Solder flux, if you purchase it on its own, typically has aN expiration date of 2 years. But what about the flux cores inside solder wire? That os gemerally considered not to expire, what gives? Okay, so it turns out that some manufacturers do list an expiration date. And actually, especially if solder wire is left exposed to open air, the flux core will go bad near the tip. But, kept in a sealed bag, many folks have reported the solder working just fine several decades later. As for solder flux, it can be restored by putting a few drops of alcohol in it if it thickens. But solder paste, due to the solder powder and flux being mixed together, it doesn’t fare so well when it expires.
A flux wetting test is useful to determine if flux is good or bad. What is that? Basically, if you drip it on to a metal surface to be cleaned and fluxed, does the flux spread evenly across the surface? Or does it tend to stick together? Proper wetting means it will spread as much as possible evenly across the surface, for a considerable surface distance.
20200723/DuckDuckGo how long does flux in solder last
20200723/https://www.evilmadscientist.com/2013/solder-expire/
20200723/http://www.circuitinsight.com/programs/54448.html
20200723/DuckDuckGo flux wetting test
20200723/https://superiorflux.com/role-of-flux/