ELECTRONIC PLUG-IN UNIT WHICH INCLUDES AN IMPROVED HEAT-DISSIPATION DEVICE

Abstract: The invention relates to an electronic plug-in unit (1000) which has two lateral edges and at least two electronic cards. This plug-in unit is characterized in that it includes a solid central core (500) made of metal lying parallel to the plane (P) of the plug-in unit (1000) in a position intermediate between two layers (100, 200) of electronic cards (110, 120, 210, 220), so as to drain away heat, by thermal conduction, to at least one lateral radiator (511, 521) forming one of the said lateral edges of the plug-in unit. The electronic plug-in unit according to the invention is particularly intended to be used in the field of mobile telephony installations.

Description: ELECTRONIC PLUG-IN UNIT WHICH INCLUDES AN IMPROVED HEAT-DISSIPATION DEVICE The invention relates to an electronic plug-in unit which includes an improved heat-dissipation device. There are many uses of electronic plug-in units. One application consists, for example, in using them to produce radiotelephone links in a rural environment within the context of mobile telephony. This type of link thus makes it possible to avoid the costly laying of cables in order to link two points which are isolated and/or difficult to access. Such electronic plug- in units make it possible to produce between two and sixteen 2 -megabit telephone links. Of course, the application mentioned is merely one example, electronic plug-in units being in fact intended for many uses which depend, in particular, on the nature of the electronic cards which they contain. An electronic plug-in unit generally comprises several specific electronic cards which are linked via connection pins, on the one hand, to related electronic modules and, on the other hand, to external electronic modules which themselves are linked to an external antenna so as to be able to transmit and/or receive information via radio waves, for example. Each card supports components such as integrated circuits, for example, which have the drawback of generating heat. To allow proper operation of the electronic cards contained in the plug-in unit, it is therefore necessary to dissipate this heat generated by the heating components and to remove it to the outside. A known solution consists in placing a fan inside the electronic plug- in unit. However, this solution cannot be envisaged as the presence of a fan involves very frequent maintenance operations, especially for cleaning or changing the fan's filters. In point of fact, the electronic plug-in units are intended to be distributed in large numbers and to be installed in isolated places access to which is sometimes difficult. It is therefore impossible to envisage carrying out very frequent maintenance operations. Moreover, the addition of such a fan results in a significant reduction in the MTBF (mean time before failure) of the plug- in unit which, for example, drops from 8 years to 2 years, approximately. This solution is therefore inconceivable. Currently, the heating components are thermally coupled to commercial heat sinks. These heat sinks are therefore preferably placed either on the edges of the electronic cards or on a side edge of the plug- in unit into which the electronic cards are inserted. Consequently, the heat dissipation is improved. Moreover, the electronic cards must be placed in the plug-in unit, along slides, in such a way that these cards are suffi- ciently far apart for natural convection to be able to occur. The distance between the cards may be about 10 centimetres. In addition, the plug-in unit is necessarily aerated, by means of small apertures made in its walls, so as to allow the heat to be removed to the outside by natural ventilation. Moreover, when several electronic plug- in units are placed in an integrating rack, they must necessarily be spaced sufficiently far apart to allow the internal heat to be removed through their apertures by convection and natural ventilation. This distance between the plug- in units is typically between 20 and 40 centimetres, approximately. Current plug- in units are therefore very bulky since they have a thickness which is generally greater than 8 cm, and they result in a waste of space in the integrating racks since they cannot be stacked one on top of another. Furthermore, the constraint of placing the heating components along the edges is sometimes not readily compatible with the architecture of an electronic card to be produced. One object of the invention therefore consists in producing a plug- in unit which is less bulky than the plug-in units used hitherto, i.e. a "flat" plug-in unit typically having a thickness of less than 10 cm. The realization of such a flat plug-in unit consists in placing the electronic cards horizontally, in the form of layers, each layer comprising one or more cards arranged side by side. The distance between two layers is very short. Preferably, it is about 2 cm. However, the fact that the electronic cards are very close to each other prevents any natural convection from occuring. Consequently, producing thinner electronic plug-in units does not allow good heat dissipation of the heat