PURPOSE: To prevent a solder fillet from generating fatigue fracture by oscillation, impact, thermal stress, etc., by putting a metallic ball near each recessed bottom having a V-shape cross section to supply cream solder and heating and cooling it with a light beam heating device to solder joining parts of the metallic parts.

CONSTITUTION: Printed boards 20,21 are assembled so that joining parts on the sides of wiring electrodes 22,23 make V-shape cross section to each other, the metallic balls 24 made of Cu are included between both wiring electrodes 22,23 and solder is melted into the spaces to form a solder fillets 25. Each metallic ball 24 is soldered at such a position that part of its spherical surface comes in approximate contact with respective wiring electrodes 22,23. When the clearances between the metallic ball 24 and each wiring electrode 22,23 are not found, a clearance where the maximum of shearing strength appears necessarily exists on the spherical surface of the metallic ball 24, therefore, when soldering is conducted through the metallic balls 24, soldered fillets 25 have strong mechanical joining.