When cured, conductive adhesive is no longer squidgy, are you actually using
a proper adhesive or are you using a thickened ink? Two possible answers
come immediately to mind:

1) By "proper adhesive" I am trying to mean those products which are usually
referred to as solvent free and based on epoxy.
If this is the case then most likely you are not achieving full cure.
The max temp for processing polyester films is usually around say 100C,
typically conductive adhesives have recommended cure schedules nearer 150C.
As a rough rule of thumb you need to DOUBLE the cure time for every 10C drop
in temp, (or half for a 10C rise) and then stick on about 10 -15% for an
engineering margin.
So from 150 to 140 is one double, 140 to 130 is  double that ....  add in
the percent fudge... turns into18x longer to cure at 100C compared to 150C.
It could be infact that your process temp is too low for the material you
are using to be initiated. Some materials need to see a min temp to start
them going. The best way to illustrate this is think of baking a cake. If
the temperature is too low the cake mix will never rise and go brown no
matter how long you leave it.

2) Some people take the ink used for making the conductor traces and let
that thicken into a paste and use that for component attach. This can give
all sorts of problems as you are pushing the formulation into areas it's not
intended for. Quite likely the deposit will remain flexible as that's what
it's designed to be and solvent evolution will take longer as you have a
much different surface to volume ratio in a short fat blob compared to long
thin patterns. The gap you are seeing is probably shrinkage caused by
further solvent loss on curing or drying.

Finally keep on mind that some of the products sold as adhesives for
polyesters are essentially modified inks and you could be seeing the sort of
thing outlined above.

Hope this helps.


-----Original Message-----
From: West, Jim <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: 25 June 1999 15
>Hi all,
>I need some help on understanding conductive adhesive.  I have an
>application that involves a membrane switch made of polyester.  We have a
>resistor in line with one of the traces that is attached by the conductive
>adhesive.  The problem I see is possibly the resistor pulling away from the
>adhesive causing intermediate readings.  Since the polyester is flexible,
we
>had the company that makes the switch apply a thicker piece of polyester to
>act as a stiffener.  But I believe this may not be good enough to support
>the resistor.  I looked under a microscope and saw that there was a small
>gap between the resistor and the adhesive and I used a tool to push on the
>adhesive and was surprised that it was soft in texture.  Does all
conductive
>adhesive remain soft or is there others that would harden?  Maybe with the
>polyester being so flexible you need the same from the adhesive.  Also, the
>switch company applies the resistor by hand and basically the adhesive gets
>smashed out under the switch.   How good is conductive adhesive when it
>comes to adhering to the resistor?  It's nothing like solder that flows to
>create a good bond.  I'm not sure how I can tell if I have a good
>connection.  If any of you have any insight on this subject I would
>appreciate your help.  Thanks for your time.
>
>Regards,
>
>
>Jim West
>Manufacturing Engineer
>