The invention relates to a method for encapsulating electronic components mounted on a carrier with controlled gas pressure as according to the preamble of claim 1. The invention also relates to a device for performing such a method.

In the encapsulation of electronic components, more particularly the encapsulation of semiconductors mounted on a carrier (such as for instance a lead frame), use is made of the so-called "transfer moulding process". The carrier with electronic components is herein clamped between mould parts such that mould cavities are defined around the components for encapsulating. Liquid encapsulating material is then introduced into these mould cavities, after at least partial curing of which the mould parts are moved apart and the carrier with encapsulated electronic components is removed. The feed of encapsulating material usually takes place by means of one or more plungers with which pressure can be exerted on a supply of encapsulating material. The plunger is displaceable in a housing into which the encapsulating material is also carried. The encapsulating material is usually placed in the mould in a non-liquid state in the form of a pellet, in the form of a package enclosed with film material, or in the form of a granulate. It is however also possible to introduce the encapsulating material in liquid form. The encapsulating material normally consists of a thermocuring epoxy or resin incorporating a filler. The plunger exerts a pressure on the encapsulating material which is simultaneously heated, as a result of which heating the encapsulating material, to the extent it is not already liquid, does become liquid. In response to the pressure applied by the plunger the liquid encapsulating material flows to the heated mould cavity and fills it with encapsulating material. For the purpose of displacing the encapsulating material, the encapsulating material is heated, after which it cures at least partially in the heated mould cavity as a result of chemical bonding (also referred to as cross-linking). In order to increase the quality of the encapsulation it is possible to apply a determined underpressure (i.e. a gas pressure lower than the ambient air pressure) in the mould cavity before starting feed of the encapsulating material. The mould cavity is here usually brought to underpressure through ventings which enable the discharge of gases during filling of the mould cavity. It is of great importance to completely fill the mould cavity with encapsulating material and to limit the feed of encapsulating material to the mould cavity.

Japanese patent 05-259652 discloses a manufacturing system for thin-film multilayer substrate that enables the filling of narrow gaps between wiring conductors with resin. Before moulding a moulding cavity is evacuated by a vacuum pump. Subsequently the inside of the cavity is filled and brought back to atmospheric pressure by interrupting the evacuation and an inactive gas is introduced such that the pressure in the cavity is set to 5-10 kg/cm . As a volatile constituent of the resin will then not vaporize, the occurrence of voids will be suppressed.

The present invention has for its object to provide a method and device for

encapsulating electronic components, wherein the chance of leakage of encapsulating material is limited, which can also enable the application of increasingly thinner liquid encapsulating materials.

For this purpose the invention provides a method for encapsulating electronic components mounted on a carrier as according to the preamble of claim 1, with the feature that the pressure in the venting connecting to the mould cavity is increased to an overpressure after the liquid encapsulating material has reached the venting connecting to the mould cavity. The overpressure can also be increased to for instance 5, 8, 10, 20 or 30 atmosphere. Another option for limiting the overpressure is to maximize it at a pressure corresponding to the pressure on the encapsulating material in the venting connecting to the mould cavity. The advantage of arranging an overpressure in the venting after the mould cavity has been completely filled is that the possibility of encapsulating material penetrating further into the venting is thus limited. It is precisely in the case of very thin liquid encapsulating material (which is being used increasingly in practice to enable forming of thinner packages and to enable filling of very narrow openings between an electronic component and a carrier) that far-reaching penetration of encapsulating material into the venting can be prevented. There is therefore less curing encapsulating material in the venting (venting bleed), this resulting in an of the carrier which is less contaminated with encapsulating material. An edge remaining free of encapsulating material moreover has the advantage that after the encapsulating process the carrier can be more easily transported with transport systems which engage precisely on the side edge, such as for instance rail conveyors. Not only does this have the advantage that the carrier remains free of cured encapsulating material at the position of the suction opening, the chance of contamination and/or blockage of the venting recessed into a mould part is also reduced. The moment in time at which the overpressure is applied is other than in the prior art. According to the prior art, the overpressure is already applied when the encapsulating material enters the mould cavity (the passage of the flow front through the gate), with the result that due to the overpressure, and also due to the vapour pressure of the encapsulating material, the encapsulating material is compressed during the filling of the mould cavity with the result that no or fewer voids occur in the encapsulation. The present invention relates to a completely different moment of increasing the gas pressure, i.e. only after the liquid encapsulating material has reached the venting connecting to the mould cavity. The mould cavity is then thus already completely filled with encapsulating material, this expressly differing from the prior art. The insight according to the present invention is that, in order to increase the chance of (or even be able to guarantee) a complete filling of the mould cavity with encapsulating material, it is desirable that the venting connecting to the mould cavity is brought to overpressure only at the moment the encapsulating material has already partially entered the venting connecting to the mould cavity. The chance of leakage of encapsulating material through the venting (flash via the venting) is thus decreased or even reduced to zero. Already applying an

