GB2280314A - A method of encapsulating a component - Google Patents

Abstract

A component is potted in encapsulant 30 on a printed circuit 12, 13 the thickness of the encapsulant being determined by a spacer 33. Pressure is applied to the encapsulant, by means of a magnet 37 attracted towards ferromagnetic base plate 31 enabling the force to be continuously applied before and during a heating process to cure the encapsulant 30. <IMAGE>

Description

A METHOD OF ENCAPSULATING A COMPONENT This invention relates to a method of encapsulating a component on a printed circuit and in particular but not exclusively to applications where the thickness of the encapsulant has to be controlled to very precise tolerances. One example of this is in the manufacture of what are commonly referred to as "smart cards" that is, cards incorporating electronic components on a printed circuit board which are carried by individuals and used for such purposes as recording or authorising transactions and/or for authorising entry to buildings.

(Note that the term "printed circuit" as used in this specification should be considered to refer to any system of conductive tracks on an insulating substrate, whether such tracks are formed by printing, etching, vapour deposition or any other technique.) It is an object of the invention to provide a method of encapsulating a component, which method gives precise control of the thickness of the encapsulant.

The present invention provides a method of encapsulating a component on a printed circuit in which a magnet is used to confine encapsulant between two planes defined by a spacing member, and in which the encapsulant is then heated to cause solidification thereof.

By employing the method in accordance with the present invention a uniform force can be applied locally to the region of encapsulant and with this force still applied the encapsulant can be heated, normally in an oven.

Preferably the printed circuit is positioned on a ferromagnetic base plate to which the magnet is attracted, and advantageously this comprises a number of pegs which locate the printed circuit and spacer member thereby ensuring alignment.

Advantageously the spacer member comprises one or more apertures, each of which corresponds to the location of encapsulant on the printed circuit. This thereby provides a uniform spacing around the periphery of the encapsulant Similarly it is advantageous if a plurality of magnets are located on a carrier sheet at positions substantially corresponding to the location of encapsulant on the printed circuit, for this can then be aligned with the printed circuit thereby aligning all the magnets simultaneously. This is particularly advantageous when the printed circuit comprises a substrate with conductive tracks comprising a number of similar electrical circuits each having components thereon to be encapsulated, for then the components required to be encapsulated on the electrical circuits can all be encapsulated simultaneously. This is particularly advantageous when the encapsulant is deposited by a screen printing process, because the screen of the screen printing process will have apertures in corresponding positions to the apertures in the spacing member, the positions of the magnets on the carrier sheet and the devices to be encapsulated on the printed circuit.

The method is particularly advantageous if the encapsulant is sandwiched between two substrates one of which comprises a printed circuit and one of which is relatively flimsy, for it ensures the flimsy substrate will be pressed firmly against the other substrate, especially where the substrate other than the printed circuit extends over a number of areas of encapsulant.

One embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which like numerals have been used to indicate like parts, and of which: Figure 1 is a perspective view of a plastic card produced in accordance with the method of the present invention; Figure 2 is a schematic sectional plan view of the card of Figure 1; Figure 3 is a cross-section through the line I - I of Figure 2; and Figures 4 to 10 illustrate various stages in the production of the plastic card depicted in Figures 1 to 3.

Referring first to Figure 1 there is shown a perspective view of the final card which has the same external dimensions as a standard "plastic card". The card contains an integrated circuit which communicates with interrogation units via an inductive link located at appropriate locations. The integrated circuit would normally contain a memory device and could be used for any number of purposes, for example recording banking transactions or recording zones of buildings etc to which entry has been gained by use of the card as an identity card.

Referring to Figure 2 there is illustrated a plan view through a section of the card 10 of Figure 1 in the plane of the card with its top surface removed to reveal the printed circuit.

From this and the cross-section along line I - I illustrated in Figure 3 it can be seen that a printed circuit 11 comprises epoxy/glass substrate 12 and conductive tracks 13, a substantial portion of which form conductive loop 14. Darkly shaded regions 15 and 16 comprise of a thermoset dielectric material, the region 15 insulating silver conductor 17 from the inductive coil 14. The purpose of dielectric layer 16 will be explained later.

An integrated circuit and capacitive components not shown in Figures 2 or 3 are contained within element 19 which is separated by cut 18 from the rest of the substrate 12 and the region 20 of the substrate 12 is lowered below the plane of the printed circuit 11, the integrated circuit and capacitive components being located in potting compound 21 sandwiched between the portion of the substrate 20 and a capping portion 22 of the same material as the substrate 12.

