Potted electronic components

This invention relates to a method of potting an electronic component, and also to potted electronic components per se. The invention is especially suitable for use in connection with the manufacture of reed relays, though it may be used for the manufacture of a variety of other electronic and electrical components. In the following, reference will mostly be made to reed relays, but it will be appreciated that the invention is not limited to such electronic components.

Electronic components for use in the construction of electronic circuits frequently are "potted" - that is to say, encapsulated within a polymerised synthetic resin material. When potted in this way, a component is hermetically sealed from the ambient and also is protected against physical damage. In addition, depending upon the resinous potting compound employed, potting may be particularly advantageous for components subjected to high voltages, to minimise the risk of insulation breakdown.

A minature reed relay usually consists of the contact elements contained within an envelope (known as the 'insert') and an operating coil within the core of which is positioned the insert. It is a common practice to encapsulate the insert together with its operating coil, so as to hold the reed relay insert and its operating coil in the required relative disposition, as well as to protect the relatively delicate coil and relay insert against mechanical damage. However, potting a reed relay inevitably increases the bulk of the relay, and this can lead to problems when it is desired to pack a number of such relays into a relatively small space on a printed circuit board.

When a plurality of reed relays are arranged very close to - one another on a printed circuit board, problems may arise due to magnetic interaction between the coils of adjacent relays. Thus, though the operating characteristics of a complete reed relay may be determined for that relay in isolation, the characteristics may significantly be altered - and particularly the operating voltage of the relay - when a number of relays are closely packed together.

In an attempt to overcome the above-stated problem, it is known to enclose a reed relay within an envelope or casing which is made of a metal having a high magnetic permeability, so as magnetically to isolate the relay within the casing. When the bulk of a completed relay is not important, magnetic isolation can easily be achieved in this way, but it has proved to be extremely difficult effectively to isolate a reed relay when the overall size of the relay is to be reduced to the smallest possible dimensions to permit very high packing densities for such relays on a printed circuit board.

Conventionally, casings for magnetic screening are made either from soft iron or g-metal using a deep drawing technique or - in the case of relatively large components - a conventional case-manufacturing technique. However, especially when using A-metal, experience has shown that it is very difficult to reduce the wall thickness of a deep-drawn casing to below 0.5 mm - and inevitably this therefore considerably increases the overall size of an electronic component potted within such a casing.

It is therefore a principal object of this invention to provide a method of constructing a potted electronic component (such as a reed relay), which permits maximum miniaturisation compatible with the required performance for the component.

Accordingly, one aspect of this invention providing a method of constructing a potted electronic component is characterised by steps of:-

• preparing a casing blank from sheet metal;
• folding the casing blank to form at least a bottom wall and two opposed side walls depending from two opposed edges respectively of said bottom wall;
• assembling with the folded blank two end walls moulded from a plastics material to complete the casing, one end wall disposed at each end respectively of the folded blank and each end wall defining an inwardly-directed component-locating portion;
• positioning within the casing the electronic component to be potted therewithin, the component being supported by inter-engagement with the component-locating portions of the two end walls; and then
• filling the free space within the casing around the component with a polymerisable potting compound.

In performing the method of this invention, the metallic part of the casing is formed by a folding operation on a pre-shaped blank. In this way, the wall thickness of the metal may be reduced to an absolute minimum value sufficient to give adequate magnetic screening - and typically in the case of a u-metal casing, the wall thickness may be reduced down to as little as 0.125 mm (i.e. about 0.005 inch) or possibly even thinner.

In order to facilitate the folding of the blank it is convenient to provide fold or score lines in the material of the blank where the material is to be folded, so as thus to give sharp corners and a casing of precisely controlled dimensions.

It is preferred for the casing blank to be prepared by an etching process, from a sheet of the material from which the casing is to be made. Simultaneous with the etching of the overall shape of the blank, the fold or score lines for folding also may be etched into the material - typically to a depth of 50% of the overall material thickness. Also, any required wording, type designations or the like may be etched into the material, at this stage.

According to a second aspect of this invention, a cased electronic component comprising a casing having a bottom wall, a pair of opposed side walls and a pair of opposed end walls, a component located within the casing and the space around the component but within the casing being filled with a polymerised potting compound, is characterised in that the bottom and side walls are formed from a metallic material appropriately folded from a blank, the end walls are formed at least partially from a plastics material and are interengaged with the side walls, and at least one electronic component including connecting leads is disposed within the casing with its connecting leads projecting from the casing through the space defined by the free edges of the side and end walls with the component being held in place by interengagement with the end walls.

