A semiconductor device having both high strength and high thermal radiation that is capable of being applied to mounting on automobiles experiencing many thermal cycles, and a manufacturing method thereof are provided. A circuit board 1a for a resin encapsulated semiconductor module device has a configuration where a silicon nitride plate 2 with a thickness of 0.635 mm has copper plates of 1.0 mm and 0.8 mm bonded to both sides thereof via active metal. A copper plate 3a is bonded to the surface side of the silicon nitride plate 2, and a prescribed circuit pattern is formed on the copper plate 3a. Tin-silver-copper cream solder layers 4a and 4b with a thickness of 200 μm are formed at a prescribed location on the circuit pattern 3a on which a semiconductor element 6 is mounted and at a prescribed location of a base plate 1 on which the circuit board 1a is disposed. Nickel particles 5 having a maximum particle size of 100 μm and an average particle size of 70 μm are dispersed in the solder 4a on the base plate 1 of good thermal conductivity. A semiconductor element (chip) 6, the circuit board 1a, and the base plate 1 are disposed on predetermined locations. Thereafter, they are set in a reflow oven (not shown in the drawings) for reflow soldering. After the inside of the reflow oven is replaced by a nitrogen atmosphere, the reflow oven is heated to 280° C. At the time when solder is melted, the inside of the oven is decompressed to 1 Pa, nitrogen is introduced, and the reflow oven is cooled to about room temperature, thereby completing the solder bonding step. After flux is washed, an outer case 7 with an insert-molded outlet terminal 8 is adhered to the base plate 1 and a predetermined connection is conducted via an aluminum bonding wire 9. Then, silicone gel 10 is injected into a package delimited via the base plate 1 and the outer case 7, and the silicone gel 10 is heat-hardened, thereby completing a resin encapsulated semiconductor device A.