generated by the components, so that the mean time before failure is, in this case too, considerably shortened. The present invention makes it possible to solve this dissipation problem since it provides a thin electronic plug- in unit which includes a central core capable of draining away, by thermal conduction, the heat generated by the heating components located on the electronic cards . The invention relates more particularly to an electronic plug- in unit which has two lateral edges and at least two electronic cards, characterized in that it includes a solid central core made of metal lying parallel to the plane of the plug-in unit in a position intermediate between two layers of electronic cards, so as to drain away heat, by thermal conduction, to at least one lateral radiator forming one of the said lateral edges of the plug- in unit. The plug- in unit according to the invention is very compact with a small thickness, typically less than 10 cm. The central core, forming a heat sink, makes it possible not only to improve the heat dissipation of the plug-in unit, by the effect of thermal conduction to the lateral radiators, but also to ensure good mechanical integrity of the plug-in unit since the electronic cards and the top and bottom covers, intended to close off the plug- in unit, are directly fixed to this heat sink. In addition, the top and bottom covers of the plug- in unit according to the invention do not necessarily have apertures since the heat is dissipated laterally to the outside by the phenomenon of thermal conduction and no longer by the phenomenon of convection, associated with natural ventilation. When they do not have apertures, the covers of the plug- in unit allow it to be hermetically protected from external attack. Furthermore, since the heat is no longer removed to the outside by natural ventilation, several plug- in units according to the invention can be stacked one on top of another in an integrating rack, which saves space. Other features and advantages of the invention will appear on reading the description given by way of illustrative but non- limiting example, with reference to the appended figures which represent: - Figure 1, an exploded view of an electronic plug- in unit according to the invention; Figure 2, a perspective view of a central core capable of being placed inside the electronic plug-in unit of Figure 1; - Figure 3, a perspective view of a central core according to an alternative embodiment; and Figure 4, a perspective view of a central core according to another alternative embodiment. An electronic plug-in unit according to the invention is denoted by the general reference 1000 in Figure 1. In the example illustrated in this figure, the electronic plug- in unit is intended to be used in the context of mobile telephony installations but, of course, the invention is not limited to such an application. It includes four electronic cards: a multiplexer card 110 capable of providing sixteen 2 -megabit links from four 2 -megabit links, a supply converter card 120, a modem card 210 and a network card 220 managing the organization of the information contained in the various cards . These electronic cards are illustrated diagrammatically in Figure 1 by rectangles without any electronic components. Of course, the number of electronic cards and their nature may be different, depending on the application for which the electronic plug- in unit is intended. The electronic plug-in unit 1000 furthermore has a solid central core labelled 500 in Figure 1. This central core 500, or heat sink, lies parallel to the plane P of the plug-in unit and in a position inter- mediate between two layers 100 and 200 of electronic cards. This is because it lies above a layer 100 which includes the multiplexer card 110 and the converter card 120 and below a layer 200 which includes the modem card 210 and the network card 220. This central core 500 is preferably made of metal in order for the heat generated by the heating components, located on the electronic cards, to be thermally conducted to at least one lateral radiator 511, 521 which itself forms one of the lateral edges of the plug-in unit. The lateral radiators 511 and 521 have a surface in contact with the outside, and therefore remove the heat to the outside. The distance between the two lateral radiators 511, 521 defines the width L of the plug-in unit according to the invention. This width is about 48 cm. The thickness of these radiators defines the thickness, or the height, of the plug-in unit. This thickness is about 4.4 cm. Finally, the length of these radiators defines the depth p of the plug- in unit. This depth is about 24 cm. At least two rails 530 are provided, formed by an additional thickness, along the longitudinal edges of at least one of the bottom and/or top faces of the heat sink 500. The heat sink shown in Figure 1 has four rails 530. At least one of these rails 530 is intended to adhere to at least one track made on one face of an electronic card so as to establish thermal contact and to pump away the heat that has built up on this card. A track 111 is illustrated on the multiplexer card 110 of Figure 1. When a single rail 530 