WO2009048330A1

WO2009048330A1 - Method and device for encapsulating electronic components with portioned liquid encapsulating material

Info

Publication number: WO2009048330A1
Application number: PCT/NL2008/050643
Authority: WO
Inventors: Albertus Franciscus Gerardus Van Driel; Joannes Leonardus Jurrian Zijl
Original assignee: Fico B.V.
Priority date: 2007-10-12
Filing date: 2008-10-09
Publication date: 2009-04-16
Also published as: NL2000930C2

Abstract

The invention relates to a method for encapsulating electronic components placed on a carrier, wherein non-portioned encapsulating material, which is first melted to form pellets before it is employed for the encapsulation, is fed to an encapsulating device. The invention also relates to a device for encapsulating electronic components in accordance with this method.

Description

METHOD AND DEVICE FOR ENCAPSULATING ELECTRONIC COMPONENTS WITH PORTIONED LIQUID ENCAPSULATING MATERIAL

The invention relates to a method for encapsulating electronic components placed on a carrier, wherein non-portioned encapsulating material is fed to an encapsulating device. The invention also relates to a device for encapsulating electronic components in accordance with this method.

For the encapsulation of electronic components, more particularly the encapsulation of semiconductors mounted on a carrier (such as for instance a lead frame), use is usually 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 the encapsulating material the mould parts are moved apart and the carrier with encapsulated electronic components is then removed. The feed of encapsulating material normally 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 plunger exerts a pressure on the encapsulating material which is simultaneously heated, as a result of which the encapsulating material becomes liquid. As a response to the pressure applied by the plunger, the liquid encapsulating material flows through runners to the heated mould cavity and fills it with encapsulating material which then at least partially cures (also referred to as cross-linking) as a result of chemical bonding.

For the purpose of encapsulating the electronic components use is normally made of a theπnocuring encapsulating material (also referred to as compound, epoxy or resin); a filler is usually also incorporated herein. The encapsulating material can for instance comprise a matrix of an epoxy resin with a filler material such as silicon oxide. By heating and placing the encapsulating material under pressure it is activated such that, once it has been displaced to mould cavities connecting onto the electronic components, it at least partially cures within a short period of time (usually 0.5 to 1 minute) such that the mould can be opened and the carrier with encapsulated components, including the remaining cured encapsulating material situated in, among other locations, runners of the mould, can be removed.

The present invention has for its object to provide a method and a device for increasing the efficiency of encapsulation of electronic components placed on a carrier.

The invention provides for this purpose a method for encapsulating electronic components placed on a carrier, comprising the processing steps of: A) feeding non- portioned encapsulating material to an encapsulating device; B) liquifying the encapsulating material in the encapsulating device; C) solidifying the liquified encapsulating material in portioned quantities; D) placing portioned quantities of encapsulating material in a mould; E) once again liquifying the portioned quantities of encapsulating material placed in the mould as according to processing step D); and F) urging the encapsulating material liquified once again as according to processing step E) into at least one mould cavity recessed into the mould. Non-portioned encapsulating material is understood to mean encapsulating material which is not divided into units such that they can be employed directly (without modification) in the encapsulating process. It is thus possible here to envisage encapsulating material in powder form, granulate, grains, varying sizes, chunks, pieces, larger pellets, slabs, pencils and so forth, or a combination of said possibilities.

