Protection of electronic circuits against static electricity

Abstract: Protection of electronic circuits (13) from static electrical discharge is provided by utilising a key mat (21), having key buttons (22) moulded therein, of a key board for the circuits, to provide an electrically insulating barrier to such discharges. The electronic circuits are housed in a housing (23) having apertures (24) through which the key buttons (22) extend. Electrical discharge from a key button through the aperture in the housing to the circuits is prevented by forming the mat to have an extent sufficient to ensure that any discharge track is longer than that for a predetermined potential. Where the direct distance to a circuit is short, the mat (21) is formed so as to ensure a discharge track greater than the direct distance and of sufficient length. For this purpose the mat may be formed with an upstanding wall (29).

Description: This invention relates to the protection of electronic circuits from undesired effects due to static electrical discharges. A preferred embodiment relates to the protection of the electronic circuits of postal franking meters. Static electrical charge is often generated in modern office environments due to the low humidity levels maintained by air conditioning and frictional engagement between carpets and the shoes of personnel working in the office. Consequently, when personnel use equipment incorporating electronic circuits, static electricity is often discharged to the equipment. In some electronic equipment such discharges may result merely in momentary mal-function of the circuits with no long term ill effects. However in other electronic equipment, such as equipment carrying out data processing functions, a momentary mal-function may result in corruption of data being stored or processed with the result that subsequent operation of the equipment would be affected by such corruption of the data. Corruption of data is of particular concern in connection with equipment such as postal franking meters in which electronic circuits carry out accounting functions related to usage of the meter for franking mail items and are required to maintain reliably accurate accounting records. The records typically include the accumulated value used in franking mail items and the value of credit remaining available for use in further franking operations. Since these accounting records provide the basis on which the user is charged for usage of the meter and on which the postal authority receives revenue for postage it is very important to ensure that these possible discharges of static electricity do not result in mal-functioning of the electronic circuits of the postal franking meter. While protection from static electricity discharges can be provided in some equipment by housing the equipment in a casing which is electrically conductive or has an electrically conductive layer entirely surrounding the circuitry, difficulties arise in using this form of protection with equipment which is required to provide interfaces such as a digital display and a keyboard between the equipment and a user. The display requires the provision of a transparent window in order that it can be viewed by the user and the keyboard requires the provision of an aperture or apertures in the casing through which the keys project. A keyboard comprising a mat of elastomeric electrically insulating material having moulded therein key buttons and which extends over the face of a printed circuit board is described in Toute l'electronique, no 507, October 1985. Paris FR, pages 54 - 57; Casella: "Connecteurs, claviers et membranes aux elastomeres silicone". The key buttons each have an electrically conductive contact for cooperation with contact pads on the face of the printed circuit board. According to the invention electronic apparatus including a printed circuit board carrying on a face thereof electronic circuits and switch contact pads; a mat of electrically insulating material having moulded therein an array of key buttons each having an electrically conductive contact, said mat extending over the face of the printed circuit board and being arranged such that the electrically conductive contacts are positioned opposite corresponding switch contact pads and are movable into electrical contact with a corresponding switch contact pad by manual operation of the key buttons; a face panel extending over the mat and having apertures therein through which the key buttons extend; said mat of electrically insulating material providing a continuous layer of electrically insulating material extending over an area of the face of the printed circuit board is characterised in that the mat has a marginal portion extending beyond the key buttons effective to prevent tracking of static electrical discharge of potential up to a predetermined magnitude from any one of the key buttons to the electronic circuits and in that the marginal portion has a form such as to ensure any possible discharge track from any one of the key buttons between the mat and the face cover to the edge of the mat has a length greater than a direct distance to said edge of the mat. An embodiment of the invention will now be described by way of example with reference to the drawings in which:- Figure 1 is an exploded view of a postal franking meter having keyboard mat and Figure 2 is a sectional view of a part of an assembled keyboard. Referring to the drawing, a mechanical sub-assembly 10 incorporates mechanisms for setting print elements of the franking meter to enable selected values of franking to be printed on mail items. The construction of such an assembly is well known in the franking machine art and does not form any part of the present invention. Accordingly it is believed to be un-necessary to describe the assembly in detail. Secured to the sub-assembly 10 is a printed circuit board 11 comprising an electrically insulating substrate 12 having upper and lower surfaces on which