WO1993013556A1

WO1993013556A1 - Multichip module cooling system

Info

Publication number: WO1993013556A1
Application number: PCT/FR1992/001228
Authority: WO
Inventors: Thierry Fromont
Original assignee: Bull S.A.
Priority date: 1991-12-30
Filing date: 1992-12-22
Publication date: 1993-07-08
Also published as: FR2685816A1; JPH06510638A; EP0619919A1; CA2123120A1

Abstract

The system is designed to cool average dissipation multichip modules. The heat conducting cover (3) associated with the substrate (1) supporting the circuits (2a, 2b, 2c, ...) coperates with flexible, low heat-resistant metal devices (5a, 5b, 5c, ...) for transferring heat from the circuits to the cover. The contact surfaces of the devices (5a, 5b, 5c, ...) are flat and have a flat/rough surface state for controlling air layers in the region of the contacts. The system offsets differences in altitude and angles between the substrate and the various circuits and diminishes thermal resistance in the region of the contacts. Application in average dissipation multichip module cooling.

Description

Cooling system for "multi-chip" module

The present invention relates to a cooling system for a “multi-chip” module comprising a substrate on which is arranged a plurality of substantially flat electronic circuits, associated with the substrate a partially flat thermally conductive cover cooperating with a plurality of heat exchange means , each in relation on one side with a circuit and on the other with the hood to transfer heat from the circuit to the hood.

- Such a cooling system is known from European patent application EP-369 115. In this application, there is described a system comprising inter alia a substrate carrying electronic circuits forming with a cover or cap a hermetic enclosure. In this enclosure are arranged vis-à-vis each circuit heat exchange means. In fact, each electronic circuit dissipates a certain amount of heat, at least part of which must be necessarily removed in order to allow nominal operation. Thus, a heat exchange means for transferring said heat from a circuit to the cover is in the present case constituted by a cylindrical metal piston whose body is housed in a cavity located in the thickness of the cover . A spring disposed in the upper cavity of the piston keeps the lower end of the latter in pressure on the circuit. The heat transfer from the circuit to the piston will be all the more efficient the larger the surface of the piston in contact with the circuit, ideally the lower end of the piston being in a plane parallel to the plane of the electronic circuit. . The axis of the cavity containing the piston is by construction perpendicular to the plane of the substrate and therefore ideally to the plane of the circuit, in this case the entire lower end of the piston is in contact with the circuit. However, in general, on the quantity of circuits mounted on the substrate, a non-negligible number of them presents an inclination by compared to the plane of said substrate, in this case only a part of the lower end of the piston is in contact with the circuit and the heat transfer is then no longer optimal. In an attempt to remedy this drawback, taking into account the mechanical rigidity of the pistons, provision is then made in the aforementioned request to give a frustoconical shape to the piston and to increase the diameter of the cavity in which said piston is housed to allow the latter to take the inclination necessary for the alignment of its lower end with the plane of the circuit and thus impose an integral contact.

Such a solution nevertheless has serious drawbacks. Indeed, for the heat exchange to take place effectively, another requirement must be satisfied: the overall thermal resistance must be of value as reduced as possible. However, in the present case, if the thermal resistance in the vicinity of the circuit and of the lower part of the piston is low (the contact surface being maximum, this implies an air gap between these two thin parts), the resistance thermal in the vicinity of the contacts between the piston and the cavity, on the other hand, is of high value and this over most of the range of variation of the inclination of the circuit. Thus, when the angle of inclination of the circuit is zero or of small value, no contact exists between the piston and the cavity, the thermal resistance is consequently high because the air gap surrounding the piston in the cavity has a not insignificant thickness, especially since the diameter of the cavity has been increased. From a certain value of angle of inclination, there is contact between piston and cavity but this contact is almost punctual and the value of the thermal resistance even reduced is still not negligible. Finally, the value of the thermal resistance is minimal but still significant for an almost linear contact of the piston with the cavity, that is to say for a value of angle of inclination equal or close to the piston. The object of the present invention is to provide a cooling system for “multi-chip” modules, preferably of medium dissipation, which is simple, effective, non-hermetic and which does not have the drawbacks of known systems.