By applying this method the encapsulating material is processed in two steps. The raw material which is fed for instance in grain form to the encapsulating device is first of all liquified, and form-retaining portions of encapsulating material are formed therefrom. These form-retaining portions of encapsulating material are then placed in the mould and liquified once again in order to fill the mould cavities therewith. The non-portioned encapsulating material will preferably be liquified by heating, which implies an active supply of heat. It is the case with this method that more freedom is created in the form of the portions of encapsulating material (pellets) which can be applied. It is thus possible for instance to manufacture pellets which cannot be applied according to the prior art due to the shape and/or dimensions. Pellets of fragile form and/or pellets which are difficult to orient can now be formed in a first phase and then introduced into the mould shortly after forming thereof. Particularly envisaged here are pellets which need to be given a less robust form than the known pellets. Small to very small pellets which are for example cylindrical with a diameter smaller than 5 mm and larger than 0.5 mm and/or elongate cylindrical pellets with a diameter to length ratio greater than 1-3 up to a ratio of even 1-20. The diameter to length ratio is preferably greater than 1-5. This makes it possible to introduce the encapsulating material into the mould in more efficient manner than according to the prior art. Smaller and elongate pellets have the advantage that they can be brought more quickly to encapsulating temperature and that the relatively small quantities of encapsulating material also cure more quickly after the encapsulation than larger quantities. It hereby becomes possible to considerably reduce the encapsulation cycle time. Yet another advantage is that in this way larger numbers of pellets can be used to encapsulate electronic components placed on a carrier than in the traditional transfer moulding process, and longer runners thus become unnecessary. With a smaller pellet the liquid encapsulating material can be introduced closer to the mould cavity than heretofore. Envisage here the melting of pellets oriented in matrix structure between mould cavities likewise placed between a matrix structure. The pellets are manufactured in the encapsulating device and it is thus possible to retain the known orientation of the pellets after forming thereof such that they do not need to be repositioned from a random orientation. This simplifies processing of the pellets and moreover reduces the chance of damage to individual pellets, with all the undesirable consequences this entails. In addition to the advantage that smaller pellets and pellets with a larger surface area to volume ratio are easier (quicker) to heat and can be placed closer to the mould cavities, this enabling a shorter cycle time, it is also advantageous that the smaller pellets will result in less consumption of encapsulating material. In addition, there are great process advantages if liquid encapsulating material is fed according to the present invention to the mould cavities placed in matrix structure because the process conditions under which the individual mould cavities are filled can hereby be identical. In mould cavities oriented in matrix form the mould cavities placed closest to the pellets are filled first according to the prior art and mould cavities located at a greater distance are filled later; this results in changing process conditions under which the mould cavities are filled, possibly also with differently encapsulated products. In the method according to the invention every mould cavity can be filled from the same short distance, with the advantage of identical process conditions and identical encapsulated products. The processing steps A), B) and C) can be performed as desired at the same or nearby location at which the processing steps D), E) and F) take place, or even be performed in the same machine or coupled machines. Alternatively, it is however also possible to envisage the processing steps A), B) and C) being performed at a different location altogether (for instance at a supplier) from where the processing steps D), E) and F) take place.

It is typically advantageous if the non-portioned encapsulating material is liquified by being heated briefly (for instance for a period of 5 to 10 seconds) to a temperature of 90-140⁰C₃ optionally to a temperature of a maximum of 180⁰C if this is done very briefly (i.e. for a maximum of several seconds). The temperature range preferably lies however between 100-120⁰C. This is important to prevent the chemical curing (cross- linking) already progressing so far that the encapsulating material can only be liquified again during too short a period to enable proper processing thereof. The period within which the encapsulating material can be processed in liquid form is also referred to as the gel time. For the purpose of solidifying the liquified encapsulating material into portioned quantities the encapsulating material can then be recooled to a temperature of less than 80⁰C. This process can be performed batch-wise. For the manufacture of the pellets (portions) of encapsulating material it is desirable to heat the encapsulating material such that the chemical curing thereof is initiated as little as possible. This while during filling of the mould cavities (so when the encapsulating material is liquified for the second time) a limited viscosity is important and a more rapid curing is moreover desirable. The portioned quantities of encapsulating material placed in the mould are therefore preferably liquified again by heating to a temperature of for instance at least 180⁰C. In respect of the interim solidifying of the encapsulating material into portioned units (pellets), it is of course a prerequisite that these are form-retaining, but that they are not recooled more than necessary. The higher the temperature during the portioned phase of the encapsulating material, the quicker they can be brought to encapsulating temperature and the more efficient use is made of energy.

