In response to a similar question Dennis Fritz [DDFRITZ@AOL.COM] Chair of the Alternate Final Finishes Committee of IPC (5-23d) said ...
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.
The committee hopes to provide stress conditions and background data so that you can develop your own models with your finish and assembly conditions.