Patent GB-1149604-A (GB)
Title: Semiconductor devices
Inventor: STEVENSON DONALD VALENTINE
IPC: H01L21/56
Filed: 1968-03-08
Abstract: 1,149,604. Injection moulding. STANDARD TELEPHONES & CABLES Ltd. 8 March, 1968, No. 11413/68. Heading B5A. [Also in Division H1] A semi-conductor element provided with surface electrodes, a set of at least two conductors to be bonded to respective electrodes, and the solder necessary for bonding are arranged to be self-jigging and are placed in an encapsulation mould with the ends of the conductors projecting from the mould cavity. The mould is heated to melt the solder, cooled to allow bonding, and encapsulant is then introduced into the mould at a temperature lower than the melting-point of the solder. In the embodiment described a power transistor die 1, 5 is provided with gold-plated emitter and base electrodes 2, 4 on one surface and with a collector electrode 3 on the other. The solder may be applied by dip-coating-passivation will prevent solder adhering to the non-electrode areas-before the transistor is separated from a large body containing many more. A die 5 is placed between resilient fingers 9, 10 and plate 8 of each contact set, the lead frame having been punched and folded from sheet copper. (If solder pre-forms are used instead of dipping only one need be applied for the two top electrodes since surface tension together with the passivation will ensure that the solder does not short between the two electrodes). The lead frame and its associated dice are then placed in the encapsulation mould (the lower half 17 only is shown) in which the plates 8 are pressed against the roof of the upper half of the mould by splines 27 on the spigots 23, 24 and the studs 20 engage the holes 14. The holes left in the encapsulation by the spigots are bolt holes for operational use in fastening the heat sink plate 8 to support bodies. After closure of the mould air is flushed out by nitrogen through ducts 30-37, the nitrogen replaced by hydrogen or forming gas, and the mould heated to effect soldering. After soldering the cavities are flushed with nitrogen and the ducts sealed by half raising ejector posts within ducts 30-33. The soldering heat causes annealing of the leads. Encapsulant is then injected from a cylinder 38 in the top half of the mould and allowed to harden. (Gas escapes along scratches provided in the mating faces of the mould). The mould may be provided with water ducts so that it may be rapidly cooled before this stage, though the leads then would not be fully annealed. The mould cavities are sealed one side by the strip 12 of the electrode frame (in the arrangement shown this allows communication between the adjacent cavities so that the number of gas and encapsulant injection ducts is greater than necessary-variants are described for encapsulation of single devices; normally moulds are made for from two to ten devices). After the mould is opened the ejector pins are fully raised and the encapsulated devices removed (a third pin for square ejection is provided in each cavity but is not shown), encapsulant removed from between the leads in the region between strip 12 and the plate 8, and surplus metal frame trimmed off to leave the individual transistors.