Because lead-free solder's soldering temperature is high, PCB's copper foils and lead plating elements can easily melt into solder in the solder dipping bath.
When Cu concentration rises, in particular, solder's melting point and viscosity also begin to rise, often resulting in poor soldering.
In order to prevent solder with different compositions from being mixed, use solder for initial use and that for adjusting Cu content from the same manufacturer, and use their solder series of the same specifications.
Be sure to indicate the following in the drawing.
① Manufacturer, ② Model of solder for initial use, ③ Model of solder for adjusting Cu content, ④ Model of wire solder