IE50182B1 - Process of encapsulating semiconductor devices by injection moulding - Google Patents

Abstract

Electrical components, e.g. semiconductor devices, are encapsulated by an injection moulding process. The devices (5), each provided with a cover (4) and mounted on a bandolier (3) are fed into a mould (2). Plastics material is injected from one or two injection points and at a specified angle in relation to the bandolier such that the pressures acting on the device (5) are substantially uniform in all directions. <IMAGE>

Description

The present invention relates to a process of encapsulating semiconductor devices by injection moulding.

It is general practice to provide electrical components and particularly semiconductor devices, such as diodes, transistors, integrated circuits, with a protective plastic encapsulation. In so doing, the encapsulation is produced by moulding the devices with a thermo-setting plastic in a moulding press. In this process known as transfer moulding process, the plastics material, e.g. in the form of mouldings, is inserted in the moulding press, liquified by heat and, with the aid of a force plug, pressed into the moulds in which sections of supporting bands (frames) with the semiconductor devices to be encapsulated being arranged thereon, are accommodated. The process is performed discontinuously. As the injection moulding material has a low viscosity steps must be taken for sealing the mould cavity. Moreover, there is required a reworking of the encapsulated semiconductor devices either chemically or mechanically for removing flash fins. In order to facilitate the removal of the encapsulation from the mould cavity, a flow agent such as wax is often added to the encapsulating material, - 2 50182 which of course, is not conductive to the quality of the encapsulating material. Up to now, these drawbacks had to be accepted because encapsulations of thermoplastic material have so far proved unsuitable in practice. The reason for this is to be seen in the fact that the viscosity of thermoplastic materials is relatively high so that injection moulding had to be carried out under high pressure (approximately seven times the pressure required for press moulding), so that injection moulding can easily cause the semiconductor devices and their connecting wires extending to the connecting conductors to be damaged even in cases where the semiconductor devices have been previously protected with a covering of quick-curing synthetic resin varnish, silicone resin or chemically high-purity photoresist. In order to enable the use of thermoplastic materials it has been proposed in the German Offenlegungsschrift (DE-OS) 28 04 956 to add an expanding agent, so that practically there is produced an encapsulation of a foamed thermoplastic material, with the individual semiconductor devices, however, previously also being protected with a special covering. Experience has shown, however, thatvthe foamed thermoplastic encapsulation is not of the expected quality, and in addition thereto, there also has to be considered the contamination by foreign ions caused by the use of foaming agents.

It is the object of the invention, therefore, to provide a process by which semiconductors can be coated with a thermoplastic material.

According to the invention there is provided a process for encapsulating semiconductor devices disposed on a continuous flat support band, comprising placing each said device and adjacent portions of the support band in a mould, and injecting a thermoplastic material into the mould so as to encapsulate the device and the adjacent portions of the support band, wherein the plastic material is injected along such a direction relative to the mould and to the support band that the device is subjected to substantially isostatic pressure - 3 50182 conditions whilst the thermoplastics material is injected.

Both the injection point and the injection angle are chosen so that the stream of material, when the injection moulding material enters into the cavities, is so directed that the crystal support as well as the electric wire connections are neither pushed out of their original position, nor damaged. When filling the cavity the forces acting upon the lead frame from both sides shall be as equal as possible. When properly selecting the two parameters it is possible to desist from covering the system crystalelectric connecting wire-connecting legs.

Another parameter is to maintain an exact temperature in the proximity of the gate mark, because owing to the small overall height of the semiconductor devices, the gate mark must be very small, so that there is a considerable danger prior to the filling of the cavity, of the material solidifying at this point. One reason for adhering to the small gate mark cross section is to be seen in the easy pulling of the gate (sprue) from the device, i.e. without any considerable mechanical pressure, which is made possible thereby, and which is necessary in the case of a fully automatic production process.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figures 1 and 2 show longitudinal sections taken through injection chambers with the semiconductor device positioned therein on a support band; and Figure 3 is the top view on to part of a supporting bandolier with the semiconductor device to be encapsulated positioned thereon.