allows thermal contact with an electronic card to be established, the second rail keeps the electronic cards in a horizontal plane, parallel to the plane P of the plug- in unit. In an alternative embodiment, it is conceivable to place a thermally conducting but electrically insulating foam on at least one rail 530. Thus, the foam makes it possible both to pump away the heat that has built up on the electronic card and to protect this electronic card from parasitic currents which are, for example, liable to produce operating faults, or even short- circuits . The heat sink 500 of the plug-in unit 1000 according to the invention also has several threaded holes 570 which are intended to receive screws for fixing the various electronic cards 110, 120, 210, 220 to this heat sink. Other threaded holes 580 are furthermore provided on the top and bottom walls of the lateral radiators 511 and 521 in order to be able to fix, by screwing, a top cover 700 and a bottom cover 800, respectively. Thus, a second function of the central core 500 of the plug- in unit consists in holding the electronic cards 110, 120, 210, 220 and the top and bottom covers 700, 800 of the plug-in unit 1000 in place. The central core 500 consequently constitutes the framework of the electronic plug-in unit. The bottom cover 800 and top cover 700 are preferably provided, respectively, with one or more stiffeners 810, 710 intended to reinforce their mechan- ical integrity. Advantageously, these covers furthermore allow the electronic cards to be protected from external attack, such as rain or electromagnetic radiation, for example . In an alternative embodiment, a few apertures may optionally be provided on the covers 700, 800 in order to allow some dissipation by convection and natural ventilation, even if this remains very small. The front panel 600 of the plug-in unit 1000 comprises an interconnection card consisting of several sockets. Some of the sockets are intended for connecting coaxial cables and other sockets are in the form of plug- in connectors. Another specific connector 610 is furthermore provided for connecting an external antenna. All the sockets on the front panel 600 are linked to the multiplexer card 110 so as to connect the latter to external electronic modules. As regards the external antenna, this is linked to the external modules which are themselves linked to the modem card 210 via the connector 610. The multiplexer card 110 is also linked to the other electronic cards 120, 210, 220 in order to communicate with them. Of course, in an alternative embodiment, it would be possible to instal a motherboard on the front panel 600 of the plug-in unit so as to interconnect all the electronic cards 110, 120, 210, 220 and to link this motherboard to external electronic modules via external connection terminals. Figure 2 illustrates a perspective view of one embodiment of the central core 500 of the electronic plug- in unit according to the invention. The same labels are used to denote the same components as in Figure 1. The heat sink 500 has two parts called, respectively, the "converter part" 520 and the "frame part" 510. The converter part 520 is wider than the frame part 510. This converter part 520 is intended to cover the converter card 120 of Figure 1. As regards the frame part 510, this is placed in a position intermediate between the two layers of electronic cards since it covers the multiplexer card 110, on the one hand, and supports the modem card 210 and network card 220, on the other hand. Advantageously, the frame part 510 allows heat dissipation by a thermal conduction effect. To do this, the heat sink 500 presents a linear increase in the mass of metal when one moves along it from a mid-position to the first lateral radiator 511. This increase in the mass of metal makes it possible, in particular, to create a thermal conduction gradient between the first lateral radiator 511 and the interface lying between the frame part 510 and the converter part 520. Such a thermal conduction gradient allows the heat to drain away to the coldest parts. In the present case, the heat drains away to the lateral radiator 511 so that it can then be removed to the outside. To do this, the amount of metal must therefore be greater near the radiator 511. This is because, the greater the mass of metal, the greater the thermal conduction becomes. Thus, the frame part 510 of the central core 500 makes it possible to drain away the heat generated by the heating components on the multiplexer card 110, modem card 210 and network card 220 to the first lateral radiator 511. To achieve this increase in the mass of metal, a generally triangular cavity 550 is made in the frame part 510, as illustrated in Figure 2. This cavity is produced in such a way that the apex of the triangle is directed towards the first lateral radiator 511. Since the cavity is narrower at the apex of the triangle, the amount of metal is therefore greater in the portion lying near the radiator 511. In an embodiment illustrated in Figures 1 and 3, this cavity 550 may also be approximately rectangular in shape. In this case, the cavity has at least one step 551 so that it has at least one narrower