125" board.

 

 

John Babcock, C.I.D.
PCB Designer III



 


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