overpressure during filling of the mould cavity with encapsulating material is precisely what must be prevented according to the present invention because - at variance with the prior art teaching - it is precisely then in the case of determined types of encapsulating material that there is an increasing chance of voids occurring in the encapsulating material. It is moreover desirable here that the gas pressure in the venting does not become greater than the pressure on the encapsulating material; the pressure exerted in the venting by the gas will thus not be transmitted to the mould cavity by the encapsulating material. The gas pressure in the venting will thus not affect the encapsulating process in the mould cavity. Pressures on the encapsulating material are usually 60 to 90 atmosphere (6-9 mPa), while the overpressure in the venting is advantageously 3 to 30 atmosphere (0.3-3 mPa).

During filling of the mould cavity with encapsulating material as according to processing step D) it is desirable to bring the gas pressure in a venting connecting to the mould cavity to an underpressure lower than 1 atmosphere. A lower gas pressure can advantageously be applied in practice of for instance between 0 - 0.5 atmosphere [0 - 50,000 Pa], or even lower, of between 0 - 0.1 atmosphere [0 - 10,000 Pa]. It will in practice generally be possible to refer to (substantially) a vacuum. The advantage of using an underpressure during processing step D) is that the chance of voids in the encapsulating material (which is evidently undesirable) can thus be reduced.

In order to accurately determine the moment at which the encapsulating material reaches the venting it is possible to measure the pressure on the encapsulating material, wherein a recorded increase in the pressure on the encapsulating material forms a control signal. The increase in the pressure on the encapsulating material does after all imply that the liquid encapsulating material is encountering an increasing resistance, this being the case when the mould cavity is filled and the encapsulating material can only escape via the venting (or the ventings) which has (have) a much smaller cross- section than the cross-section of the mould cavity. The increasing pressure on the encapsulating material will thus be a control signal for actively increasing the pressure in the venting connecting to the mould cavity. The pressure on the encapsulating material can be determined by measuring at a single central location the pressure with which the encapsulating material is urged into a plurality of mould cavities. This is for instance possible using a direct measurement with a sensor adjacently of a mould cavity (load sensor) or by an indirect measurement (for instance in the form of a measurement of the pressure exerted by a plunger with which the encapsulating material is displaced to the mould cavity enclosing the electronic component). An indirect measurement has the advantage, among others, that the sensor does not have to be in contact with the encapsulating material. It is thus possible for instance to measure the force with which a plunger beam is displaced. A plurality of plungers are generally displaced

simultaneously using a plunger beam. The thus recorded increase in the pressure on the encapsulating material can for instance result, with a determined time delay, in active increase of the pressure in the venting connecting to the mould cavity. There is therefore more certainty that the encapsulating material has reached the venting before the pressure in the venting increases. The time delay can for instance ensure that the encapsulating material has entered the venting over a limited distance; it is after all necessary to prevent the pressure in the encapsulating material already building up at a stage when the mould cavity is not yet completely filled with encapsulating material; this could result in an avoidable, incomplete filling of the mould cavity. The time delay can reduce the risk of incomplete filling of the mould cavity.