For this, the cooling system of the kind mentioned in the preamble uses as a heat exchange means a flexible metallic device with low thermal resistance and is remarkable in that the surfaces of the flexible metallic device in contact at both ends, under pressure determined, with the circuit on the one hand and the cover on the other hand are flat and have a surface condition of flatness and roughness such that it allows the control of the two air spaces between on the one hand the circuit and the first of the two ends and on the other hand, when it exists, between the cover and the second of the two ends, the flexible metallic device with low thermal resistance compensating for the differences in altitude and angles between the substrate and the different circuits and minimizing thermal resistance in the vicinity of said air spaces, while the differential expansions are automatically compensated by the movement of gli relative to the circuit.

Thus the device chosen as a heat exchange means, by virtue of its flexibility, easy deformation ensuring an integral and systematic application of its contact surfaces with the surfaces of the circuit on the one hand and of the cover on the other hand. These contact surfaces are flat and machined or treated so as to effectively control the air gaps existing at the contacts and obtain thermal resistances of very low value. The low thermal resistance of the device itself results in an overall thermal resistance also of low value and therefore an efficient transfer of heat. The elasticity of the various devices advantageously guarantees on the one hand a good application of these on the circuits under a relatively low pressure imposed by the cover during its association with the substrate and on the other hand compensation for the differential expansions between the substrate and the cover. A good yield is thus obtained which authorizes use of the system with “multi-chip” modules of average dissipation without internal presence of a fluid at the “chips” and therefore without it being necessary to impose a hermetic closure. . The efficiency can also be improved by using greases at the level of the contacts with the circuits and / or by providing means for extracting heat at the level of the cover, for example a circulation of fluid on the upper surface of the latter.

According to a preferred application, the flexible metal device with low thermal resistance is a bellows heat pipe of which the evaporator, a metal plate, is the part in contact with the circuit, the condenser, another metal plate, the part in contact with the hood and the body connecting the evaporator to the condenser forms a metal bellows, the assembly containing a heat transfer fluid.

In this way, the choice of using a heat pipe or heat tube which is a static device of very high thermal conductivity and therefore of very low thermal resistance allows efficient transfer of heat from a circuit to the hood. The choice of using a heat pipe also has an important advantage. In fact, with this type of device, the more the power dissipated and therefore the energy to be transferred increases, the more the overall thermal resistance decreases, and this in a non-linear manner, while in addition the thermal resistance decreases when the temperature of the condenser. increases (within reasonable limits of course,

• ie prohibiting the drying of the heat pipe). Also, the bellows body gives this device the desired flexibility, under a determined pressure, allowing the deformation necessary to compensate for differences in altitude or inclination. In addition, the plasticity due to the choice of material chosen for the bellows, for example a copper alloy, makes it possible to "limit the bearing force and therefore the pressure of the heat pipe on the circuit.

In a preferred embodiment, each bellows heat pipe is fixed by its condenser to the cover, the assembly being associated with the substrate under a pressure determined by means of fixing means. Thus, to each circuit corresponds a position of a bellows heat pipe which will come naturally and without adjustment to appear and be applied to the associated circuit. The flexibility of the heat pipe, unlike the rigid systems of the prior art, necessarily results in application of a good quality planar contact which favors the decrease in thermal resistance, while the various expansions are automatically compensated for by the sliding movement of the evaporator relative to the circuit, movement of small amplitude allowed because of the freedom left between heat pipe and circuit. The pressure that the heat pipe must exert on the circuit, which as has been seen previously is of limited value due to the plasticity of the material of the bellows, is determined by the means for fixing the cover to the substrate. Thus a bearing force for example of around 300 to 400 g is generally sufficient, this can be easily obtained when the cover is closed on the substrate by means of screws. Furthermore, since no fixing is provided between a heat pipe and its associated circuit, it follows that no degradation of the circuit is caused, as is often the case in the systems of the prior art, the circuits being , during the removal of the cover, found in their initial mechanical state. According to an additional characteristic of the cooling system according to the invention, each bellows heat pipe also comprises, located along its evaporator, a wick allowing use in the horizontal direction. This wick is preferably made of a fine mesh metal mesh which will thus allow capillary action to raise the heat transfer fluid when the cooling system is used with the heat pipes in a horizontal position _

The following description with reference to the accompanying drawings, all given by way of example, will make it clear how the invention can be implemented.

The icrure 1 schematically represents a section of a cooling system for a "multi-chip" module according to the invention.

Figure 2 shows in section an example of a flexible metallic device with low thermal resistance usable in the cooling system according to the invention.

Ficrure 3 shows in section another example of a flexible metallic device with low thermal resistance usable in the cooling system according to the invention.

