Shelf life of solder is a concern that is raised by customers on a somewhat regular basis. Solder pastes typically have a well defined shelf life, however the useable life of a solid form of solder may be much longer and harder to define. This was the topic of one of my first blog entries, although I think Eric Bastow has a better explaination than I did. Here is Eric's explanation:
“One of the issues surrounding solid forms of solder (no incorporated flux) is determining the usable (or “shelf”) life of the solder. Solder manufacturers have to draw a “line in the sand” somewhere to establish a time limit on the duration of their “warrantee” period. For better of for worse, in our document driven world, many electronics manufacturers live and die by the solder manufacturers stated shelf life, and will even petition the solder manufacturer to recertify the solder after the shelf life has expired so that it can continue to be used. Otherwise, it is excluded from further use and discarded.
There are many solder alloys in existence and each alloy “ages” in a unique way. However, the most common issue with aged solder is oxidation. Is there a level of oxidation at which a solder is no longer usable? With normal storage and handling and use of a flux, the author believes that the answer may be “no” based upon the results of an experiment.
60In/40Pb spheres, 300 microns in diameter, were reflowed in air or nitrogen onto ENIG coupons with two different activity level no-clean fluxes; reduced activity ROL0 (passes J-STD-004A SIR un-activated) and ROL1. The spheres were oxidized to four different levels; fresh, 4 days at room conditions, 5 hours at 85C & 85%RH and 3 minutes of violent shaking. The appearance of the “fresh” was shiny; the “4 days at room conditions” and “5 hours at 85C & 85%RH” were very similar in their slightly dull appearance, and the “3 minutes of violent shaking” were noticeably darkened.
The experiment was performed with a reflow profile that had a peak temperature of 231C. After reflow, the diameter of the wetted spot was measured.
Interestingly, the determining factors were the flux type and reflow environment (air or nitrogen). For a given flux and reflow environment, there was no statistical difference in the size of the wetted spot among the different levels of oxidation.“