patterns of electrically conductive tracks are deposited. The electrically conductive pattern 13 deposited on the upper face of the substrate includes an array of pairs of switch contact pads 14. It will be appreciated that for clarity the conductive tracks of the pattern 13 shown in the drawing are merely exemplary and that in practice the pattern would consist of a larger number of conductive tracks and would be more complex. The conductive patterns on the upper and lower faces of the substrate are interconnected by conductive connections extending through the thickness of the substrate 12 and electronic components, such as integrated circuit components are mounted on the lower face of the substrate and electrically connected to the conductive patterns to form electronic circuits for carrying out accounting and control functions for the postal franking meter. The particular arrangement and interconnection of the components is not relevant to the present invention and accordingly it is believed to be un-necessary to describe the construction and operation of the electronic circuits. A mounting 15, comprising a moulding of electrically insulating plastics material, is secured to the substrate 12 of the printed circuit board by means of projections 16, 17 which engage respectively in slots 18, 19 in the substrate. When the mounting 15 is secured to the substrate, firstly the projections 16 are engaged in the slots 18 and then the projections 17, which are resilient and of barb-like form are urged into the slots 19 to retain the mounting on the substrate. A liquid crystal display panel 20 is held by the mounting 15 such that it is tilted relative to the plane of the substrate. Electrical connections, not shown, are provided between the display panel 20 and the electronic circuits formed on the printed circuit board 11. A mat 21 having an array of key buttons 22 moulded therein is laid over the upper face of the printed circuit board. The mat is formed of electrically insulating material which is relatively easily deformable to permit the key buttons to be depressed by manual operation thereof. The interior of each key button is provided with an electrically conductive contact pill 30 (see Figure 2). The key buttons and pairs of switch contact pads 14 are so positioned relative to one another that for each pair of contact pads the contact pill 30 of the corresponding key button is aligned to lie over the contact pads 14 of a pair such that, when the key button is depressed by manual operation thereof, the contact pill bridges between the contact pads of the pair so as to electrically connect the pair of contact pads. An outer face panel 23, comprising a plastics moulding, extends over the mat 21 and the liquid crystal display panel 20. The panel has apertures 24 through which the key buttons 22 extend and is provided with a transparent window 25 to enable viewing of data displayed by the display panel. An overlay sheet 26 may be secured to the face panel on which indications of the functions of the key buttons are provided. The face panel 23 and the printed circuit board 11 are secured to the mechanical sub-assembly 10, with the mat 21 sandwiched therebetween, by means of screws 27 and this assembly is then housed in a housing (not shown) which is preferably formed as a moulding of plastics material with an electrically conductive layer on its inner surface. The face panel 23 forms the upper wall of the housing. Usually in electronic apparatus using an array of key buttons formed in a mat of resilient material, the mat has an extent which is merely large enough to accommodate the area required for the array of key buttons. With a mat of such limited extent any static electrical discharge from a user touching the key buttons is able to track along a relatively short path from any of the key buttons through the corresponding aperture 24 to the electrically conductive pattern on the upper face of the printed circuit board. While the face panel 23 can be secured to the remainder of the housing for the machine in a manner, for example by ensuring a sufficiently wide overlap, which prevents static discharge to the electronic circuits within the housing, the apertures in the face cover which are necessary for the key buttons 22 can enable static discharge to occur from the exterior of the housing to the interior thereof. However in accordance with the present invention, the mat is formed to have an extent such as to extend beyond the key buttons. Thus the mat 21 comprises not only the area of the array of key buttons but also an additional marginal area 28 extending to the edges of the printed circuit board. Accordingly the mat 21 provides a continuous insulating layer over the upper face of the printed circuit board and as a result any discharge track extending from any of the key buttons through the apertures 24 and between the mat and the face panel 23 to a conductive track on the printed circuit board is so long that discharges of static electricity of relatively high potential to the electronic circuits are prevented. The dimensions of the mat and in particular the additional marginal area have been chosen such that, based on a value of 1 KV/mm, the length of any possible discharge track is not less than that which is able to withstand the value of static potential for which protection is required. Using the basis of 1 KV/mm provides a margin of tolerance sufficient to take account of deleterious effects due to the possible presence of foreign particles in the space between the mat and the face cover. The degree of protection required may differ for different applications. In relation to certain items of electronic equipment the authorities in some countries specify a minimum discharge potential which the equipment is required to withstand. This minimum potential may be of the order of 15 KV or for example in relation to franking machines for use in the United Kingdom the minimum potential is 21 KV. Postal meters provided