Patent GB-861581-A (GB)
Title: Improvements in and relating to semi-conductor devices
IPC: H01L21/00
Filed: 1957-05-29
Abstract: 861,581. Semi-conductor devices. PHILCO CORPORATION. May 29,1957 [June 8, 1956], No. 17043/57. Class 37. Method of providing, in, for example, germanium or silicon PNP and NPN transistors, diodes, &c., a connection of low thermal impedance between a region 22 of recrystallized semi-conductive material underlying a metal contact 14 integral therewith and a body 30 of low thermal impedance having an end surface and a surface extending away from the end surface which is readily wettable by the metal of the contact 14 when in liquid form, comprises converting the metal of the contact 14 to liquid form by heating without melting the semi-conductive material underlying the contact 14, holding the end surface of the body 30 against the contact 14, whereby the metal of the contact is caused to flow away from the recrystallized region and on to the surface of the body 30, advancing the body, e.g. under its own weight, into substantial contact with the recrystallized region as the contact metal flows away therefrom without displacing the contact metal on to the semiconductive material surrounding the recrystallized region, and cooling the body 30 and the recrystallized region to solidify the contact metal and bond the body to the recrystallized region. The body 30, which is of copper and may be coated with nickel, is provided with a truncated-cone stud 32 the end of which is similar in configuration to but slightly smaller than the recrystallized region. The end and sides of the stud 32 and a small portion of the adjoining surfaces of the body 30 are provided with a flux such as zinc chloride and with a coating 48 containing the same metal as the collector contact 14 and preferably another metal serving to lower its melting-point, and in which the metal of the contact 14 is readily soluble. In a specific example the contact 14 is of indium and the stud 32 and adjacent portions are provided with a thin coating 48 of indiumcadmium alloy in eutectic proportions. A metal pellet 49 of the same materials as the coating 48 may be provided on the end of stud 32 and in contact with the contact 14. The stud 32 is preferably exactly coaxially aligned with the recrystallized region by means described. The collector lead 38 is soldered to the body 32 with a lead-tin solder. The entire assembly is immersed in a bath consisting of propylene glycol and ¢% of indium trichloride and maintained at a temperature of between 160 and 180 C. until contact 14 melted and stud 32 moved against the recrystallized region. The assembly is then removed and allowed to cool at room temperatures. A nickel ribbon 80 is soldered to the emitter contact 12 with indium-cadmium eutectic solder, the other end of the ribbon being spot-welded to the lead wire 40. A second ribbon 82 is soldered to the base tab 28 and spot-welded at its other end to the lead wire 42. The leads 38, 40 and 42, which are of Dumet metal, traverse a glass bead 36 provided with a steel eyelet 34. A nickel sealing ring 46 is soldered to the body 30 and the eyelet 34. The assembly is then cleaned by electrolytic etching in a stream of sodium hydroxide, followed by rinsing with pure water. The covering member 86, preferably of nickelplated copper, is then cold-welded to flange 47 in atmosphere of dry air. The resultant transistor is characterized by a temperature drop of about 0.6 C. per watt dissipated between the collector junction and the body 30, providing a maximum collector dissipation of at least 55 watts in ambient temperatures as high as 65 C., and of at least 110 watts in ambient temperatures of about 30 C. The method of the invention can be applied to the connection of a body of lowthermal impedance to the emitter recrystallized region and connection of bodies of low thermal impedance to both collector and emitter recrystallized regions can be effected simultaneously.