Abstract: The rate of decrease of an oxidation suppressing element in a solder bath is measured during a soldering operation. Soldering is then carried out while replenishing the solder bath is then replenished during soldering so as to maintain the surface level of molten solder in the bath with a replenishment solder alloy which supplies the oxidation suppressing element to the solder bath at at least the rate at which the oxidation suppressing element is consumed during soldering. When the oxidation suppressing element is P, the concentration of P in the replenishment solder alloy is preferably 60-100 ppm.

Abstract: In this invention, the rate of decrease of an oxidation suppressing element in a solder bath during use is measured, and a solder alloy including the oxidation suppressing element in the same or greater proportion than the rate of decrease in the amount is suitably added in accordance with the decrease in the solder bath. As a simple method, in flow soldering of an Sn—Ag based or Sn—Ag—Cu based solder alloy, to compensate for a decrease in the P content of the solder bath which is observed during operation, a solder alloy containing 60-100 ppm by mass of P in an Sn—Ag based or Sn—Ag—Cu based solder alloy for replenishing a solder bath is supplied to maintain not only the P content but also the surface level of molder solder bath.