In addition to providing the bottom wall and two opposed side walls for the casing, it is preferred for the blank to be folded also to provide end wall sections, folded round to lie at 90° to the side walls, at each edge thereof extending perpendicular to the bottom wall, when folded. Such end wall sections should extend towards each other but preferably define a gap therebetween. These sections then may serve to support the plastics material end walls, for example by engaging grooves appropriately formed in the end walls. Moreover, the end wall sections will enhance the magnetic screening provided by the casing, by continuing that screening at least partially around the end wall regions.

For the preferred form of casing described above, each end wall section advantageously includes a bendable tab, which may be deformed out of the general plane of the end wall section when an end wall has been engaged with the two end wall sections, so as thereafter to restrain withdrawal or removal of the end wall, prior to potting.

Conveniently, the component-locating portion of each end wall comprises an inwardly-directed groove formed in the end wall and extending towards the bottom wall from the edge of the end wall opposed to the bottom wall, the groove stopping short of the bottom wall such that a component located by the grooves of the two opposed end walls will be held clear of all the metallic walls of the casing.

Depending upon the nature of the electronic component to be located within the casing, the component may be held in the required position prior to potting merely by inter-engagement of its own connecting leads with the end wall component-locating portions. Particularly where more than one component is to be positioned within the casing - for example, a reed relay insert, its operating coil surrounding that insert and possibly also a shunt diode - it may be advantageous to employ a known form of lead frame, with the electronic components soldered or otherwise electrically connected to the lead frame. The lead frame is then itself held in the required position by the component-locating portion of the end walls. After potting has been completed, the support strip of the lead frame may be removed, to leave connecting pins projecting from the casing. The use of such lead frames is known in the integrated circuit art and will not therefore be described in further detail here.

This invention also extends to a cased electronic component whenever constructed by a method as described above.

The invention will in the following be explained in greater detail and a preferred manufacturing method thereof will be described reference being made to the accompanying drawing which shows the components employed in producing a potted reed relay, the components being separated one from another for clarity.

The constructional method described below is intended for the production of reed relays which may be mounted at 0.2 inch (5.08 mm) centres, while still giving adequate magnetic separation to prevent interaction of adjacent relays, notwithstanding the very high packing densities which may be achieved.

Initially, a casing blank is formed from a sheet of µ-metal of approximately 0.005 inch (0.127 mm) thickness, by an etching technique. The etching should be performed both to give the overall shape required for the blank, and also fold lines and such wording and other designations as are required on the completed product, which fold lines and wording should be etched typically to a depth of not more than 50% of the metal thickness. Conveniently, a plurality of such blanks are all etched at the same time, from a relatively large sheet of the material.

Following separation of the blanks from the sheet of material, the blank is folded to produce the casing-part 10 shown in the drawings. This casing-part includes a bottom wall 11, two side walls 12 and 13 connected to the bottom wall 11 by fold lines 14 and 15 respectively and, at each end of the casing, a pair of end wall sections 16 and 17 connected to the respective side walls 12 and 13 by fold lines 18 and 19 respectively. The bottom wall 11 and side walls 12 and 13 are rectangular in shape, and the end wall sections 16 and 17, though generally rectangular, include tabs 20 adjacent their free ends, for a purpose to be described below. Also, there is a gap between the two end wall sections 16 and 17, as shown in the drawings.

The casing is completed by means of two end walls 21, each of which includes a pair of grooves adapted for inter-engagement with the end wall sections 16 and 17. Once so engaged, the tabs 20 are turned over to restrain withdrawal of the end walls, as illustrated on the right-hand end of the construction shown in the accompanying drawing. Each end wall 21 also includes a component-locating groove 22, extending towards but not up to, the bottom wall 11.

Separately, a component sub-assembly 23 is manufactured for positioning within the space defined by the casing. This sub-assembly includes a reed relay insert 24 carrying its operating coil 25 directly therearound, the insert 24 and coil 25 being carried by a lead frame 26, which may also be produced by etching typically from nickel-iron, which then is tin plated. The relay insert leads 26 are welded to pins 27 of the frame, and the coil ends are soldered to pins 28 of the frame. Also, a diode 29 is soldered across the pins 28.

Pins 27 of the lead frame each have lugs 30 adapted to be received in the grooves 22 of the two end walls 21 respectively of the casing. These lugs 30 are so disposed that with the sub-assembly pushed fully home within the casing, none of the components carried by the lead frame, nor the pins of that lead frame contact the metallic casing parts. Then, an appropriate resinous potting compound is run into the casing so as wholly to surround the components therewithin. After curing of the potting compound, the salvage bar 31 of the lead frame is broken away, to leave the pins 27 and 28 projecting from the casing, through the potting compound, so completing the relay.

Though in the foregoing exclusive reference has been made to the production of miniaturised potted reed relays, it will be appreciated that the manufacturing technique may be employed for the production of other potted components, especially where miniaturisation is of particular importance. Other changes and variations equally are possible without departing from the spirit and scope of this invention, as defined by the following claims.