Hello all,

        We discovered a problem with the soldering of surface mount resistor
pots on a board with Tg around 175 C, on a developing product.  Can anyone
help in identifying the root cause?

        The resistor pot is ceramic with thickfilm silver, then Sn/Pb is
screened and reflowed before we receive it from the vendor.  The board has
electroplated Ni, electroplated gold (50 uin), Sn63/Pb37 screen printed,
component placed, and finally IR-Convection reflow.  The flux used is
water-soluble.

        What we see is approx. 10% fail due to separation at the
thickfilm-ceramic interface of the resistor pot.  What we suspect is excessive
leaching of the silver out of the thickfilm, especially since its gone through
two reflows and is pure silver with some binders.  But, the reflow profile
looks standard as well as the wash process.  The solder joints look good too.
So, if it is excessive leaching, I don't know what adjustments could be made.

        Thanks in advance for any insight you can offer.

Glenn Pelkey
Quality/Reliability Engineer

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