The printed circuit board 11 and element 19 are sandwiched between two outer sheets 23 and 24 of PVC thermoplastics material and two intervening layers (not shown in Figures 3 or 4), of polyester which is coated on both sides with a thermally activated catalyst adhesive by which the laminated structure is adhered. This polyester acting as a reinforcing layer preventing element 19 "breaking out" of the PVC layers 23 and 24.

The fabrication process of the card illustrated in Figures 1, 2 and 3 begins with a substrate sheet 12 of copper-clad epoxy/glass which is etched to form a number of identical printed circuits 13, each as illustrated in Figure 4. On top of each printed circuit is printed a thermoset dielectric material indicated by the shaded regions 15, 16 which is cured in place.

The function of circular part 16 is explained below. The linear part 15 serves as an insulator to separate printed conductive link 17 for the inner end of coil 14 defined by part of the printed circuit 13. Separated from a main part of the substrate by lines of weakness not shown are a number of strips (not shown), each carrying printed patterns 25 (only one of which is illustrated), with apertures 26 therein, the patterns defining areas which ultimately become the top reinforcing caps 22 of the elements 19.

The substrate comprising the plurality of printed circuits, or patterns, is placed on a bed of a screen printing machine (not shown) and a screen placed over it. A squeegee is then used to print a low ionic epoxy encapsulant/adhesive material onto positions 27 as shown in Figure 5. This is a mixture of a resin and a catalyst which sets hard when cured. Suitable materials are, for example, available from Ablestick, Encaremix, or Dexter Hisol. The substrate is then placed in a "pick-and-place" machine which places components comprising of capacitors 28 and silicon chips 29, shown in Figure 6, onto the epoxy which acts as an adhesive to hold them in place. The silicon chips 29 at this stage are "naked", that is to say they are not encapsulated. A notable feature of this process is that the epoxy is applied to areas where there is no copper layer, this being unnecessary because of the adhesive attachment of the components. A saving of 35 microns in thickness is achieved: a significant achievement in this technology where reduction of thickness is of crucial importance, for this particular process, the entire assembly not exceeding 760 microns. An advantage of using epoxy adhesive is that if suitably selected it remains in its adhesive state for approximately one week and there is therefore no need to clean down the equipment, providing it is in regular use.

The sheet substrate with its plurality of circuits and respective components positioned on it, is then baked at 100"C until the epoxy has gelled, i.e. set but not hardened. This takes place under a flow of nitrogen to prevent oxidation of the copper. The sheet is then placed on the work-holder of a wire bonding machine where it is held in position by a vacuum.

Suitable machines for this purpose are commercially available. Wire connections are then made between contacts on the individual components to appropriate parts of the printed copper circuitry. This is done by an ultrasonically assisted diffusion welding process. The sheet is then placed back in the screen printer with a different stencil in place. This stencil is much thicker, its thickness being selected so that the same epoxy encapsulant/adhesive now to be deposited over the components is sufficient to cover them completely. An epoxy adhesive being selected which does not tend to trap bubbles. Notably, this material is the same as that which was used for the adhesive. It does not have to be the same but it preferably has similar physical characteristics. After the removal of the stencil, the sheet is as shown in Figure 7, the components being potted in encapsulant 30.

The substrate 12, with a plurality of circuits 13 and associated regions of encapsulant 30, is then mounted on a ferromagnetic stainless steel backing plate 31, as shown in Figure 8, by means of pegs 32 extending from the backing plate 31 through tooling holes in the substrate. Note that for clarity Figure 8 only illustrates one section of the substrate corresponding to one circuit.

A copper spacing member 33 having a plurality of apertures 34 (each corresponding to each of the regions on the sheet having encapsulant 30 deposited thereon) is located over substrate 12 again by means of pegs 32 co-operating with tooling holes in the spacing member. Previously placed on the spacing member 33 are each of the now separated strips 35, previously referred to. Regions of the strips, defined by printed patterns 25, are to form reinforcing caps 22 for the encapsulant, as will be explained, and are aligned with the apertures 34 in the spacing member 33. A fibreglass sheet 36 is then laid over the assembly, which sheet is again located by means of the pegs 32. On the fibreglass sheet 36 are captive magnets 37, only one of which is seen from the partial section shown in Figure 8. These magnets are located at positions corresponding to the encapsulant 30 and are attracted to the ferromagnetic stainless steel backing sheet 31 thereby sandwiching the assembly. This is shown in more detail in Figure 9 which corresponds to the ringed region "A" of Figure 8.

The patterns 25 are thus pressed into contact with the spacing member 33 which is pressed down onto the circular part 16 of the dielectric material. The portions of the strip 35 defined by the patterns 25 are pressed onto the, still soft, epoxy encapsulant/adhesive. The thickness of the encapsulant is therefore determined by the thickness of: dielectric ring 16; spacing member 33; and ring pattern 25, each of which are manufactured to precise thickness tolerances. During this compression process the encapsulant spreads out as shown in Figure 9, but not as far as the edges of the spacer sheet. It is prevented from doing so by its meniscus acting against the inner edges of the copper ring pattern 25 and dielectric ring 16, which meniscuses thereby define the radius of the encapsulant.

