Abstract: An improved process for the fusing a tin-lead solder mixture deposited to form a finished circuit on a printed circuit or wiring board. The method comprises the rapid or flash fusing of the circuit board having the tin-lead solder mixture deposited thereon by immersing into the vapor of an inert fluid having a boiling point just above the melting point of the solder alloy. Problems with "weak knee" conditions are avoided by adjusting the heating time-at-temperature to that which allows the desired tin-lead solder alloy to form only as a thick "mush" rather than as a molten fluid. By so doing, the solder alloy is formed without the necessary degree of fluidity that causes it to draw away from the edge or rim of plated holes in the board.