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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