Hi Glenn - We have run into this type of "opportunity" before with some
silver finished components. Two issues to investigate:1) component thermal
stability and finish requirements; 2) reflow profile and process control.
There has been considerable industry documentation of silver finishes
leaching into the solder joints thus creating component failures. This
failure mechanism was one of the forcing functions that resulted in barrier
metallizations (such as copper or nickel) being used in surface mount chip
capacitor terminations. The ANSIJ -STD-002 specification includes a
Resistance to Dissolution test (test D) that might be useful. I also
suggest that you review the finish stackup/requirements so see if you have
adequate finish thickness.  The second issue of reflow profile/process
control is just making sure that you are not above the solder reflow
temperature for an extremely long time period or using excessive
temperatures. The silver is diffusing into the solder and long reflow time/
high reflow temperatures just increase the diffusion rates. I think that
the use of Sn62/Pb36/Ag2 is highly overrated as one method of combating
silver diffusion - we solder many silver finished components with Sn63
without incident. The common thread seems to be process controls for having
good success with either solder alloy. Good Luck.

Dave Hillman
Rockwell Colllins
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[log in to unmask] on 10/21/97 10:52:53 AM

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cc:    (bcc: David D Hillman/CedarRapids/Collins/Rockwell)
Subject:  [TECHNET] Assy:  Solder Failures




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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