GB2299456A

GB2299456A - Insulation of component leads

Info

Publication number: GB2299456A
Application number: GB9506506A
Authority: GB
Inventors: Jean Maurice Wing; Steven John Entwistle
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1995-03-30
Filing date: 1995-03-30
Publication date: 1996-10-02
Also published as: GB9506506D0

Abstract

A member of conductive material, such as a heat sink (10), defines apertures (14) through which the pins of electronic components extend into a printed circuit board. The apertures (14) are insulated by a method including the insertion of a strip (32) of insulating material (such as a prepreg) and the heat sink (10) between a pair of platens so that insulating material is pressed into the apertures (14). The resultant heat sink is improved in that the production process is simplified and made more reliable, and the resultant insulation sleeves (33) are located more strongly against axial displacement as they are additionally secured to a face of the heat sink by the adhesion of the strip (32) which can also be reinforced by an internal filamentous substrate.

Description

INSULATION OF COMPONENT LEADS The invention is concerned with the insulation of component leads where they pass through a member of conductive material.

Electronic components commonly have leads, in the form of pins, by which they are mounted on a printed circuit board. Some components generate heat in use, some at a rate which needs to be dissipated in a controlled or regulated manner such that the components and peripheral materials are not damaged, their reliability is not impaired, or the components exceed the temperature range over which they are designed to operate.

For this reason printed circuit boards commonly have heat sinks (sometimes referred to as a thermal plane) positioned between the heat generating components and the printed circuit board. Such heat sinks are usually metal plates of suitable thermal capacity and conductively for dissipating the heat. Accordingly a heat sink is formed with a pattern of apertures which correspond to the positioning of the pins of the components. These apertures are usually cylindrical bores which are formed through the heat sink, for instance by punching or drilling.

It is of course necessary to insulate the component pins, where they pass through their respective bores, from electrical contact or discharge with the conductive material of the heat sink.

At the present time component pins are insulated from their respective bores in the heat sink by depositing a layer of insulating material within each aperture. This is achieved by injecting an appropriate resin, typically a two-part epoxy, into each bore. With this technique it is essential to fill each bore with the resin to ensure that there are no voids or gas bubbles. Each bore is typically overfilled and, after the resin has hardened, it is then necessary to trim both ends of each plug of solidified resin level with the opposed faces of the heat sink. A concentric bore is then drilled through each resin plug to leave space for the respective pin to pass through the heat sink into the printed circuit board.

The dielectric integrity of a heat sink manufactured in this manner is reliant both on the successful individual filling of each bore without producing any voids, and on the mechanical bonding between the cylindrical outer surface of each resin plug and the respective bore in the heat sink. Such bonding is reliant on the roughness of each bore and, in practice, the integrity of a whole heat sink can be compromised by a single resin plug either falling out, or being pushed out by a component pin during assembly.

The present invention provides an improved process for insulating the bores in a heat sink, or other conductive member, and an improved resultant product.

According to one aspect of the invention a method of insulating apertures through a member of conductive material includes locating a strip of pressure-formable insulating material adjacent one face of the member to overlie the aperture, pressing the strip and the member together wish a force adequate to cause parts of the strip to flow into and fill the apertures, and subsequently boring through the filled apertures to permit respective component leads to pass through the member. In this manner all of the apertures are filled simultaneously in a manner which avoids the production of voids or bubbles in the plugs of insulating material that have been proposed into the apertures.

Preferably the insulating material is selected so that it will adhere to the walls of the aperture. In this manner the adherence of each plug of insulating material to the walls of its aperture serves also to hold the remaining plugs in position through the residual material of the strip. Altematively the insulating material may be selected so that it will adhere to both the one face of the member and to the walls of the apertures. This feature of the invention ensures the integrity of all of the plugs of insulating material which are effectively both joined together and are secured to the face of the member.

The thickness of the strip must be selected, relative to the size and density of the apertures, to provide sufficient material for filling the apertures during the pressing operation. To achieve this result it may be necessary to have the strip of a thickness which will leave an excessive thickness of the insulating material adhered to the face of the member. In this case the method may include reducing the thickness of the strip after it has been pressed against the member. The strip is preferably made from a prepreg material and desirably includes a filamentous substrate such as a fabric made from glass fibres. Alternatively the strip can be formed from a thermoplastic material, in which case the strip would be heated to facilitate the flowing of the insulating material into the apertures.