In yet another preferred variant of the method according to the invention, during feed of the portioned encapsulating material to a mould cavity recessed into the mould the liquified encapsulating material is urged by a plunger directly against the carrier to which the electronic components for encapsulation are connected. Owing to the reduced supply of required heat to the encapsulating material it is possible to dispense with a so- called cull bar (a heated melting plate against which the pellet is pressed by a plunger). A significant advantage hereof is that the contact surface of a press that can be used for encapsulation can be utilized more efficiently; owing to this invention more components can now be encapsulated in an existing encapsulating press. Once again, this becomes possible due to the greater freedom resulting from the invention in respect of the pellet shapes and pellet dimensions to be applied. The short encapsulation cycle time depends on, among other factors, the relatively small quantity of encapsulating material which must be heated, the relatively large surface area along which the encapsulating material can be heated (length to diameter ratio of the portioned pellets) and the relatively limited distance over which the encapsulating material has to be displaced.

It is also desirable that the plunger, after the at least partial curing of the encapsulating material, is also used to exert a force on the cured encapsulating material for the purpose of releasing an encapsulated electronic component. The plunger is in this way also employed as ejector.

The present invention also provides a device for encapsulating electronic components mounted on a carrier, comprising; a container for holding non-portioned encapsulating material; heating means for liquifying non-portioned encapsulating material coming from the container; at least one cavity for receiving and allowing the liquid non- portioned encapsulating material to solidify; mould parts which are displaceable relative to each other and which in a closed position define at least one mould cavity for enclosing an electronic component mounted on a carrier; feed means for liquid encapsulating material connecting to the mould cavity; and displacing means for displacing portioned quantities of encapsulating material to the feed means for encapsulating material connecting to the mould cavity. Using such a device the method as described above can be performed in order to thus realize the advantages likewise stated above. The cavity(cavities) for manufacturing the form-retaining quantities of encapsulating material (pellets) and the mould parts displaceable relative to each other can be combined in a single machine frame, although it is on the other hand also possible to only couple them by means of for instance the displacing means for the encapsulating material. The displacing means for the encapsulating material can optionally also comprise a buffer for holding a (limited) quantity of portioned encapsulating material. A limited quantity of portioned encapsulating material must be envisaged as a quantity of a maximum of 2-4 times the quantity required for a single encapsulating operation. In order to adjust the capacity of the different processing steps to each other, it may also be desirable to perform one or more processing steps plurally and in parallel. Most desirable here is that the capacity-limiting operation (generally the encapsulating of the electronic components between the mould parts) forms the most costly part of the device.

Such a buffer must also for this reason be of limited size because the chemical curing of the encapsulating material begins after the portioning. This despite the temperature of the portioned encapsulating material being markedly lower during the transfer than during feed of the encapsulating material to the mould cavities.

The cavity for receiving and allowing the liquid non-portioned encapsulating material to solidify can be provided in a preferred variant with cooling means with which the liquid encapsulating material can be actively cooled. This technical measure speeds up the throughput time in the forming of the portioned quantities of encapsulating material and slows down the (chemical) curing of the encapsulating material, which lengthens the period within which it has to be processed.

In yet another embodiment variant the cavity for receiving and allowing the liquid non- portioned encapsulating material to solidity is provided with a plurality of pellet- forming spaces, wherein the relative orientation of the pellet-forming spaces corresponds to the relative orientation of the receiving spaces for portioned pellets of encapsulating material forming part of the feed means for liquid encapsulating material. The relative orientation of the portioned pellets required during encapsulation can thus advantageously already be obtained during the manufacture of these portioned pellets. It is however desirable here that the relative orientation of the portioned pellets is retained during the transfer thereof.

