Something similar it happen to us a while ago, and I decided to measure
the test vehicle before and after each process, by using material with
the grain in long direction (24) and separately material with grain in
shorter direction (18).This allowed me to identify precisely the
movement during each process and also to find out what lot will give the
least movement (average) within the less variation (range). I found huge
differences between these 2 lots, and it was not until the panels
reached legend that operators could identify that there is a problem
(the film made originally for grain direction didn't fit the other
panels, for cross grain direction). Also the range of the movement was
greater on crossgrain direction than on grain direction. A decision was
made to have supplied only panels with longer side in grain direction.

So, a mix of panels with grain in both directions might be your cause
(for some material suppliers you must reinforce this spec, in the top of
standard specs).
Julian

PS: The bake before/after lam/hal did not make a big difference in
equalizing these differences.

Ed Cosper wrote:
> 
> I have been experiencing some unusual material movement recently on .059 FR-4. A distortion in some cases in excess of .020" over 24 in. has been experienced. It manifests itself after hot air level and is commonly detected at legend or fabrication when the tooling holes no longer fit the pins.
> 
> We have evaluated all of our internal processes to insure we are not inducing any excessive stress. The highest temperature exposure is 490 deg. F during H.A.L. . We have found that pre-baking the material does control the problem. Detailed analysis indicated that the material is in fact completely cured from the manufacturer.
> 
> My concern is that although this is a random occurrence, I have established a notable increase in frequency on non pre-baked ridged material. Although more prevalent from one supplier, since we have been checking I have also seen an increase in other suppliers as well.
> 
> Is anyone else experiencing this concern?
> 
> Ed Cosper
> [log in to unmask]
> 
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