Bing,

Be careful what you conclude from your "evaluations".Your colleague was
basically right. Moisture is rapidly (5-30 minutes) absorbed by both the
polyimide and adhesives used to make typical flex circuits. However,
prebaking must be done correctly or you gain very little. The
time/temperature has to be appropriate for each configuration of layers
and materials. For example, a multilayer circuit made with Upilex and FR
adhesive could take >24 hours at 275F to be dry enough to avoid
delamination. My experience has always been that the hydration rate is
much greater than the drying rate, so you must move quickly from
prebaking to reflow. The change is by no means permanent, and fluxing
parts in batches and leaving them damp while each sheet is processed can
also lead to disaster.

Other factors in FPC delamination are the choice of materials that can
have a huge influence on the delamination resistance, and your reflow
temperature profile. The quality of the FPC fabricator's lamination will
also be a factor to consider.

I'd need to know a lot more about your case to be of more specific help.

Andy Magee
Flex Guru
[log in to unmask]

Bing wrote,

Hello technetters !!!
Hope you all had a nice weekend . . .
I'm not sure if I'm the only one encountering this kind of problem
since nobody answered my first question regarding my problem on
delamination on FPC's.
I had series of evaluations performed to qualify the effect of
pre-baking in the elimination of delamination of FPC's  but to no
avail. A colleague of mine said that one of the causes of this is the
moisture embedded/trapped in between the FPC layers which then
causes such delamination when subjected to heat (reflow). Pre-baking
is needed to evaporize the moisture so that when it reaches the
reflow station, moisture is already gone and no delamination will
then be created/contributed  at the reflow station. But this
explanation proved me wrong. In the series of my evaluations,
delamination still occured (even with the raw FPC's) at the reflow
station.
Please, I need your ideas about this one ...
Thanks in advance and have a nice week ahead of you...

Bing

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