PURPOSE: To reduce the quantity of bubbles within solder that fixedly joins together a leadframe and a radiator.

CONSTITUTION: Solder 3 is printed in the form of cross on a soldering region 2 of a radiator 1 using a metal mesh plate, and a leadframe is superimposed on the soldering region 2 of the radiator 1. An assembly consisting of the leadframe and the radiator 1 is heated in a heating furnace. Because of heating, the solder 3 radially spreads from the center owing to surface tension. Thereby, the solder 3 printed in the form of cross comes close to each other at the center but spreads outsides. When the solder 3 spreads and is flattened, bubbles remaining in the solder 3 are discharged, and hence the quantity of bubbles within the solder is decreased.