According to another aspect of the invention a member of conductive material has apertures for accommodating component leads, and the apertures are insulated by insulating material which adheres both to the aperture walls and to an adjacent face of the member. Preferably the insulating material within several apertures is formed integral with a strip of the insulating material adhering to the face of the member. The insulating material adhering to the face of the member is preferably reinforced by a filamentous substrate.

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a plan view of part of a heat sink, drawn full size, showing typical arrangements of cylindrical apertures; Figure 2 is a vertical section through any two of the apertures shown in Figure 1, but drawn to enlarged scale and showing a known construction of printed circuit board using a heat sink with insulated apertures; Figures 3,4 and 5 show the heat sink of Figure 2, to an increased scale, and illustrate the conventional method of insulating the apertures in the heat sink; Figure 6 is a schematic section illustrating a method of insulating the apertures of a heat sink in accordance with the present invention; Figure 7 corresponds with Figure 6 but illustrates the effect of deforming the strip of insulating material under pressure; and Figure 8 is a vertical section, corresponding in scale to Figures 6 and 7, but illustrating a heat sink provided with insulated apertures in accordance with the present invention.

A portion of a typical heat sink 10 is shown in Figure 1. It is formed from a sheet of conductive material and is designed specifically for use in conjunction with a complementary printed circuit board. The heat sink 10 is typically formed with large apertures, such as aperture 11, and slots 12 which are designed to minimise the bulk of the heat sink 10 whilst leaving conductive paths for dissipating heat. The heat sink 10 is also formed with a linear array of apertures 13 and a much larger grid of apertures 14.

The apertures 13 and 14 extend through the heat sink and are provided to allow the passage of conductors from various electronic components as hereafter described.

In Figure 2, a typical electronic component 15 is shown with its conductors 16 and 17 passing through apertures 14 formed in the heat sink 10 which is secured by a thin layer of adhesive 18 to the back of a printed circuit board 19. Conductive strips 20 and 21 are formed in the usual manner on the lower surface of the printed circuit board 19 and are provided, as shown, with respective holes through which the conductors 16 and 21 extend, electrical contact being achieved by the use of solder 22 in the usual manner.

Although the electrical component 15 has an insulated casing, its conductors 16 and 17 are uninsulated and it is necessary to ensure that they do not make electrical contact with the conducting material of the heat sink 10. For this reason both of the apertures 14 are provided with an insulating lining 23. The conventional manner of producing the insulating linings 23 is illustrated in Figures 3, 4 and 5. The insulating material is typically a two-part epoxy resin which, after mixing, is injected into the apertures 14.

Due to the smallness and close spacing of the apertures 14, this process is currently achieved by injecting the resin separately into each of the apertures 14, leaving plugs 24 as shown in Figure 3. The plugs 24 typically have uneven ends and these are trimmed off, as shown in Figure 4, so that their ends are flush with the upper and lower faces of the heat sink 10. Subsequently each of the plugs 24 is drilled to provide through holes 25 for the conductors 16 and 17 as shown in Figure 5. This process is particularly time consuming because each of the apertures 14 is filled separately. Each plug 24 needs to be checked to ensure that it does not contain any bubbles or other voids, and it is then necessary to drill the holes 25 coaxially through the very small plugs 24 without pressing the plugs out of the apertures 14.

This process is very substantially improved by the method taught by the present invention which will now be described with reference to Figures 6,7 and 8.

In Figure 6 the heat sink 10 is placed between a pair of platens 30,31 which are mounted with their pressed surfaces parallel to each other as shown. A strip of insulating material is positioned between the platen 31 and the lower face of the heat sink 10, as shown in Figure 6. The strip 32 is positioned so that it lies under all of the apertures 14 in the group.

The platens 30 and 31 are then pressed together, as shown in Figure 7, so that the insulating material forming the strip 32 is compressed and some of the insulating material flows into the apertures 14 to form plugs 33. As the upward movement of the insulating material in the apertures 14 is limited by the upper platen 30, the top surfaces of the plugs 33 are moulded flush with the top surface of the heat sink 10. If desired, the top platen 30 may be provided with unshown grooves or apertures to ensure that air displaced from the apertures 14 can escape. The material for each strip 32 can readily be checked to ensure that there are no internal bubbles or voids before it is used in the process. For this reason the process ensures that the plugs 33 are formed without the presence of any air bubbles or voids.Furthermore, the presence of the platens 30 and 31 simplifies the process by avoiding the operation that has been described with reference to Figure 4. The insulating material forming the strip 32 is chosen so that it will adhere to the walls of the apertures 14 and to the undersurface of the heat sink 10.

