Bob, you asked:

>WOULD ANYONE CARE TO ADVISE THE GUIDELINES FOR FLEXIBLE
>PRINTED WIRING PERTAINING TO PRE-BAKE BEFORE ASSEMBLY.
>DURING THIS LAST AND LONG HOT SUMMER, WE SEEM TO HAVE
>EXPERIENCED UNFAVORABLE STORAGE CONDITIONS AND OUR FLEX
>CIRCUITS ARE "BLISTERING", THAT IS, THE COVERLAY IS LETTING GO
>OF THE BASE MATERIAL DURING PROCESSING.  WE BELIEVE THIS TO
>BE A MOISTURE RELATED PROBLEM. WE WAVE SOLDER AND IR
>REFLOW CIRCUITS IN OUR PROCESS. YOUR ADVICE AS TO BAKE
>TEMPERATURE, DURATION, AND SUGGESTED WORKING TIME
>ALLOWABLE BETWEEN BAKE AND ASSEMBLY WOULD BE
>APPRECIATED.

You will likely receive many opinions. You did not state what the material
was, but it is probably a Kapton (polyimide film) type material. Unfilled
resins of this material will absorb about 1% by weight of water. Keep in
mind that laminated layers may see some absorption between film layers,
too.

FR-4 boards will absorb most of the moisture that baking removes within 24
hours. Weigh some samples of your flex cirucits before and after baking,
you will have your own data. Should be able to do it with a 10 mg balance.

Regarding storage, I suggest that you make a dessicator in which to keep
them, for instance buy a 10 gallon aquarium, apply rubber gasket material
to the top, and get a glass cover made to fit the length and width.

Put a few pounds of indicating Drierite (changes to a pink color after
absorbing moisture, costs about $5/lb) underneath a metal standoff shelf in
the bottom of the aquarium. Keep the lid on, it will last for weeks or even
months, and baking the Drierite will regnerate it.

If you do this, you will never need to bake, and the shelf life is multiplied.

Of course, depending on your production rate and the size of the part, it
may be impractical, so my advice may be worth what you paid.

One other word - on the net, ALL CAPS IS THE EQUIVALENT OF YELLING.

Jerry Cupples
Interphase Corporation
Dallas, TX