In Figure 1 is proposed a section of a cooling system for "multi-chip" module comprising a substrate 1 on which is arranged a plurality of electronic circuits 2a, 2b, 2c, ..., substantially flat. A partially flat cover 3 thermally conductive is associated with the substrate 1 by means of fixing means 4, for example screws. The cover 3 cooperates with a plurality of heat exchange means 5a, 5b, 5c, ..., each in relation on one side respectively with a circuit 2a, 2b, 2c, ..., and on the other with cover 3 to transfer heat from the associated circuit to the cover.

The heat exchange means 5a, 5b, 5c, ..., is a flexible metallic device with low thermal resistance. According to a preferred application, this device is a bellows heat pipe more precisely described with the figure 2. In accordance with the idea of the invention, the surfaces at the two ends of the device 5a, 5b, 5c, ..., in contact on the one hand with the circuit 2a, 2b, 2c, ..., respectively, and on the other hand with the cover 3 are flat and are machined or treated so as to have a surface condition of flatness and roughness such that it allows the control of the two air spaces between, on the one hand the circuit 2a , 2b, 2c, ..., and the first of the two ends and, on the other hand, when it exists, between the cover 3 and the second of the two ends, the flexible metallic device with low thermal resistance compensating for the differences d altitude and angles between the substrate 1 and the different circuits 2a, 2b, 2c, ..., and minimizing the thermal resistances in the vicinity of said air spaces, while the differential expansions are automatically compensated for by the sliding movement compared to the circuit. Thus these planar surfaces are in intimate contact minimizing the air space and thereby the thermal resistance. The surface condition of the devices is intended to be harmonized with that of the circuits for which the flatness / roughness is known from their specification, thereby enabling the volume of air to be controlled between the contact surfaces. In addition, this volume of air can be further minimized by adding thermal grease to the contacts. The bearing force of the cover 3 on the devices 5a, 5b, 5c, ... makes it possible to maintain sufficient pressure to ensure the minimum contact force necessary allowing the deformation of the flexible body of the devices 5a, 5b, 5c,. .., and thus the compensation of the inclinations. This bearing force is determined by the fixing means 4 of the cover 3 on the substrate 1, its intensity of the order of 300 to 400 g per circuit, is limited due to the plasticity of the bellows thus advantageously reducing the forces applied on the circuits. Of the three devices 5a, 5b, 5c shown in FIG. 1, only the device 5a has not undergone deformation, the upper surface of the circuit 2a being parallel to the plane of the substrate. The intentionally exaggerated distortion of devices 5b and 5c shows that any inclination is automatically and fully compensated while maintaining the entire upper surface of the devices 5a, 5b, 5c, ..., in close contact with the cover 3 and therefore without increasing the thermal resistance.

In a preferred embodiment, the devices 5a, 5b, 5c, ..., are bellows heat pipes, bellows heat pipes of which the evaporator, a metal plate, is the part in contact with the circuit, the condenser, another metal plate, the part in contact with the cover and the body connecting the evaporator to the condenser forms a metal bellows, the assembly containing a heat transfer fluid.

The devices 5a, 5b, 5c, ..., are preferably fixed by their condenser to the cover 3, any fixing means that can be used: glue, solder, screws, etc ... (depending on the fixing means chosen, the air gap between condenser and cover will exist or will not exist).

In addition, for modules with higher dissipation, pumping and heat extraction means can be associated with the cover 3, means which can consist, for example, of an intimately linked radiator (either integrated or added) to the cover or a circulation of a fluid on the upper surface of the latter.

In Figure 2 is proposed in section an example of a flexible metallic device with low thermal resistance, here a bellows heat pipe, which can be used in the cooling system according to the invention.

The heat pipe 5 is mainly composed of three metal parts: the evaporator 51, the condenser 52 and connecting the condenser to the evaporator, the bellows 53. The evaporator 51 is constituted by a metal plate, for example made of copper, which has the. shape of a disc in the upper part of which a recess was made to receive the lower part of the bellows 53. The lower part of this disc is, .in operation, applied to the circuit to be cooled, it must be perfectly flat and its surface condition harmonized with that of the circuits to be cooled. The condenser 52 which has, here, the same shape and the same characteristics as the evaporator 51, is symmetrically opposite the latter relative to the median plane perpendicular to the axis XX 'of the bellows, it receives in its recess the upper part of the bellows 53. The condenser is in contact and preferably is fixed to the cover 3. The bellows 53 can be glued, welded or brazed, avoiding burrs, to the hollow parts of the evaporator 51 and condenser 52 to form a hermetic device containing the heat transfer liquid 54. A person skilled in the art will be able to easily define the diameter and the thickness of the metal plates, the thickness of the bellows and the composition of the heat transfer liquid. However, it should be noted that the thickness of the bellows must suit the desired flexibility. Too much thickness decreases. flexibility while a thickness that is too small increases the thermal resistance, the latter possibly being reduced by choosing an adequate heat transfer liquid and / or by increasing the diameter of the metal plates in contact with the circuit to be cooled.

