Thanks Everyone!

Yes.  The fab notes already call out the required impedances on their
various layers with those layer and dielectric thicknesses.  Just did
not know if this board thickness tolerance they are requesting was
normal and if this was going to throw our impedance calculations off.

John Babcock


-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Tim
Reid
Sent: Tuesday, June 26, 2007 8:52 AM
To: [log in to unmask]
Subject: Re: [DC] Board Thickness Tolerance

John,
The overall board thickness tolerance & impedance tolerance are two
different things. The 
fab vendor will adjust the prepreg/core thicknesses (trace layer
distances from adjacent GND 
planes) as well as the trace widths to get your impedance requirements.
The overall board 
thickness tolerance is something the fab shop balances with what
material they happen to 
have in stock at the time. I've never noticed a board shop wanting to
use 12% before though; 
10% is standard for us. 

Regards,
Tim




Date sent:      	Tue, 26 Jun 2007 08:21:04 -0700
Send reply to:  	"(Designers Council Forum)"
<[log in to unmask]>,
             	Babcock John <[log in to unmask]>
From:           	Babcock John <[log in to unmask]>
Subject:        	[DC] Board Thickness Tolerance
Originally to:  	Designers Council Forum
<[log in to unmask]>
To:             	[log in to unmask]

> 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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