In this manner it will be noted that the plugs 33 are supported, not only by their adherence to the apertures 14 as has been the case with the prior art, but are additionally located by the bond between the strip 32 and the bottom surface of the heat sink 10. The platens 30 and 31 have surfaces formed from a material which would not bond with the insulating material forming the strip 32.

As shown in Figure 8, the through holes 25 are bored through the plugs 33 as before, thereby leaving each of the apertures 14 with an insulated sleeve to ensure that the conductors 16 and 17 cannot short against the conductive material of the heat sink 10.

It should be noted that the adherence of the strip 32 to the undersurface of the heat sink 10 provides a very substantial support for the plugs 33 and prevents them from being individually displaced from their respective apertures 14. This considerably improved support for the plugs 33 brings the added advantage that a greater force can be used when drilling the through holes 25, thereby additionally speeding the production process.

In many applications the residual thickness of the strip 32, as shown in Figure 8, will be sufficiently thin to enable the heat sink 10 to be mounted directly to the back of the printed circuit board 19 using a suitable adhesive. However, there may be cases in which the depth or density of the apertures 14 will necessitate the use of a thicker strip of insulating material 32 to ensure full penetration of the plugs 33 into the apertures 14. In this event the thickness of the residual strip 32 can be reduced, as required, by any appropriate process.

The strips 32 are preferably formed from a prepreg sheet which is partially cured. The prepreg material will desirably be as specified within BS 4584 Section 102.12 or MIL-S-13949H/12B. The prepreg preferably contains a filamentous substrate such as a woven cloth of glass fibres or other suitable materials. In this event, the fibrous substrate will tend to remain in the deformed strip of insulating material 32 rather than penetrate any significant distance into the plugs 33. Consequently the bores 25 will penetrate the fibrous layer, but the fibrous layer will effectively key the bottom of each of the drilled plugs 33 permanently to the strip 32, thereby providing an even stronger construction.

The process can be adapted from that described to enable the strips 32 to be formed from a range of other insulating materials. For instance, the strip 32 could be formed from a thermoplastic material to which heat will be applied during the pressing operation illustrated in Figures 6 and 7. With some materials it may be appropriate to achieve such heating by the abrupt application of pressure between the platens 30 and 31 as shown in Figure 7.

Claims

1. A method of insulating apertures through a member of conductive material, including locating a strip of pressure-formable insulating material adjacent one face of the member to overlie the aperture, pressing the strip and the member together wish a force adequate to cause parts of the strip to flow into and fill the apertures, and subsequently boring through the filled apertures to permit respective component leads to pass through the member.

2. A method, as in Claim 1, in which the insulating material is selected so that it will adhere to the walls of the apertures.

3. A method, as in Claim 1, in which the insulating material is selected so that it will adhere to both the one face of the member and to the walls of the apertures.

4. A method, as in any preceding Claim, including reducing the thickness of the strip after it has been pressed against the member.

5. A method, as in any preceding Claim, including making the strip from a prepreg material.

6. A method, as in Claim 5, in which the prepreg material includes a filamentous substrate.

7. A method, as in any of Claims 1 to 4, including making the strip from a thermoplastic material and heating the strip to facilitate the flowing of the material into the apertures.

8. A method of insulating apertures through a member of conductive material substantially as described herein with reference to Figures 6 to 8.

9. A member of conductive material having apertures insulated by the method of any of Claims 1 to 8.

10. A member of conductive material having apertures for accommodating component leads, in which the apertures are insulated by insulating material which adheres both to the aperture walls and to an adjacent face of the member.

11. A member, as in Claim 10, in which the insulating material within several apertures is integral with a strip of the insulating material adhering to the face of the member.

12. A member, as in Claim 10 or 11, in which the insulating material adhering to the face of the member is reinforced by a filamentous substrate.

13. A member of conductive material, having apertures for accommodating component leads, substantially as described herein with reference to Figure 8.

14. A printed circuit, comprising a printed circuit board and associated electronic components, and including a heat sink formed as a member of conductive material, in accordance with any of Claims 9 to 13, with at least some of the component leads passing through its insulated apertures.