Hi Glenn-

If I'm reading your problem correctly, I just experienced a similar problem
with a thick film resistor network.  Is the failure ocurring at the lead
attachment point to the ceramic substrate?  Does it appear to be an
adhesive or cohesive failure?  Is the pad on the ceramic essentially gone?

If the pad is gone and the lead attach solder is intact then you are having
a mechanical failure.  The Tce mismatch between the part and your board is
putting stress on the connection and it is failing.  The part should be
able to withstand this so it sounds like you have a vendor part problem.

The problem we had was due to the supplier not putting down a thick enough
pad on the thickfilm substrate.  So when the supplier did his lead attach
process (solder dip), essentially the entire pad dissolved leaving a weak
mechanical connection.  The pad size also makes a difference. Corrective
action was to increase the pad thickness (and control the thickness better
than he had been doing) and increase the mounting pad size.

If this is the failure mode then the other 90% which "passed" test are
suspect and could give you intermittent failures down the line.

-Gregg
[log in to unmask]

At 08:52 AM 10/21/97 PDT, [log in to unmask] wrote:
>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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