portion 552 on the side facing the first lateral radiator 511. Thus, the end 552 of the cavity 550 lying near the first lateral radiator 511 has a width 11 less than the width 12 of the opposite end lying near the interface between the frame part 510 and the converter part 520. The fact of producing a cavity 550 in the central core 500 is advantageous because this cavity makes it possible to reduce the quantities of metal used and consequently to reduce not only the cost but also the weight of the electronic plug- in unit into which the core 500 is inserted. Moreover, when there is no cavity it is not possible to create a sufficient thermal conduction gradient so that the heat drains away less effectively and takes place independently, towards both lateral radiators . In another embodiment, it is also possible to envisage increasing the mass of metal by increasing the thickness of the frame part 510 of the heat sink 500, the thickest portion 553 lying near the first lateral radiator 511. In this case, it is preferable for the frame part 510 of the heat sink 500 also to have a cavity, for the cost and weight reasons explained previously. This cavity is approximately rectangular. This variant is illustrated in the perspective shown in Figure 4. With regard to the converter part 520, this has three branches 522, 523, 524 which are connected to the top of a second lateral radiator 521. The heat generated by the electronic components on the converter card lying beneath this converter part 520 is removed in a conventional manner, by connecting the heat-generating electronic components to the internal wall of the second lateral radiator 521 via a metal piece, not illustrated in Figures 1 to 4. Consequently, whereas the first frame part 510 drains heat away to the first lateral radiator 511, in order to allow the heat to be removed in the first direction, the second converter part 520, which has no central core, allows heat to be removed to the second lateral radiator 521 in the opposite direction to the first. Preferably, the non-cavitied parts of the central core 500 are in superposition with the heating components on the electronic cards so as to optimize the thermal conduction effect. The heat sink 500 and the lateral radiators 511, 521 are made as a single casting. The metal from which the heat sink 500 is made must be a good thermal conductor. Thus, the heat sink may be made, for example, of copper, bronze or aluminium. However, copper is difficult to cast, it is brittle since it twists easily, and it is expensive. Likewise, bronze is a very expensive material. Consequently, the preferred metal for making the heat sink 500 is aluminium. The aluminium is melted and poured into a mould. However, this metal is known to shrink as it cools. Consequently, the dimensions of the mould are designed so that the finished heat sink has the desired dimensions. The mould thus has dimensions which are 4 mm greater than those of the finished heat sink 500. After the aluminium heat sink has cooled and has been removed from the mould, it is then tapped by means of a tool such as, for example, a hammer. In one embodiment, when the production rate is high, it is conceivable to make two different pieces separately: on the one hand, the central part of the heat sink 500, i.e. the frame part 510 and the converter part 520, and, on the other hand, the lateral part of the heat sink, i.e. the lateral radiators 511, 521 which are identical. Moreover, the surfaces of the rails and of the internal walls of the lateral radiators are ground flat, for example by means of a grinding wheel or a milling machine, so as to produce good contacts with, respec- tively, the tracks on the electronic cards and the metal pieces provided for connecting the heating components on the converter card to the lateral radiator 521. Grooves 590 are provided in the upper and lower walls of the two lateral radiators 511, 521. These grooves are intended for sliding on the top and bottom covers of the plug- in unit, respectively, which are then fixed to the radiators 511, 521 by screwing. The thickness of the lateral radiators, i.e. the thickness of the electronic plug-in unit according to the invention, is less than 10 cm. In general, it is at least equal to 44 mm and the distance between the two layers of electronic cards is between 15 and 20 mm. The plug- in unit according to the invention which has just been described makes it possible to dissipate powers of about 15 to 20 watts, i.e. temperature rises of between 15 and 20 °C with respect to the normal operating temperature, which is about 55 °C. On the other hand, if the power increases and reaches a value of about 30 watts, because of the use of a larger number of heating components on the electronic cards for example, it will be necessary to increase the thickness of the plug- in unit so that the heat is distributed over a larger volume. This thickness is then multiplied by n, n being an integer greater than or equal to 2. In addition, in this case the lateral radiators are thicker and consequently have a greater area of contact with the outside, so that a more effective removal of the heat to the outside is ensured.

