Abstract: A water soluble solder paste, which does not adversely impact the surface insulation resistance (SIR) of a circuit board, is comprised of powdered solder, a water soluble flux (containing an activator within a water soluble vehicle), a hydrophobic surfactant preferentially absorbed over the water soluble vehicle, and a solvent within which the surfactant and flux are dissolved. The hydrophobic surfactant is chosen to be preferentially absorbed by the circuit board surface over the water soluble flux vehicle to prevent the vehicle, which is hydrophilic from becoming trapped in the board surface, thereby adversely affecting the board SIR.

Abstract: A water-soluble soldering flux which does not adversely impact the surface insulation resistance of a circuit board is comprised of a water-soluble vehicle, an activator and a hydrophobic surfactant preferentially adsorbed at the board surface. At soldering temperatures, the activator reacts to clean metallized areas on a circuit board and promote solder wetting. The hydrophobic surfactant, which is preferentially adsorbed at the board surface, acts to block the diffusion of hydrophilic vehicle molecules into the board which would otherwise degrade the board's surface insulation resistance.