1. Field of the Invention
The present invention relates to a semiconductor device. More specifically, the present invention relates to a protector for protecting a semiconductor pellet from thermal stress.
2. Description of the Prior Art
A resin sealed semiconductor device is more advantageous as compared with a hermetically sealed apparatus in view of its compactness and low cost in manufacture. One example of a resin sealed semiconductor device is disclosed in Japanese Utility Model Laying-Open Gazette No. 155455/1981, in which a structure for enhancing moisture resistance is disclosed, although the disclosure is of less interest to the present invention.
FIG. 1 is a sectional view of an internal structure of a resin sealed semiconductor apparatus according to the prior art in which a semiconductor basic element is shown comprising a semiconductor pellet 1 soldered between protector plates 2 and 3. The protectors 2 and 3 are then soldered to outer electrodes 4 and 5, respectively, while the electrode 5 is mounted to an insulating plate 7 fixed to a copper radiator base 6. The radiator base 6 and a wall 8 of polybutylene telephthalate resin together form a vessel, filled with epoxy resin 9 sealing the semiconductor basic element. When the semiconductor pellet is a thyristor, for example, the same comprises P, N, P layers with a passivation layer G of glass coated on the junction portions 1a between the respective layers, so that the junction portions may be protected in a stabilized state.
In such resin sealed semiconductor apparatus, molybdenum and tungsten having substantially the same coefficient of thermal expansion as that of the semiconductor pellet 1 and having a larger mechanical strength are used as a material for the protectors 2 and 3 against thermal stress. These protectors 2 and 3 protect the semiconductor pellet 1 from thermal stress due to external temperature cycle, the Joule heat caused by a current and the like.
The size of the lower protector 3 is selected to be usually larger than the upper protector 2 on the upper surface of the pellet having a gate, because the larger protector 3 is adapted to be grasped by a jig so that the semiconductor basic element may be held during manufacturing. However, when soldering the protectors 2 and 3 to form a basic semiconductor element by allowing same to pass through a furnace containing hydrogen atmosphere of a predetermined temperature, it is possible that excessive solder might adversely affect the characteristics of the formed basic element. More specifically, since the upper protector 2 is smaller than the upper main surface of the pellet 1, the solder material 10 which may consist of 92.5% Pb, 5% In and 2.5% Ag, for example, interposed therebetween does not flow out to spread beyond the upper surface area of the pellet 1; however, since the lower protector 3 is larger than the lower main surface of the pellet 1, an excessive portion of the solder material 11 interposed therebetween could spread beyond the lower surface of the pellet on the protector 3, thereby causing a small protuberance 11a as shown. Such a protuberance 11a could disadvantageously cause an electrical discharge between the same and the end portion 1a of the pellet.