Fellow techsters,

       For what it's worth, I ran into this exact issue about a dozen
       years ago. Turns out, the laminate material was "stress relieving"
       in the various thermal excursions in the board fab process. The
       culprit was the tensioning mechanism in the laminators resin
       coating process. The glass cloth was being S-T-R-E-T-C-H-E-D  in
       the B-staging (dip & dry) operation. Subsequent bakes by the board
       fab'er (me), caused erratic shrinkage and voluminous ingestion of
       Rolaids.

       Now imagine if Mr Rohr had .4mm pitch components on this size
       board. He would be unable to stencil solder paste since since even
       though the "master pattern" matched his stencil, the actual boards
       would vary in size "all over the bloody parking lot"!
       An ugly scenario that assembly folks cannot live with...

       Regards

           marko <[log in to unmask]>

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
[log in to unmask]
Phone (414) 857-7904
FAX   (414) 857-7392