A key factor when working with water washable solder pastes is to be aware of the impact of humidity on solderability. Water wash paste residues must be compatible with water cleaning systems so they can be prone to moisture pick-up. Moisture absorption can result in paste spattering during reflow and also solder balling, which are soldering defects. Paste printed on a ceramic coupon exposed to 60%RH for 4 hours shows no solder degradation. Next, a series of paste over prints exposed to 80%RH shows no solder degradation up to 24 hrs. The profile to the right shows the reflow profile condition for the test coupons.