Ed,
We had similar problems many years back and introduced a bake on all
incoming laminate sheets. We had to bake in a large oven at 160C (320F)
for 6 hours to make sure all the material reached a temperature above
Tg. It would be worth having an independent Tg check on your different
suppliers' material to make sure it is fully cured.

Even if the cure is complete, there is still the inherent stress in the
laminate caused during the cool down cycle. This was the other reason
for the bake and was equally important.

Nowadays we buy pre-cut panels which the supplier bakes prior to
despatch and have no problems with stability although we still get some
uniform shrinkage at HASL depending on the copper pattern and laminate
thickness.

I think the rule is someone has to bake it unless the laminate suppliers
have a very long cool down cycle perhaps using an air cooling system.
Regards
Paul Gould
Teknacron Circuits Ltd

Ed Cosper writes
>
>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
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