Lead-free solder is replenished to a solder bath to receive a work having copper, the work to be processed subsequent to a soldering process using an air knife or a die. The replenished lead-free solder includes Sn as a major composition and at least 0.01 to 0.5 mass percent Ni. By replenishing the lead-free solder having the above composition, the density of the solder bath, being sharply changed for using one of a HASL device and a die, is quickly restored back to within an appropriate range.
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