The whole assembly is now placed in an oven and cured at a temperature of 150"C.

This fully gels the encapsulant/adhesive both under the components and the encapsulant portion. After removal from the oven the fibreglass sheet 36 and associated magnets, and the backing member 31 are removed from the assembly, the remaining assembly being held together by the now cured encapsulant 30. This remaining assembly is placed on a rule die which forms cuts 18 in each circuit, one of which can be seen in Figure 2. These cuts are configured so that their free ends correspond with the slots 26 (see Figure 4) in the strips 35.

Note at this stage the ends of each cut are located on the copper pads 34 of Figure 2 which prevent tearing. The cutter presses through the assembly as illustrated by dotted lines 41 in Figure 9, leaving the element 19 with a reinforcing capping portion 22 formed by the cutting process on a limb, also formed by the cutting process, of the substrate 11, as is best seen from Figure 2. The spacing member 33 and remaining portions of the strips 35 are lifted off.

It will be noted from Figure 2 that the electrical connections to the element run parallel to an edge of the card, in which direction the card is most resistant to bending, as opposed to across the hinge line which runs across the corner of the card where it is most susceptible to bending.

Using another rule die, cruciform shapes are cut out of the assembly to give each printed circuit the shape illustrated in Figure 2. This removes the epoxy/glass substrate from those areas which are to become the corners of the finished cards. It is notably these corner parts which are most subject to the type of manipulation which encourages de-lamination.

The printed circuit 1 with reinforced element 19 is now placed, as shown in Figure 10, between two outer sheets 37 and 38 of thermo plastics material in the pvc family with the inter-position of polyester layers 39 and 40 coated on both sides with a thermally activated catalyst adhesive. The assembled sandwich is placed in a press where it is heated to cause lamination. During this stage the elements 19 imbed themselves in each of the sheets of thermo-plastic material in such a way as to tend to centralise themselves between opposite faces leaving the plane of the substrate sheet 12 on the central axis as shown in Figure 3. The press now opens and the assembly is removed to z cutting machine where the individual cards as illustrated in Figures 1 and 2 are cut out.

Although in the specific embodiment illustrated each electronic element is formed integrally with the printed circuit, each element could alternatively be formed on a separate part of the printed circuit which is subsequently connected to the main part of the printed circuit, the element being retained in an aperture in the main printed circuit formed by a circular or similar cutter by being "snapped" into the aperture, the end faces of the element being of slightly greater diameter than the aperture. The elements would be formed by a process very similar to that disclosed except they would be formed on' a substrate having a far greater density of elements, from which substrate they would eventually be cut. The elements can be cut from the substrate having a limb extending from the element by which they are connected to the main portion of the printed circuit of a card by normal soldering of conductive tracks on the limb to tracks on the printed circuit or by similar techniques.

The card shown in the illustration shows an electronic element connected to an inductive loop 14 in a contacless card. The electronic element could alternatively be connected by means of a limb to an electrical contact or other component of a card.

Claims (10)

1. A method of encapsulating a component on a printed circuit in which a magnet is used to confine encapsulant between two planes defined by a spacing member, and in which the encapsulant is then heated to cause solidification of the encapsulant.

2. A method as claimed in claim 1 wherei^. the printed circuit is positioned on- a ferromagnetic base plate to which the magnet is attracted.

3. A method as claimed in claim 2 wherein the base plate comprises a number of pegs which locate the printed circuit and spacing member.

4. A method as claimed in any preceding claim wherein the spacing member comprises one or more apertures each of which corresponds to the location of encapsulant on the printed circuit.

5. A method as claimed in any preceding claim wherein a plurality of magnets are located on a carrier sheet at positions substantially corresponding to the location of encapsulant on the printed circuit.

6. A method as claimed in any preceding claim wherein the printed circuit comprises a substrate with conductive tracks thereon, the conductive tracks comprising a number of similar electrical circuits each having components thereon to be encapsulated.

7. A method as claimed in any preceding claim wherein the encapsulant is deposited by screen printing.

8. A method as claimed in any preceding claim wherein the encapsulant is sandwiched between two substrates, one of which comprises the printed circuit, and one of which is relatively flimsy.

9. A method as claimed in claim 8 wherein the substrate other than the printed circuit extends over a number of areas of encapsulant.

10. A method substantially as hereinbefore described with reference to Figures 8 and 9 of the accompanying drawings.