Problem: Even if the strength of a solder composition close to a SnBi eutectic composition was improved, it was brittle, so when it was used for small electronic devices such as mobile phones or notebook computers, the resistance to drop impacts when the small electronic equipment was dropped was low. Therefore, interface peeling often took place between the soldered surface and a printed circuit board, resulting in the devices being destroyed.
Means for Solving the Problem: When soldering using a solder paste containing a SnBi-based low-temperature solder, at least one type of solder composition selected from a Sn-Ag, a Sn-Cu, and a Sn-Ag-Cu solder composition is diffused into the solder paste by simultaneously supplying at least one type of preform selected from a Sn-Ag, a Sn-Cu, and a Sn-Ag-Cu solder composition, whereby resistance to drop impacts is improved.