Mary: As you are probably starting to realize there can be a long grocery
list of possible causes. Let me give you a few things to look into or
consider that could be potential issues.
1. Defluxing/Cleaning Operation:
        A. The cleaning step could actually be mobilizing or releasing
material/contaminants from or on the assembly.

        B. Is AK225AES a bipolar cleaner ? If not try one that is, there
could be certain types of contaminants being left           behind.

        C. Are these assemblies the only ones being processed thru this
cleaner ? If not consider a separate cleaning               operation after
this one prior to potting.

2. Silicone Potting Material and Handling:
        A. How are the assemblies handled and stored prior to potting ? Are
they being re-contaminated?

        B. Is the primer and potting material within it's opened/unopened
shelf life and stored correctly ?

        C. If it is multiple component, is it being proportioned and mixed
properly using the right type of containers and             mixing apparatus
?

        D. Is the silicone properly cured and bonded ? Check the shore A
hardness and peel strength of the potting material to       the resistor
network. Verify the primer is the right one for the potting material.


Again there can be many, many causes for your problem and there may be more
than one thing contributing to it. Good luck!



Regards

Michael Barmuta

Staff Engineer

Fluke Corp.

Everett Wa.

425-356-6076


-----Original Message-----
From: Davis, Mary M. [mailto:[log in to unmask]]
Sent: Friday, August 06, 1999 1:12 PM
To: [log in to unmask]
Subject: [TN] Leakage in resistor networks


Lately, we are experiencing leakage (low resistance values and noise) at
temperature in laser trimmed resistor networks which have been potted in
silicone.  As received, parts test out O.K.  The parts are hand soldered
into an assembly using RMA flux, cleaned in a vapor degreaser in AK225AES,
coated with silicone primer and then potted in silicone. I know resistor
networks are very sensitive to chloride contamination. I suspect a sensitive
enough test will find a number of sources for something as common as
chloride but how much is too much? Any suggestions on how or where to look.
We are going to try testing parts at various stages of our process to try to
isolate the problem but I suspect we are looking for something that is very
subtle.

Your suggestions will be greatly appreciated.

Mary M. Davis
Materials & Process Engineer
ELDEC
[log in to unmask] <mailto:[log in to unmask]>
425-743-8327

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