In yet another preferred embodiment of the device according to the invention at least one of the displaceable mould parts is provided with feed means for encapsulating material, these feed means comprising plungers directed toward a side of a mould part facing an opposite mould part. The encapsulating material can thus be pressed directly against the carrier; a cull bar is unnecessary; this makes the encapsulating device more efficient and there is less chance of contamination of the mould parts resulting from the liquid encapsulating material.

When the plungers themselves are oriented directly toward the at least one mould cavity for enclosing an electronic component mounted on a carrier, the plungers can be employed, after the at least partial curing of the encapsulating material in the mould cavity, as ejectors for the purpose of pressing the encapsulating material out of a mould part. The plungers have thus acquired a dual function; the feed of encapsulating material and the release of the encapsulated components.

hi an alternative embodiment variant the displacing means for displacing portioned quantities of encapsulating material to the feed means for encapsulating material connecting to the mould cavity and the at least one cavity for receiving and allowing the liquid non-portioned encapsulating material to solidify are combined in a single assembled construction part.

It is also advantageous for the displacing means for displacing portioned quantities of encapsulating material to the feed means for encapsulating material connecting to the mould cavity to comprise at least one manipulator with which portioned quantities of encapsulating material can be displaced from the cavity in which they are manufactured to the feed means for liquid encapsulating material connecting to the mould cavity.

Using such a manipulator the pellets (form-retaining parts of encapsulating material) can be gripped, displaced and set down in the mould. Envisage here for instance gripper heads with one or more grippers or vacuum cups.

The 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 perspective view of the encapsulating device according to the invention; figure 2 is a schematic perspective view of an alternative embodiment variant of a part of the device shown in figure 1; and figures 3A-3F show embodiment variants of form-retaining portioned parts of encapsulating material as can be applied in the encapsulating device according to the invention. Figure 1 shows an encapsulating device 1 provided with a container 2 for holding non- portioned granular encapsulating material 3. Arranged around a dispersion outlet 5 on the underside of container 2 are heating means 4 with which the granular encapsulating material 3 is converted into liquid encapsulating material 6 which can be discharged from dispersion outlet 5. Also arranged in dispersion outlet 5 are cooling channels 7 with which the dispersion outlet 5 can be cooled such that the outflow of liquid encapsulating material 6 can be stopped. Encapsulating device 1 is also provided with a multiple cavity 8 provided with a matrix of pellet-forming spaces 9 into which the liquid encapsulating material 6 is carried by displacing dispersion outlet 5 and/or cavity 8 in X and Y direction (see arrows Pi). In the filled pellet-forming spaces 10 the liquid encapsulating material 6 will solidify into portioned form-retaining encapsulating parts 11 , referred to below as pellets 11. Pellet-forming spaces 9, 10 are formed by continuous openings in a moulding plate 12, on the underside of which is positioned an ejector plate 13. Placed on ejector plate 13 are pins 14 by which the pellet-forming spaces 9, 10 are sealed on the underside. Ejector plate 13 is vertically displaceable (see arrow P₂) by a drive 15 (ejecting cylinder), whereby after solidifying of the encapsulating material in pellet-forming spaces 9, 10 the portioned pellets 11 can be pushed upward by pins 14 (ejector pins) until they protrude at least partially above moulding plate 12.

Once pellets 11 have been pushed up by ejector pins 14 of ejector plate 13, they can be gripped by a manipulator head 16 which also forms part of encapsulating device 1. Manipulator head 16 is displaceable (see arrows P3) for this purpose in at least two directions (at least the X and Z directions). For this purpose manipulator head 16 is provided on the underside with grippers 17 with which the individual pellets 11 can be gripped in the matrix orientation in which they come out of moulding plate 12. Grippers 17 can be given a mechanical form, although in the embodiment variant shown in this figure grippers 17 are embodied as vacuum cups connecting to a channel system 18 with which underpressure can be generated and ended.

