Hi Jim,
For your type of application, I typically use a range of tolerances,
depending on what we can live with, of +/-.010 down to +/-.007. Will
also depend on how board is panelized, whether scored, mousebite,
routed, or combo. When MechEng ask for what clearance to allow for the
board, my reply is 30 mils min, to allow for variances in molding
process. (Think they always listen?)
Has any draft been taken into account? That's been a problem in the
past, no so much now with the right design software. May also want to
have the molded part measured to verify various points along the
periphery of where the board will line up in housing, just to make sure.
Do you use GD&T for the board outline, or typical bi-lateral
tolerancing?
Sincerely,
Roger
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of James Verrette
Sent: Wednesday, August 09, 2006 9:13 PM
To: [log in to unmask]
Subject: Re: [TN] Routed edge tolerance
We are not planning to press fit the board into the housing, we need to
make sure we have enough clearance, I need to make sure the mechanical
guys are using realistic numbers and so we can still be as close as
possible to a "plain vanilla" PCB. I am starting to think +/-.005 is
too tight. I did some measurements on some boards and found a few
measurements outside the +/-.005. Most were due to the optical system
picking up stray fibers. Std deviation was about .0025. So I guess if
I was practicing 6 sigma design I would design for +/- .0075.
________________________________
From: James Verrette
Sent: Wednesday, August 09, 2006 8:06 PM
To: [log in to unmask]; 'TechNet E-Mail Forum'
Subject: RE: [TN] Routed edge tolerance
It is FR-4 in molded polycarbonate housing.
________________________________
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Wednesday, August 09, 2006 7:44 PM
To: James Verrette; [log in to unmask]
Subject: Re: [TN] Routed edge tolerance
Hi Jim,
I do not know what kind of housing you are putting your PCB in, but from
your outline description I am guessing some cast housing. You are also
not giving us a clue about your application. You are also talking about
punching instead of routing, so i am guessing CEM-3 instead of FR-4 as
the PCB material.
In that case, your routing tolerances are your least problem; much more
worrisome are likely the different thermal expansions of the PCB and the
housing as well as swelling of the CEM-3 with moisture absorption.
GM ran into this with their windshield wiper controls: single-sided
CEM-3 PCBs in some combination of routing and housing tolerances
requiring press-fitting the PCBs. The result: windshield wiper failures
for some of the more high-end [read: high-powered=running hotter] SUVs
within as little as 1 year after sale due to solder joint failures
caused by PCB warping-over 4 million vehicles were ordered by NHSTA to
be recalled.
Werner
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