Claims: CLAIMS 1. Electronic plug-in unit (1000) which has two lateral edges and at least two electronic cards, characterized in that it includes a solid central core (500) made of metal lying parallel to the plane (P) of the plug-in unit (1000) in a position intermediate between two layers (100, 200) of electronic cards (110, 120, 210, 220), so as to drain away heat, by thermal conduction, to at least one lateral radiator (511, 521) forming one of the said lateral edges of the plug-in unit. 2. Plug- in unit according to Claim 1, characterized in that the central core (500) presents a linear increase in the mass of metal on going along it from a mid- position to the first lateral radiator (511) so as to create a thermal conduction gradient. 3. Plug- in unit according to either of Claims 1 and 2, characterized in that the central core (500) has a generally triangular cavity (550) , the apex of the triangle being directed towards the first lateral radiator (511) . 4. Plug-in unit according to Claim 3, characterized in that the cavity (550) includes at least one step (551) so that it has at least one narrower portion (552) on the side facing the first lateral radiator (511) . 5. Plug-in unit according to either of Claims 3 and 4, characterized in that the non-cavitied parts of the central core (500) are in superposition with heating components on the electronic cards. 6. Plug- in unit according to one of Claims 1 to 5 , characterized in that the thickness of the central core (500) varies and in that the thickest portion (553) is located near the first lateral radiator (511) . 7. Plug-in unit according to one of Claims 1 to 6, characterized in that it has two parts: a first part (510) capable of draining away heat to the first lateral radiator (511) in a first direction, and a second part (520) , which has no solid central core, capable of removing heat to the second lateral radiator (521) in the opposite direction to the first. 8. Plug-in unit according to one of Claims 1 to 7 , characterized in that the central core (500) and one or both of the lateral radiators (511, 521) are made of aluminium and form a single casting. 9. Plug- in unit according to one of Claims 1 to 8 , characterized in that the central core (500) has at least two rails (530) formed by an additional thickness, at least one of these rails being in contact with a track (111) provided on an electronic card (110) so as to remove heat which has accumulated on the said electronic card. 10. Plug- in unit according to one of Claims 1 to 9 , characterized in that a foam which is thermally conduct- ing and electrically insulating is fixed to at least one rail (530) . 11. Plug-in unit according to one of Claims 1 to 10, characterized in that the central core (500) has several threaded holes (570, 580) intended to allow the elec- tronic cards (110, 120, 210, 220), on the one hand, to be fixed by screwing and two covers, a bottom cover (800) and a top cover (700), on the other hand, which are intended to close the plug-in unit. 12. Plug- in unit according to one of Claims 1 to 11, characterized in that it has a front panel (600) on which sockets are provided, these allowing the electronic cards (110, 120, 210, 220) to be connected to external electronic modules.

Inventors: BARBIER JEAN-PAUL, EVEILLARD BERNARD, FERAL DOMINIQUE, FONTCHASTAGNER MARC, BARBIER, JEAN-PAUL, EVEILLARD, BERNARD, FERAL, DOMINIQUE, FONTCHASTAGNER, MARC
Applicants: BARBIER JEAN PAUL, EVEILLARD BERNARD, FERAL DOMINIQUE, FONTCHASTAGNER MARC, T R T LUCENT TECHNOLOGIES, BARBIER, JEAN-PAUL, EVEILLARD, BERNARD, FERAL, DOMINIQUE, FONTCHASTAGNER, MARC, T.R.T. LUCENT TECHNOLOGIES
Application year: 1998
Main classification: H05K7
IPC codes: H04Q1, H05K7