Once manipulator head 16 has gripped pellets 11 , head 16 is displaced to a press 18 likewise forming part of encapsulating device 1. This press 18 comprises two mould parts 19, 20 displaceable relative to each other. The mutual displacement of mould parts 19, 20 is realised as according to arrow P₄ by a schematically represented drive 21. There are a number of drive mechanisms for presses known in the prior art which can be applied in encapsulating device 1. The lower mould part 20 is provided with a moulding plate 26 with moulding spaces 22, between two of which a plunger space 23 is located in each case in which a plunger 24 is movable. Plunger spaces 23 and plungers 24 are relatively positioned in the matrix pattern in accordance with the matrix of the pellet-forming spaces 9 and the matrix orientation of grippers 17. Pellets 11 are placed in plunger spaces 23 by manipulator head 16, after which they are liquified by heating means not shown in this figure. The lower mould part 20 is provided with a plunger plate 25 which bears plungers 24. By moving plunger plate 25 toward moulding plate 26 using a drive 27 (see arrow P₅) the plungers 24 press the liquid encapsulating material to moulding spaces 22.

Figure 2 shows a part of an encapsulating device embodied in a manner other than the part with corresponding functionality of encapsulating device 1 shown in figure 1. Figure 2 shows particularly an alternatively embodied container 30 for holding large pellets 31; in this situation the pellets 31 form the non-portioned encapsulating material for the encapsulating process. Container 30 connects onto a melting block 32 which can be heated for the purpose of liquifying non-portioned encapsulating material coming from container 30. Melting block 32 is provided with a multiple channel system 33 for dispensing the liquid non— portioned encapsulating material in a plurality of flows. From the multiple channel system 33 in melting block 32 liquid encapsulating material is thus carried into pellet-forming spaces 9, 10 of moulding plate 12. In the filled pellet- forming spaces 10 the liquid encapsulating material 9 will solidify into portioned form- retaining encapsulating parts (portioned pellets). After the solidification into pellets the ejector pins 14 of ejector plate 13 are pressed upward as according to arrow P₂ by ejecting cylinder 15 such that the pins can be readily gripped from the top side.

Figures 3A-3C show a number of portioned pellets 40, 41, 42 with shapes which in prior art encapsulation could not be applied or could only be applied with very many problems. Pellet 40 of figure 3 A is elongate such that under normal conditions it would be very susceptible to damage. Pellet 41 according to figure 3B is likewise very elongate but has in addition a projecting head 43, whereby pellet 41 would be very difficult or impossible to position in a traditional encapsulating device. This is also the case for pellet 42 of figure 3C, which is provided with two protruding edge parts 44 with which pellet 42 can connect to a profiling of a moulding plate 26.

Figures 3D-3F also show portioned pellets 45, 46, 47. Pellet 45 in figure 3D is again elongate but is also provided with a triangular cross-section. Pellet 46 of figure 3E is provided with a square cross-section and also with two protruding edge parts 48 for a precise positioning and a good connection to a moulding plate 26. Figure 3F shows a portioned pellet 47 with an even more complex form; this pellet 47 has an H-shaped cross-section. It will be apparent from these examples that using the method according to this invention a very great freedom results in respect of the choice of form of pellets of encapsulating material.

Claims

1. Method for encapsulating electronic components placed on a carrier, comprising the processing steps of: A) feeding non-portioned encapsulating material to an encapsulating device;

B) liquifying the encapsulating material in the encapsulating device;

C) solidifying the liquified encapsulating material in portioned quantities;

D) placing portioned quantities of encapsulating material in a mould;

E) once again liquifying the portioned quantities of encapsulating material placed in the mould as according to processing step D); and

F) urging the encapsulating material liquified once again as according to processing step E) into at least one mould cavity recessed into the mould.

2. Method as claimed in claim 1, characterized in that the non-portioned encapsulating material is fed in grain form to the encapsulating device.

3. Method as claimed in claim 1 or 2, characterized in that the non-portioned encapsulating material is liquified by heating.

4. Method as claimed in any ofthe foregoing claims, characterized in that the non-portioned encapsulating material is liquified by applying pressure.