The present invention makes it possible to also process very liquid encapsulating materials because the chance of leakage (flash, bleed) of very liquid encapsulating material through the venting is limited. It thus becomes possible to process

encapsulating material having a dynamic viscosity during filling of the mould cavity of 2 - 9 Poiseuille ([Pa.s] = [kg/m.s]). It is noted here that the application of encapsulating material of increasingly lower viscosity is desirable so as to thus also enable sufficient filling with encapsulating material of relatively small spaces and spaces with a very limited height (this being a result of the market demand for ever increasing

miniaturization)

It is also possible to combine the present method with arranging a film material on the venting connecting to the mould cavity, wherein the overpressure is exerted on the side of the film remote from the encapsulating material. It is then desirable here for the underpressure to be exerted during encapsulation on the side directed toward the encapsulating material. This prevents the feed channel for the overpressure being contaminated with encapsulating material. The chance of gas being blown in undesired manner into the mould cavity is moreover also reduced in this way. By having the film seal an underpressure connection connecting to the venting when the overpressure is exerted on the film, the venting (which is active earlier in a processing cycle when the mould cavity is brought to underpressure when not yet completely filled) will not become contaminated with encapsulating material either.

The increase of the pressure in the venting can take a menu-controlled form such that it also depends on a number of process variables. The process variables to be entered into the menu can for instance consist of: the type of encapsulating material, the processing temperature, the type of carrier material, the closing pressure of the mould parts, the minimum quality level and so forth.

The present invention also provides a device for encapsulating electronic components mounted on a carrier, comprising: mould parts which are displaceable relative to each other and which in a closed position define at least one mould cavity for enclosing an i

electronic component, feed means for liquid encapsulating material connecting to the mould cavity and at least one venting connecting to the mould cavity for suctioning gas from the mould cavity, wherein the device is also provided with a pressure conduit connecting to the venting, whereby a gas can be introduced into the venting at an overpressure of at least 3 atmosphere. Using such a device the advantages can be realized as already described above in respect of the method according to the invention. It is also noted here that this device makes it less necessary to take measures to give the size of (a part of) the venting a variable form in order to thus combine a good suction capacity with a reduced chance of leakage through the venting. A venting with a passage of variable dimension is a structurally complex construction which is therefore expensive and sensitive. The present invention makes such complex measures unnecessary under most conditions. This means that the present invention makes fewer requirements in respect of the maximum size of the suction opening. With a larger size of the suction opening that is now possible owing to the present invention a better suction is possible and the dynamic control behaviour of the pressure in the mould cavity can optionally be realized better than is possible with a smaller suction opening. The feed means are usually provided with at least one plunger and the pressure conduit can connect to a (gas) pressure source as desired, for instance a compressed air system, a pressurized gas container or a booster.

The device can also be provided with a control with which the pressure in the venting can be controlled between an overpressure and an underpressure. This underpressure will here desirably be less than 0.5 atmosphere, preferably less than 0.1 atmosphere. The invention can also be provided with a control with which this underpressure can be actively controlled. Active control is understood here to mean a dynamic control option such that more and less underpressure is present as desired in the mould cavity during the feed of encapsulating material. In this way the encapsulating process can possibly be better controlled. These measures contribute toward an improved quality of the encapsulated electronic components.

The device can also be provided with at least one valve connecting to the venting and controllable by the control. The venting can thus be closed as soon as an overpressure is applied in the venting through the pressure conduit. Contamination of the underpressure system is inter alia thus prevented. For the purpose of detecting the pressure on encapsulating material during the feed of the encapsulating material, the device is desirably provided with a pressure sensor connecting to the mould cavity and connecting to the control. In a specific embodiment variant the feed means for encapsulating material are provided with a plunger which also comprises a pressure sensor connecting to the control for detecting the pressure on encapsulating material during the feed of the encapsulating material. With these measures the pressure on the encapsulating material can be detected in real time during the encapsulating process, whereby the overpressure can be applied to the venting at a suitable moment in time. In yet another embodiment variant the plunger is provided with a position detector, this position detector connecting to the control. Provided the starting position is known, the position of the plunger forms an accurate measure for the filling level of the mould cavity. The position of the plunger can be readily determined by means of position detectors. External measurement (i.e. a measurement outside the mould cavity) is thus representative of the filling level of the mould cavity.

The device is provided with a pressure source connecting to the venting for generating a medium pressure of at least 3 atmosphere in the venting connecting to the mould cavity. Such a pressure source can for instance be formed by a pressure conduit, a gas bottle, a booster or any other source with which a gas pressure of at least 3 atmosphere can be generated.

The present invention will be further elucidated on the basis of the non-limitative exemplary embodiments shown in the following figures. Herein:

figure 1 shows a schematic view of a device according to the invention, figure 2 shows two graphs with the variation of the pressure on the

encapsulating material and the pressure in the suction chamber, and

figures 3 A and 3 B show a part of a variant of an encapsulating device during successive stages of a method for encapsulating electronic components according to the invention.

