Janice,

Your question about shelf life involves so many variables - the finish, was
that process in control during fab, the time, the temperature, the humidity
of storage, the assembly conditions of flux, solder temperature and time, and
more.

The Alternate Final Finishes Committee of IPC (5-23d) has nearly completed a
round robin "conditioning" study of 4 final board finishes with bare copper
and reflowed tin/lead as controls.  This study is to develop models of
solderability after various hold times from 8 to 1000 hours, at humidities of
30 to 95% relative, and temperatures from 50 to 95C.  While this will not
make an aging model for anyone, the results should allow you to compare the
finishes on your boards to those in the study.   If you wish to compare your
results to what we stress the finishes to, perhaps you can develop a model.

From an early analysis of the data, we see that finishes like reflowed
tin/lead gradually decline in solderability with increase of time,
temperature, and humidity.  Immersion silver seems to behave the same way,
but not necessarily on the same slope.  However, surprisingly, some finishes
like OSP seem to solder the same with increasing time temperature and
humidity until they reach a point of degradation and then become very
difficult to solder.  This degradation mechanism is unknown to our committee.


The testing and analysis will be done in April (probably just about the time
of IPC Expo).  The conclusions may take longer.

Our committee hopes to provide stress conditions and background data so that
you can develop your own models with your finish and assembly conditions.
Hopefully we can help you more in a couple of months.

Dennis Fritz
MacDermid, Inc
Chairman - Alternate Final Finishes 5-23d

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