5. Method as claimed in claim 3 or 4, characterized in that the non-portioned encapsulating material is liquified by brief heating for 5 to 10 seconds to a temperature of90-140°C.

6. Method as claimed in claim 3 or 4, characterized in that the non-portioned encapsulating material is liquified by very brief heating for a maximum of 5 seconds to a temperature of a maximum of 18O⁰C.

7. Method as claimed in any ofthe foregoing claims, characterized in that for the purpose of solidifying the liquified encapsulating material into portioned quantities the encapsulating material is cooled to a temperature of less than 80⁰C.

8. Method as claimed in any of the foregoing claims, characterized in that the quantities of encapsulating material portioned during processing step C) are cylindrical with a diameter to length ratio of between 1-3 and 1-20, diameter to length ratio greater than 1-5, preferably a ratio greater than 1-5.

9. Method as claimed in any of the foregoing claims, characterized in that the portioned quantities of encapsulating material are cylindrical with a diameter smaller than 5 mm.

10. Method as claimed in any of the foregoing claims, characterized in that the portioned quantities of encapsulating material placed in the mould are liquified again by heating.

11. Method as claimed in claim 10, characterized in that the portioned quantities of encapsulating material placed in the mould are liquified again by heating to a temperature of at least 180⁰C.

12. Method as claimed in any of the foregoing claims, characterized in that during feed of the portioned encapsulating material to a mould cavity recessed into the mould the liquified encapsulating material is urged by a plunger directly against a carrier to which the electronic components for encapsulation are connected.

13. Method as claimed in claim 12, characterized in that the plunger, after the at least partial curing of the encapsulating material, is also used to exert a force on the cured encapsulating material for the purpose of releasing an encapsulated electronic component.

14. Device for encapsulating electronic components mounted on a carrier, comprising; a container for holding non-portioned encapsulating material; heating means for liquifying non-portioned encapsulating material coming from the container; at least one cavity for receiving and allowing the liquid non-portioned encapsulating material to solidify; mould parts which are displaceable relative to each other and which in a closed position define at least one mould cavity for enclosing an electronic component mounted on a carrier; feed means for liquid encapsulating material connecting to the mould cavity; and displacing means for displacing portioned quantities of encapsulating material to the feed means for encapsulating material connecting to the mould cavity.

IS. Encapsulating device as claimed in claim 14, characterized in that the cavity for receiving and allowing the liquid non-portioned encapsulating material to solidify is provided with cooling means (active cooling of liquid encapsulating material).

16. Encapsulating device as claimed in claim 14 or 15, characterized in that the cavity for receiving and allowing the liquid non-portioned encapsulating material to solidify is provided with a plurality of pellet-forming spaces, wherein the relative orientation of the pellet-forming spaces corresponds to the relative orientation of the receiving spaces for pellets of encapsulating material forming part of the feed means for liquid encapsulating material.

17. Encapsulating device as claimed in any of the claims 14-16, characterized in that at least one of the displaceable mould parts is provided with feed means for encapsulating material, these feed means comprising plungers directed toward a side of a mould part facing an opposite mould part.

18. Encapsulating device as claimed in claim 17, characterized in that the plungers are oriented toward the at least one mould cavity for enclosing an electronic component mounted on a carrier.

19. Encapsulating device as claimed in any of the claims 14-18, characterized in that the displacing means for displacing portioned quantities of encapsulating material to the feed means for encapsulating material connecting to the mould cavity and the at least one cavity for receiving and allowing the liquid non-portioned encapsulating material to solidify are combined in a single assembled construction part.

20. Encapsulating device as claimed in any of the claims 14-19, characterized in that the displacing means for displacing portioned quantities of encapsulating material to the feed means for encapsulating material connecting to the mould cavity comprise at least one manipulator with which portioned quantities of encapsulating material can be displaced from the cavity in which they are manufactured to the feed means for liquid encapsulating material connecting to the mould cavity.