Unbeknowst to me, the refrigerator where I store my solder paste and fluxes that I use for SIR (Surface Insulation Resistance) testing was being moved. One of my colleagues showed up at my desk with 2 jars that he had just removed from the refrigerator. In the time it took him to walk 50 feet, a significant amount of moisture had condensed on the outside of the jars. They were simply wet...as if somebody had just dunked them in a tank of water.
The incident impressed upon me the importance of allowing the paste/flux temperature to rise to room temperature before opening their containers. If I had removed the lids of either of the containers when they were handed to me (still cool), moisture would have quickly condensed on the surface of the paste/flux. As a rule, solder pastes and solder flux (tacky flux), be they no-clean (rosin/resin based) or water washable, do not react well to moisture. Moisture-contaminated paste or flux may:
exhibit reduced viscosity
spatter during reflow
produce excessive oxidation of the solder joint
CONCLUSION: Always allow your solder pastes and fluxes to equilibrate to room temperature before opening their containers. Often, this means planning ahead - sometimes removing them the night before you plan to use them. This is especially important as the northern hemisphere heads into the summer months.