To improve the manufacturing yield and reliability of a semiconductor device by washing the semiconductor device with water and, after dipping the device in a methanol solution maintained within a specific temperature range, drying the device.

After the side face of the semiconductor substrate of a diode assembly 1a is etched, the stains which adhere to the assembly 1a at soldering time and the etchant are removed by ultrasonically cleaning the assembly 1a by dipping the assembly 1a in pure water contained in an ultrasonic cleaning tank. Then the assembly 1a is set on a jig 14 so that the electrodes having larger areas of semiconductor chips 2 formed in truncated cones may come on the bottom sides. After the assembly 1a is dipped in a methanol solution 16 which is cooled to 3-10°C by means of a temperature controller 17 and contained in a bath 15 together with the jig 14 for about 10 seconds, the assembly 1a is dried by passing the assembly 1a through an infrared tunnel furnace so as to evaporate the methanol.