ESD-PROTECTED WORK STATION

Abstract: An electronic apparatus (1, 3) has a supporting structure (1) which supports electronic units (3). An electrically conducting table-top (5) is mounted at the supporting structure and arranged at the same electrical potential as this.

Description: ESD-protected work station TECHNICAL FIELD The present invention relates to an ESD (electrostatic discharge) -proteced work station at electronic equipment, which has a supporting structure for supporting and/or enclosing electronic units. BACKGROUND ART Electronic equipment normally comprises electronic units which are sensitive to and may be destroyed by electrostatic discharges . Examples of such electronic units are semi¬ conductor components, both discrete components and inte¬ grated circuits. An example of components which are parti¬ cularly sensitive to electrostatic discharges are so-called CMOS circuits, but also semiconductor components of other types and units which do not consist of or comprise semi¬ conductor components may be destroyed or damaged by the voltages - often very high - which occur in case of electro¬ static discharges. Electrostatic discharges occur, for example, in those cases where either a component or unit or an operator is electro¬ statically charged and the operator touches the com¬ ponent/unit. Alternatively, such a discharge may take place if an electrostatically charged component is placed on top of a conducting surface, or by placing a component on top of an electrostatically charged surface. In all of these cases, there is a considerable risk that the electronic component/unit is destroyed by the discharge that takes place through it . In large electronic equipment, such as electronic cabinets which comprise a plurality of printed circuit-boards (PCBs) , each one with a plurality of electronic components, PCBs or other units will be withdrawn, handled and reinserted into the cabinet during manufacturing and commissioning as well as during maintenance and repairs. At present, the handling of withdrawn PCBs is normally performed on temporary work surfaces, such as chairs, benches or floors, which are often charged and cause a discharge through the PCB when this is placed on the surface. Special work stations are sometimes arranged but such work stations are, in practice, seldom or never used since they are often arranged far from the equipment in question and their use would give rise to considerable loss of time and extra work. For the above reasons, the frequency of damage, caused by electrostatic discharges, to units included in electronic equipment is high, which causes considerable additional costs and other inconvenience to manufacturers and/or users . The temporary work station or work surfaces mentioned above also entail a considerably increased risk of mechanical damage to PCBs or other components withdrawn from a piece of equipment. SUMMARY OF THE INVENTION The present invention aims to provide a work station of the kind mentioned in the introductory part of the description, in which damage to electronic units, caused by electrostatic discharges, is practically completely eliminated. The invention also aims to provide, without significant addi¬ tional cost, a work station which is convenient and which is always readily available and with the aid of which the work is simplified and also the risks of mechanical damage to electronic units which are in process are greatly reduced. What characterizes a work station according to the invention will be clear from the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS In the following the invention will be described with reference to the accompanying Figures 1 - 3, which show equipment in the form of an electronics cabinet with a work station according to the invention. Figure la shows the cabinet seen from the front and Figure lb the cabinet seen from the side. Figure 2 shows in more detail the work station itself, Figure 2a showing the work station seen from the front, Figure 2b the work station seen from the side and Figure 2c the work station seen from above. Figure 3 shows a section through the table-top itself. DESCRIPTION OF THE PREFERRED EMBODIMENTS The electronics cabinet shown in figure 1 comprises a supporting metal construction in the form of the cabinet 1 itself. In the embodiment shown, the cabinet has closed walls, roof and back. The front surface 2 of the cabinet is shown open in the figure and with the table-top in lowered position. The cabinet may be provided with a door (not shown in the figure) for closing the front of the cabinet. The cabinet is provided with supporting and connecting members for a plurality of printed circuit-boards (PCBs) 3 and possibly also for additional PCBs or other devices than the shown PCBs. A cover plate 4 is mounted in the cabinet, approximately in the plane of the front surface, and to the cover plate a sheet metal table-top 5 is attached with the aid of a hinge 6. All the parts now mentioned are made of metal and are metallically connected to each other. Consequently, they will be at the same electrical potential. The table-top 5 is shown in its working position - the lowered position. When the table-top is not used, it is intended to be raised towards the cover plate 4 and be kept in raised position with the aid of a suitable locking device 9. In this position, the cabinet door (if any) may be closed. In the lowered position, the table-top is supported by two metal chains 7 and 8. Figure 2 shows only the cover plate 4 itself and τ.ne raoie- top 5. The table-top is provided with a turned-up front edge 5a to increase its stiffness and to reduce the risk of PCBs or other units, placed on the table-top, falling down from the table-top. The locking device 9 engages a lug 4a of the cover plate . The table-top is suitably given such dimensions as to be adapted to the units, arranged in the cabinet, which may be placed on the table-top. Figure 2c, showing the table from above, shows as an example how the table-top is dimensioned to accomodate two adjacently positioned PCBs 3a and 3b - dashed in the figure - of so-called double Europe format. A grounding band 10 is electrically connected to the cover plate 4 with the aid of a flexible electrical conductor 11 and a screw 12. The band 10 may be fixed around the wrist of an operator who is to work with units withdrawn from the cabinet. The band is electrically conducting, for example by being provided with woven-in metal wires, and is provided with some suitable fastening device, for example Velcro ® fastening, to be fastenable in a simple manner around the wrist of an operator. The band 10 or the conductor 11 may possibly be made with such a high resistivity as to prevent an instantaneous discharge of a charged operator. Figure 3 shows the section through the table-top 5 designated A - A in Figure 2b. On its top side, which forms the work surface, the table-top 5 is provided with a so- called ESD mat or electrostatic charge-diverting mat. Such a mat, known per se, is electrically conducting but has high resistance. This will cause charged objects which come into contact with the mat, for example by being placed thereon, to be discharged, but sufficiently slowly to prevent damage to sensitive electronic units or components . An ESD mat may, for example, be made of plastic material mixed with suitable material which gives the mat the desired conductivity. When the work station is not to be used, the table-top 5 is raised towards and locked to the cover plate 4 and the cabinet door (if any) is closed. When work is to be carried out on units included in the cabinet, the table-top 5 is lowered - after opening the door (if any) - and the operator fastens the grounding band 10 around one of his/her wrists. This causes the operator to be electrically connected to the cabinet, and thereafter both his/her hands are free for work. Thereafter, PCBs and other units arranged in the cabinet may be contacted by and withdrawn from the cabinet by the operator without the risk of static discharges arising. Since the table-top is also electrically connected to the cabinet, and therefore at the same potential as this, withdrawn PCBs or other units may be placed on the table-top without the risk of discharges. Any spare PCBs or other replacement components may be placed on the table-top enclosed in their protective coverings. Thereafter, without the risk of electrostatic discharges, they may be removed from their protective coverings and be handled by the operator. The work station according to the invention, as described above, is an integrated part of the electronics cabinet and is therefore always in position, conveniently arranged and instantaneously available. In practice, it will therefore be used by the persons who are to work with PCBs or other electronic units arranged in the cabinet. It practically completely eliminates the risk of damage to electronic units caused by electrostatic discharges. By offering a con¬ venient work station, which is always immediately available, it facilitates work on the units of the cabinet and con¬ siderably reduces the risk of mechanical and other damage to the units. These advantages are obtained in a simple manner and at only an insignificant additional cost. A work station according to the invention may advantageously be used with an electronics cabinet which has completely or partially open walls . A work station according to the invention may be arranged also with other types of electronic apparatus than the elec¬ tronics cabinet described above, for example in equipment comprising open equipment frames supporting electronic units or with apparatus boxes enclosing such units. In the equipment described above, the table-top is made of metal and provided with a so-called ESD mat which is electrically connected to the table-top. Alternatively, the table-top may be made of other materials, electrically conducting or non-conducting. If non-conducting materials are used, however, the work surface of the table-top should be provided with an electrically conducting mat, coating or surface layer with a suitable resistivity which is electrically connected to the supporting structure of the apparatus. In the embodiment described above, the table-top is electrically connected to the supporting structure - the cabinet - of the apparatus by making the suspension device - the hinge 6 - of the table of metal. Alternatively, the. electrical connection between the work surface and the apparatus can be arranged with the aid of a wire connection or in some other way. The table-top described above is raised in the rest position and is lowered to its working position, which makes possible the simple device - the chains 7 and 8 - to hold the table in the working position (if desired, of course, other members may be used for this purpose) . Alternatively, the table-top may be lowered in the rest position, which makes it possible to omit the locking device 9, and be raised to its working position. The members for holding the table-top in the working position must then be designed in some other suitable way. The cover plate 4 described above may be eliminated and the table instead be directly fixed to the cabinet in some suitable way. Also, the grounding band 10 - 11 may be fixed directly to the cabinet instead of to the cover plate. The above description shows how the table-top is arranged at the front of the apparatus (the cabinet) . Alternatively, it may be arranged at the side of the apparatus, on the inner side of a door or in some other way. The above description is directed to an embodiment of the invention in which the table-top constitutes an integral part of the apparatus. Alternatively, the table-top may constitute a separate unit which is stored in a suitable manner, preferably adjacent to the apparatus, for example inserted into a compartment at the side of the cabinet or in the cabinet door. When using the table-top, it is fixed to the front, for example by being hooked to the front of the apparatus, and is connected to the potential of the supporting structure of the apparatus by direct metallic connection, by a wire connection or in some other way.

