Polyimide Film, like DuPont Kapton, absorbs up to 50% of the materials own weight in water in a very short time.  It is always a good idea to bake Flexible Circuits prior to processes that have temperatures above 200F.

Chuck Brummer
Acuson
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Mick Thwaite wrote:

We are just starting to use flex PCB's in our production area. Is it true that the PCBs have to be dried prior to using as they absorb moisture quickly? Is it also true that hot air is better than IR reflow?, and Does anybody have any good ideas on the materials to use for the vacuum jig to hold down the panel during wire bonding?We have used vacuum holds as small as 1.00 mm but still find that the PCB moves during bonding.....   Mick Thwaite