I have some Kester rosin core wire solder (96-Sn/4-Ag) that has exceeded its shelf life. According to Kester's knowledge base, their Shelf Life Policy statement states that:
"Flux cored solder wire has a limited shelf life determined by the alloy used in the wire. For alloys containing more than70% lead, the shelf life is two years from date of manufacture. Other alloys have a shelf life of three years from date of manufacture."
I would like to know the mechanism of degradation of the alloy.