Claims: CLAIMS 1. An ESD-protected work station at an electronic apparatus (1,3) which has a supporting structure (1) for supporting and/or enclosing electronic units (3), characterized in that it comprises an electrically conducting table-top (5) mounted at or adapted to be mounted at the supporting structure (1) and at the same electrical potential as this structure. 2. A work station according to claim 1, characterized in that the table-top (5) is mechanically and electrically connected to the supporting structure and constitutes an integral part of the apparatus . 3. A work station according to claim 2, in which the apparatus has a front surface (2) via which the electronic units are accessible, characterized in that the table- top (5) is arranged at the front surface of the apparatus. 4. A work station according to claim 2 or 3, characterized in that the table-top (5) is hinged to the supporting structure (1) . 5. A work station according to any of the preceding claims, characterized in that the table-top (5) has a worktop (5b) of electrically diverting material. 6. A work station according to claim 5, characterized in that the worktop (5b) consists of an electrostatic charge diverting mat. 7. A work station according to any of the preceding claims, characterized in that it comprises a contact member (10,11), which is electrically connected to the supporting structure (1) , for electrically contacting an operator during work at the work station. 91/15097 9 8. A work station according to claim 7, characterized in that the contact member consists of an electrically conducting wrist band (10) which is electrically connected to the supporting structure (1) with the aid of an electrical conductor (11) . 9. A work station according to any of the preceding claims in an electronics cabinet (1) which supports and encloses electronic units (3) and which has a front surface (2) , via which the electronic units are accessible, characterized in that the table-top (5) is fixedly connected to the cabinet and is hinged at the front surface of the cabinet. 10. A work station according to claim 9, characterized in that the table-top has a folded position, in which it is substantially parallel to the front surface (2) , and a working position, in which it makes an angle with the front surface, the table-top being provided with members (7,8) for fixing the table-top in the working position.

Inventors: BENDA STEN, HELLSTRAND BENGT, MATTISSON LARS, STRYCHARCZUK JANUSZ, BENDA, STEN, HELLSTRAND, BENGT, MATTISSON, LARS, STRYCHARCZUK, JANUSZ
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1991
Main classification: H05F1
IPC codes: H05F3