Hans M. Rohr wrote:
> 
> Many of the boards we manufacture are 11" X 17".  Our customer requires that we
> hold the dimensional tolerance on their corner tooling holes to +/- 0.005".
> When we  started producing these boards several years ago, we had a problem
> holding this tolerance.  We found that baking the panels prior to drilling
> allowed us to achieve this tolerance.  We found that if shrinkage occured, it
> was during the final cure of the LPI solder mask (310 deg. F for 45 minutes).
> Baking panels at 310 deg. F for two hours at temperature solved the shrinkage
> during LPI cure.
> 
> Several weeks ago we started experiencing shrinkage of 0.015" - 0.020" over a
> 16" Y dimension.  The X dimension shrinkage is about 50% of the Y direction
> shrinkage or less.  The annular ring mis-registration is 0.001" or less.  We
> have confirmed that the baking oven is properly calibrate and working correctly.
> This shrinkage occures on more than one lot of laminate but does not happen on
> all lots.  Furthermore, the shrinkage is not consistant within the lot.  The
> boards are not warped or twisted.  The problem is not related to a specific
> board design.  It occurs with and without extensive ground planes.  The laminate
> is produced by one of the larger U.S. manufactures.
> 
> This is a first for us.  Has anyone seen anything like this?  I would appreciate
> any inormation (fact or theory) on the possible cause.
> 
> Hans Rohr
> Printed Circuits of America__________________________________________________________________________________
Hans:

I've also encountered two cases of laminate instabillity in the past six months.  
The first case was discovered after LPI final cure.  The second case was discovered 
at LPI exposure (after tack dry).  In both cases the hole to pad misregistration 
was similar when "good" and "shrunken" panels were compared.  When the hole to hole 
locations were compared, there was a .015 to .020 inch change in panel size.  The 
material exhibiting instability does not comnprise the entire laminate lot.  Rather 
there is variation within the lot.

I have contacted the laminator (a major US concern) and they have been 
investigating their process for a possible cause.  They did admit that other PWB 
manufacturers had encountered similar problems.

Please contact me directly if you wish to trade some detailed information.

Don Vischulis
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Phone (414) 857-7904
FAX   (414) 857-7392