The invention provides a preparation method of a semiconductor device. In one enforcement manner, a first base plate provided with a first welding material convex point and a second base plate provided with a second welding material convex point are overlapped and then arranged in a furnace by mutually and temporarily fixing the welding material convex points. After the furnace is exhausted to form a decompression atmosphere, carboxylic acid gas is introduced. After the carboxylic acid is introduced, the temperature in the furnace is raised; and by exhausting gas in the furnace, the decompression atmosphere is formed in a temperature range which is higher than a reduction temperature of the carboxylic acid gas to an oxidization film and is lower than a fusion temperature of the welding material convex points. The temperature in the furnace is raised to the temperature range which is higher than the fusion temperature of the welding material convex points, and the first welding material convex point and the second welding material convex point are jointed by fusing.