with a key mat extending over the printed circuit board as hereinbefore described have withstood test discharges in excess of 21 KV. When the window 25 is formed separately from the face panel and is secured to the face panel, air gaps may extend between the window and the face panel which would allow a relatively short track for electrical discharge from the exterior of the panel to the liquid crystal display and thence to the other parts of the electronic circuits. Accordingly it is necessary to ensure that the window is secured to the face panel, for example by adhesive, in such manner that there is a continuous unbroken seal between the edge of the window and the panel so that there are no air gaps through which an electrical discharge could track. It will be appreciated that, with the juxta-position of the array of key buttons and the display panel as shown in the drawing, the length of discharge track from the row of key buttons nearest the display panel to that portion of the printed circuit board lying below the display panel would be shorter than any discharge track extending across the marginal portions 28 of the mat 21 to the underlying printed circuit board. If as a result the degree of protection provided against static electrical discharges is insufficient the length of track from this row of keys is extended by moulding the mat 21 such that the marginal portion 28 extends, as a wall 29, upwardly from the general plane of the mat around the mounting 15 as far as, or toward, the display panel 20. The face panel 23 is formed with a rib 31 projecting from its lower surface (Figure 2) toward the mat and extending between the wall 29 and the row of key buttons 22 nearest the wall 29. Accordingly the shortest tracking path from the key button 22 to the conductive tracks 13 on substrate 12 of the printed circuit board 11 extends from the key button through the aperture 24 down to the free edge of the rib 31, then between the rib 31 and the wall 29 across the top of the wall and down to the conductive tracks 13. Thus the wall 29 and the rib 31 are effective to ensure that the length of any tracking path from the key buttons 22 to the conductive tracks 13 is greater than the direct distance to the edge of the mat. The length of any possible discharge track is dependent upon the height of the wall and this can be formed to be sufficient to provide the required degree of protection against static discharge. In Figure 2 the wall 29 and the rib 31 are shown spaced apart. However if desired the wall may extend in contact with the rib.

Claims: Electronic apparatus including a printed circuit board (11) carrying on a face thereof electronic circuits (13) and switch contact pads (14); a mat (21) of electrically insulating material having moulded therein an array of key buttons (22) each having an electrically conductive contact (30), said mat (21) extending over the face of the printed circuit board (11) and being arranged such that the electrically conductive contacts (30) are positioned opposite corresponding switch contact pads (14) and are movable into electrical contact with a corresponding switch contact pad by manual operation of the key buttons; a face panel (23) extending over the mat (21) and having apertures (24) therein through which the key buttons (22) extend; said mat (21) of electrically insulating material providing a continuous layer of electrically insulating material extending over an area of the face of the printed circuit board (11) characterised in that the mat (21) has a marginal portion (28) extending beyond the key buttons (22) effective to prevent tracking of static electrical discharge of potential up to a predetermined magnitude from any one of the key buttons (22) to the electronic circuits (13) and in that the marginal portion (28) has a form (29) such as to ensure any possible discharge track from any one of the key buttons (22) between the mat (21) and the face cover (23) to the edge of the mat (21) has a length greater than a direct distance to said edge of the mat. Electronic apparatus as claimed in claim 1 further characterised in that the marginal portion (28) includes a wall (29) extending away from the face of the printed circuit board (11) and the face cover (23) includes a rib (31) extending toward the mat (21) between the wall (29) and the key buttons (22) such that a static discharge track from the key buttons (22) to the circuits (13) on the circuit board (11) is constrained to extend between the wall (29) and the rib (31). Electronic apparatus as claimed in claim 1 or 2 further characterised in that the marginal portion (28) is effective to prevent static electrical discharge up to 15 KV. Electronic apparatus as claimed in claim 1 or 2 further characterised in that the marginal portion (28) is effective to prevent static electrical discharge up to 21 KV. Electronic apparatus as claimed in any preceding claim further characterised by a display panel (20) mounted on the printed circuit board (21) at a location outside the area covered by the mat (22) and projecting from the face of the printed circuit board (21) and in that the marginal portion (28) of the mat (21) extends away from the face of the printed circuit board (21) to or toward the display panel (20). Electronic apparatus as claimed in claim 5 further characterised in that the display panel (20) is located within the housing (23) and the housing includes a transparent window (25) to enable viewing of the display panel and wherein the transparent window (25) is sealed to the housing (23) in such a manner as to substantially preclude any air-gap between the window and the housing. Electronic apparatus as claimed in any preceding claim characterised in that it is a part of a postal franking meter.

Inventors: BARRATT STEPHEN CHARLES, GRANT BRIAN WILLIAM, BARRATT, STEPHEN CHARLES, GRANT, BRIAN WILLIAM
Applicants: NEOPOST LTD, NEOPOST LIMITED
Application year: 1989
Main classification: G07B17
IPC codes: H01H13, G07B17