Figure 1 shows a device 1 for encapsulating electronic components 3 mounted on a carrier 2. Carrier 2 is clamped between two mould parts 4, 5 such that electronic components 3 are here received in a mould cavity 6 which is left clear in top mould part 4. An encapsulating material 8 which has become liquid due to heating can be urged through a runner 9 to mould cavity 6 by means of a plunger 7. In order to now obtain the proper filling of mould cavity 6 with encapsulating material 9, gas (generally air, possibly with gases released from encapsulating material 9) can be suctioned via an suction chamber 11 and a venting 12 to the environment (see arrow Pi) by activating a pump 10 during the filling. Device 1 is further provided with an intelligent control 13, here shown in the form of a computer, which can preferably be programmed with a plurality of menus in order to provide, subject to determined parameters to be entered, a control behaviour of the underpressure in mould cavity 6 appropriate for the relevant conditions.

A valve 14 controllable by intelligent control 13 is placed in venting 12, which valve can close venting 12 on the side directed towards mould part 4. Device 1 is further provided with a source 15 for applying gas pressure (here a gas bottle) which is connected to suction chamber 11 via a pressure conduit 16. This suction chamber 11 is connected to mould cavity 6 by a narrow passage opening 17, which is also referred to as venting. As soon as encapsulating material 9 has completely filled mould cavity 6 and reaches venting 17, intelligent control 13 will close controllable valve 14 and also open a control valve 18 for the purpose of opening pressure source 15. The pressure gas from gas bottle 15 will then enter suction chamber 11 and the part of venting 17 still free of encapsulating material 9 at a pressure of at least 3 atmosphere. Flow of encapsulating material 9 further into venting 17 is thus counteracted, this being precisely the sought-after effect. In order to determine the moment at which pump 10 must be stopped and (possibly a short time later) the moment at which the pressure in the suction chamber must be increased by feeding gas through pressure conduit 16, there is placed in plunger 7 a pressure sensor 18 which connects via a signal line 19 to intelligent control 13. As soon as mould cavity 6 is filled with encapsulating material 9 and the encapsulating material reaches venting 19, the resistance encountered by the encapsulating material will increase, with the result that the pressure on encapsulating material 9 increases. The increase in pressure on encapsulating material 9 is detected by pressure sensor 19, with the result that intelligent control 13 can control the position of valves 14, 18 and the operation of pump 10. Figure 2 shows two graphs 20, 21 which represent the parallel progression of the pressure in the course of time for respectively the pressure exerted on encapsulating material 9 by a plunger 8 (Puansfer, graph 20) and the pressure in suction chamber 11 (Pvenung, graph 21). The upper graph 20 shows that the pressure Ptr _ansfer exerted on encapsulating material 9 remains limited for a longer time during filling of mould cavity 6 with encapsulating material 9 and then suddenly increases sharply as soon as mould cavity 6 is completely filled with encapsulating material 9. Second graph 21 shows that the pressure P _vent ing is expressly lower than 1 atmosphere (underpressure) during filling of the mould cavity, but that as soon as the pressure Ptransfer exerted on encapsulating material 9 suddenly begins to increase (as soon as the mould cavity is filled with encapsulating material), the pressure in suction chamber 11 (Pventing) transposes rapidly to an overpressure of several times the atmospheric pressure. Figure 3 A shows a part of a variant of an encapsulating device according to the invention, wherein a venting 30 is separated from a pressure conduit 31. Another change made here is that a film layer 33 is arranged in a top mould part 32. Not only does this film layer 33 shield wall 35 of a mould cavity 36 from encapsulating material, venting 30 is hereby also shielded as will be elucidated hereinbelow. As soon as encapsulating material 37 reaches a venting 38, the suction through venting 30 will be discontinued. Gas will then be blown in through pressure conduit 31 , as shown in figure 3B, whereby film 33 releases from top mould part 32 at the position of pressure conduit 31. Film 33 here forms an additional resistance to encapsulating material 37 penetrating further into venting 38. An additional advantage of film 33 is that it shields venting 30 from the pressure gas fed through pressure conduit 31 and also from encapsulating material 37. Venting 30 is thus less likely to become contaminated than if film 33 were not applied.