Michael,
Join the club............... Conformal coating of silicone is porous to
sulfur, the sulfur attacks the exposed silver end terminations were the
epoxy meets the termination. Usually see black clouds in silicone conformal
coating around termination.

You need to find the source of sulfur in your system. For us it was an EPDM
rubber grommet. We also went from Dale parts (which have no Pd content) to
KOA parts with have 4% minimum Pd. We also went to a acrylic/polyeurathane
conformal coating. Our problems have been gone for 3 years.

Please contact me off line for additional details.

Mark



                    Michael Bell
                    <Michael.Bell@       To:     [log in to unmask]
                    GPC.CO.NZ>           cc:
                    Sent by:             Subject:     [TN] End Cap/Resistor problems
                    TechNet
                    <[log in to unmask]
                    RG>


                    04/16/02 05:30
                    PM
                    Please respond
                    to "TechNet
                    E-Mail
                    Forum.";
                    Please respond
                    to Michael
                    Bell






Hey all,

I have just had a few questions raised regarding a customers product and
it's reliability.  Presently, they are potting a control circuitry into a
siliconne rubber mould.  This is sealed and then installed in oven control
systems.  What appears to be happening is, over it's lifetime, for some
unknown reason, the ends of the SMT caps is corroding and giving
intermittant faults.  If you pearce the siliconne rubber mould and press on
the end cap with a probe, it then forms a contact and the fault goes away.
The PCB itself is an FR4 PCB with HASL finish, and has an operating current
of around 6mA.  The product is washed once inside the oven control, but
shouldn't the siliconne rubber protect this sufficently enough???  Since
the
rubber is a permeable compound, is it possible for water to make it's way
into the mould and onto the surface of the PCB and then not escape????
Would this lead to the corrosion of the end caps???


Cheers

Mike

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