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Humidity and Solder Paste Do Not Mix

Condensation on side of cold solder paste jarUnbeknowst 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 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 Condensation on bottom of cold paste jarremoved 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.