The reference numeral 1 either with or without an apostrophe, refers to an arrow indicating the direction of the main stream of the injection moulding material, the reference numeral 2 indicates the cavity to be filled by injection - 4 50182 moulding, 3 indicates the supporting band and integral lead frame with the semiconductor device 5 thereon which is provided with the cover 4, while Figure 1 shows an arrangement in which the injection is carried out in the direction as indicated by the arrow 1 under an angles of about 65°, typically 60° to 70°, on one side past the edge of a supporting band 3 (see also Figure 3). In the course of this, the band 3 is fed in horizontally. Both the injection angle The injection point P may also be chosen so that the injection angles lies in the plane of the lead frame extending parallel in relation to the direction of movement of the supporting band 3, with the injection being effected through between the connecting legs of the frame contained in the band 3 (see Figure 3, arrows 1').

Figure 2 shows an arrangement in which the injection is effected from the side towards the band 3.

Accordingly, the injection point P is in the plane of the support band, and the injection angle ot, when referred to the plane of the supporting band 3, is zero. The narrow side of the bandolier offers only a small working surface; it divides the stream of material into two halves encapsulating the semiconductor devices 4 from both sides.

Figure 3 is the top view on to a part of a band with the semiconductor device 4 to be encapsulated, positioned thereon. The arrows 1 and 1* indicate the directions of flow of the injection moulding material referred to in connection with Figure 1. The reference numeral 6 indicates the terminals of the semiconductor device with the connecting legs.

The coating of the individual semiconductors secured to the band 3 is carried out, independently of the shape as explained with reference to the drawings, in a conventional type of injection moulding unit, e.g. in a heating (hot) channel mould with the feed being so designed as to enable a continuous introduction of the supporting band 3 into the mould cavity. This mould itself is designed in accordance with the intended way of injection as described, for example, in connection with Figures 1 or 2 hereinbefore.

The injection cycle is performed as follows: With the aid of a peripheral mechanism the supporting band is placed below the injection moulding unit and is thus simultaneously surrounded by the parts of the mould cavity, the transfer (pot) plunger is lowered and, via the injection opening, fills the cavities. Thereupon the mould cavity is reopened, the bandolier is further transported by a certain distance, and the entire process is restarted.

In order to supply the heat required by the heating channel mould in an optimum way, it is necessary to distribute the heat to as many as possible cartridge heaters, so as to ensure a uniform distribution of temperature.

The temperature sensor should be arranged within the area of the greatest accumulation of heat.

The process permits a fully automatic encapsulation of semiconductor devices by injection moulding and no longer requires either a chemical or mechanical - 6 50182 reworking. The output rate is much higher than with the hitherto employed (transfer-moulding) process. Moreover, when correspondingly dimensioning the heating channel, the gate mark system, as well as the number of housing cavities, the consumption of material can be kept at a substantially lower level, and in addition to the foregoing, it should be noted that the scrap of injection moulding material can be reworked in the course of the process.

Claims (7)

1. A process for encapsulating semiconductor devices disposed on a continuous flat support band, comprising placing each said device and adjacent portions of the support band in a mould, and injecting a thermoplastics material into the mould so as to encapsulate the device and the adjacent portions of the support band, wherein the plastics material is injected along such a direction relative to the mould and to the support band that the device is subjected to substantially isostatic pressure conditions whilst the thermoplastics material is injected.

2. A process as claimed in Claim 1, wherein the injection angle is from 60 to 70° relative to the support band, and wherein the injection point is so positioned that the main flow of material sweeps past the respective component to be encapsulated, and is deflected by the side walls of said cavities lying in the direction of flow.

3. A process as claimed in Claim 1, wherein the injection direction is in the plane of the support band.

4. A process as claimed in Claim 1 or 2, wherein that said injection is carried out from two sides.

5. A process as claimed in any one of Claims 1 to 4, wherein the injection moulding material is a polycarbonate or polybutylene terephthalate. - 7 50182

6. A semiconductor device encapsulation process substantially as described herein with reference to the accompanying drawings.

7. A semiconductor device encapsulated by a process as claimed in any one of the preceding claims.