in the chamber 21 is thoroughly exhausted the exhaust valve 32 is closed and the nitrogen gas supplying valve 34 is opened to fill the chamber with the nitrogen gas then the chamber 21 is opened and the power cell units 10 are taken out the heating in the hydrogen gas atmosphere allows direct reduction of the joint surfaces of the insulated circuit board 11 the first semiconductor chip 13 and the second semiconductor chip 15 before the solder melts thus cleaning of the surfaces is performed in the embodiment in particular due to the planar shape of the letter u the hydrogen gas extends over the central parts of the joint surfaces of the insulated circuit board 11 the first semiconductor chip 13 and the second semiconductor chip 15 before the solder melts therefore the cleaning effect on the surfaces is enhanced fig6 a and 6b are images of scanning acoustic tomography sat obtained in the experiment that was conducted for solder joining a semiconductor chip and an insulated circuit board using a sn 13wt sb solder plate according to the soldering process in the method of the invention the observed field of each of the images is over whole the solder joined surface the dark part indicates solder and the white part indicates a void fig6.