Jim,

If your board is cost sensitive and you do not suffer from very tight
tolerance requirements on the housing, I would suggest specifying +/-0.010".

I've seen local board shops here struggling a bit with the +/-0.005"
(especially when you mention the complex curved outline) and, assuming, that
they are at least as good as in other parts of the world, that could mess up
your yield.

And, finally, I can only join the voices that warned you against punching.
In case someone does provide you with a good explanation why some "special
punching process" would yield good results - there is reason to consider
(maybe there are things we haven't met yet), but not the standard punching.

The shearing and tearing of glass fibers during punching leaves you with
very rough edges and damage to the glass/resin interface further into the
board. Such damaged interface welcomes the capillary diffusion of water
along the glass bundles, resulting in nasty shorts.



Yehuda Weisz



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of James Verrette
Sent: Wednesday, August 09, 2006 4:54 PM
To: [log in to unmask]
Subject: [TN] Routed edge tolerance



I am trying to figure out what the minimum clearance would need to be

between a PCB and a housing that it fits into.  The piece I am not sure

on is the tolerance of the routed edge of the PCB.  The PCB is about

.050 thick and the outline is quite complex with curved edges and

cutouts.  What is the maximum variation in the routed edge that I can

expect from the typical mid-high volume board fabricator?  I have been

told +/-.005 hole to edge and +/-.005 edge to edge.  But what is the

yield at this tolerance?  It is just for straight line routs?  The board

is 4 layers and cost sensitive, so we don't want to over specify the

edge dimension tolerance and drive up cost or limit ourselves to select

few board shops.



I have also seen quotes from Asian sources recommend punching instead of

routing.  Where is a good source of layout design guidelines for this

process? Does it yield good results? I have some knowledge of

singualtion punching, but not in the board fabrication process.



Jim Verrette

Electrical Engineer





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