As you surmised, the condition you described is most likely moisture
     related. The prebake requirement is entirely dependent on the part
     construction and adhesive type.

     Typically something like 250F for 30 min. is used. Some combinations
     do better with a 275F bake for 2-4 hours. If the dielectric is Upilex
     in a multi-layer part it can take a very long time (24 hrs) to bake
     out the moisture due to the low transmission rate of the film.


     Typical Values              Kapton HN        Upilex S

     Water Absorption            2.8%             1.2%
     wt. %

     Water Vapor Permeability    54               1.7
     g / sq. m / mil



     Other possible causes:

     Insufficient adhesive thickness.
     Improper surface cleaning prior to coverfilm lamination.
     Improper lamination cycle.
     Relaminated parts. (Looks ok, but has no real bond strength.)
     Undercured adhesive.
     Local "hot spots" on the part during IR.
     Solder temperatures exceeding those recommended for the adhesive.


     The more you can tell us about the parts the closer we can come to
     being specific for you.

     Andy Magee - Applications Engineer
     Rogers - Circuit Materials Unit
     Phone: (602) 917-5237
     Fax: (602) 917-5256
     E-Mail: [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: PRE-BAKE FOR FLEXIBLE CIRCUITS
Author:  BOB (BOB HOENE) {NAME:BOB
HOENE|EMS:INTERNET|MBX:[log in to unmask] at MCIMAIL
Date:    9/11/95 11:01 AM


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.