By way of example of the conventional power semiconductor device, a power transistor has been employed.
Since this type power transistor is used for supplying a large amount of electric power, consideration is sufficiently given to heat radiation.
FIG. 6 is a sectional view of the power transistor. In the power transistor, a frame 11 to which a transistor element 10 is assembled is arranged in such a manner that the bottom face of an island 11A is exposed outside of a border 12A of a resin body 12. In the thus composed power transistor, a screw 15 is inserted into a through-hole 12B of the resin body 12 and screwed to a radiating plate 16. Due to the foregoing, heat generated in the transistor 10 is quickly released through the frame 11, so that the effect of heat radiation is enhanced.
However, in this resin sealing type power transistor, the frame 11 is exposed outside. Therefore, it is very difficult to maintain an insulating condition of the frame 11. A method for maintaining the insulation is described as follows. In the case of fixing the frame 11 to the radiating plate 16 with a screw, it is necessary to interpose an insulating plate 17 such as a mica plate between the frame 11 and the radiating plate 16. However, it is complicated.
Due to the circumstances described above, a construction in which the frame 11 is completely accommodated in the resin body 12, that is, a full package structure is commonly used (shown in Japanese Examined Patent Publication No. 5-12857).
With reference to FIG. 7, a method of manufacturing a power transistor of the full package structure will be explained below.
Reference numeral 20 is a metallic mold, which is composed of an upper metallic mold 20A and a lower metallic mold 20B. In a void 21 formed by the upper and lower metallic molds 20A, 20B, an island 11A of the frame 11 is set downward under the condition that the island 11A is spaced from the lower mold surface. Then a thermosetting resin is injected into the void 21 from a resin inlet formed on a mating face of the upper and lower metallic molds 20A, 20B.
While the injected resin is filled into the void 21 of the metallic mold 20, it is hated (at about 200.degree. C.) by a heater block not shown in the drawing. Therefore, the injected resin is hardened by the reaction of a hardening agent contained in the resin.
Then the resin body 12, which has been hardened, and the frame 11 are separated from the metallic mold 20, and laser beam marking is effected on the surface 12B of the resin body 12.
In the frame 11, a bend portion 11C is formed, and the distance H1 between the island 11A of the frame 11 and the lower metallic mold 20B, and distance H2 between the surface 11B of the frame 12 and the upper metallic mold are determined to be H1&lt;&lt;H2. When distance (wall thickness) H1 between the island 11A of the frame 11 and the lower metallic mold 20B is reduced as described above, in the same manner as that shown in FIG. 6, when this transistor of the full package structure is assembled onto the radiating plate 16, it is possible to effectively release generated heat from the frame 11 to the radiating plate 16. The characteristic of the power transistor is greatly affected by distance H1 between the island 11A and the lower metallic mold 20B. That is, the shorter the distance H1 is, the more the radiation is enhanced.
Since laser beam marking is effected on a surface of the resin body 12 formed in the void 21, an entire surface of the metallic mold 20 for forming the void 21 is generally subjected to mirror finish on which the maximum surface roughness is not more than 1 .mu.m. The reason why the entire surface of the metallic mold 20 is subjected to mirror finish is described as follows. When a resin surface of mirror finish is subjected to laser beam marking, a surface of the marked portion becomes rough in the manner of satin finish, so that the contrast of the marked portion is improved as compared with a portion in which laser beam marking is not effected. (This technique is disclosed in Japanese Unexamined Patent Publication No. 4-257246.) Other reasons why the metallic mold of mirror finish is used are described as follows. Compared with a metallic mold of satin finish in which voids are formed by carving by means of electric discharge machining, a metallic mold of mirror finish in which a plurality of metallic mold members that have been individually made are combined, can be easily machined in the case where a resin body 8 having a step portion shown in FIG. 4 is formed, and further when the metallic mold has been worn away, the metallic mold can be partially replaced, which is economical.
However, when wall thickness H1 is reduced as described above in the case of a conventional full package type power transistor, the following problems may be encountered.
FIG. 8 is a view showing a condition in which molten resin J flows into a clearance formed between the island 11A and the lower metallic mold 20B and hardens when molten resin J comes into contact with the lower metallic mold 20B, the temperature of which is raised when it is heated by a heater block not shown.
When molten resin J is injected into the metallic mold 20 having small clearance H1 described above, pressure of molten resin J is raised because clearance H1 is so small. Since the surface of the lower metallic mold 20B is a surface subjected to mirror finish and further the injection pressure is high, a hardening acceleration portion A which comes into contact with the metallic mold 20 and starts hardening slides in the flowing direction of molten resin J, so that the portion A is pushed forward and joined to another hardening acceleration portion A which is pushed forward from another side.
In this way, a confluence B is formed. The confluence B appears on a surface of the resin body 12 and cloudy pattern is formed, so that the appearance of the surface is deteriorated. Further the confluence B becomes porous because of the cloudy pattern, which could be a cause of the ingress of water (shown in FIG. 9). Water enters the porous confluence B from the cloudy pattern that has appeared on the surface of the resin body 12. Finally, water reaches the island 11A and comes into contact with the transistor 10 assembled onto the surface of the island 11A. Due to the foregoing, the transistor element 10 is damaged in some cases.
When the resin body 12 that has been sealed with resin is released from the lower metallic mold 20B, since an adhesive force between the island 11A and the resin is low at the confluence B, an adhesive force between the surface of the lower metallic mold 20B and the resin overcomes the adhesive force between the island 11A and the resin, so that the resin is drawn to the lower metallic mold 20B side, and a blister C is generated (shown in FIG. 10). One of the reasons why the blister C is generated is that the resin is not completely separated from the surface of the metallic mold 20 because the surface of the metallic mold 20 is subjected to mirror finish on which the surface roughness is not more than 1 .mu.m. However, it is considered that the main reason is the deterioration of adhesion between the island 11A and the resin 12 which is caused by the cloudy confluence B.
When the blister C occurs in the resin boy 12, not only the appearance is deteriorated but also a sufficiently large area of the contact surface can not be ensured between the radiating plate 16 and the resin body 12 because of the blister C.