In addition, so that the heat pipes can be used in a horizontal position, a wick 55 can be added, in this case it will be located along the evaporator 51. Such a wick is preferably made of a wire mesh with a fine mesh. .

According to a second embodiment shown in Figure 3, each device 5a, 5b, 5c, ..., consists mainly of an upper metal plate 56 in contact with the cover 3 and a lower metal plate 57 in contact with the associated circuit, the plates 56 and 57 being connected together by a metal bellows 58 and the assembly containing a good fluid 59 conductor of heat, for example a liquid metal in the desired temperature range such as gallium or mercury. The upper plate 56 further comprises a plunger 56a (maintained by or integrated into the plate 56 as drawn in FIG. 3) partially bathing in the fluid 59.

The fluid 59 thus compensates for the differences in inclination and the variations in the difference between the plunger 56a and the bottom plate 57 induced by the differences in altitude between the circuit 2a, 2b, 2c, ..., and the cover 3 and allowed by the metal bellows 58, while the overall thermal resistance of the device 5a, 5b, 5c, ..., is kept of low value. The metal plate 56 of each of the devices 5a, 5b, 5c, ..., is preferably fixed to the cover 3, any means that can be used: glue, welding, screws, etc., ... (depending on the method of fixing chosen , the air gap between the plate 56 and the cover 3 will exist or will not exist).

This simple and efficient cooling system advantageously operates in the conventional temperature ranges of electronics, from minus a few tens of 'C to more than 100 ° C. It is preferably used in combination with "multi-chip" modules of average dissipation, of the order of a few tens of / cm.

Claims

Claims:

1. Cooling system for a “multi-chip” module comprising a substrate on which a plurality of substantially flat electronic circuits is arranged, associated with the substrate a partially flat thermally conductive cover cooperating with a plurality of heat exchange means each in relation on one side with a circuit and on the other with the cover to transfer heat from the circuit to the cover, the heat exchange means being a flexible metallic device with low thermal resistance, characterized in that the surfaces of the device flexible metal in contact at both ends, under a determined pressure, with the circuit on the one hand and the cover on the other hand are flat and have a surface condition of flatness and roughness such that it allows the control of the two blades of air between the circuit on the one hand and the first of the two ends and on the other hand, when it exists, between the cover and the second of the two ends s, the flexible metallic device with low thermal resistance compensating for the differences in altitude and angles between the substrate and the different circuits and minimizing the thermal resistances in the vicinity of said air spaces, while the differential expansions are automatically compensated by the sliding movement relative to the circuit.

2. Cooling system for "multi-chip" module according to claim 1, characterized in that the flexible metallic device with low thermal resistance is a bellows heat pipe of which the evaporator, a metallic plate, is the part in contact with the circuit, the condenser, another metal plate, the part in contact with the cover and the body connecting the evaporator to the condenser forms a metal bellows, the assembly containing a heat transfer fluid.

3. Cooling system for "multi-chip" module according to claim 2, characterized in that each bellows heat pipe is fixed by its condenser to the cover, the assembly being associated with the substrate under a pressure determined by means of fixation.

4. Cooling system for “multi-chip” module according to claim 3, characterized in that each bellows heat pipe also comprises, located along its evaporator, a wick thus authorizing use in the horizontal direction.

5. Cooling system for "multi-chip" module according to claim 4, characterized in that the wick consists of a wire mesh with a fine mesh.

6. Cooling system for "multi-chip" module according to claim 1, characterized in that the low thermal resistance flexible metal device mainly ^consists' of an upper metal plate in contact with the cover and a metal plate lower in contact with the circuit connected to each other by a metal bellows, the assembly containing a fluid which is a good conductor of heat, the upper metal plate further comprising a plunger partly bathing in the fluid, the latter thus compensating for the differences in inclination and the variations of the difference between the plunger and the lower metal plate induced by the differences in altitude between the circuit and the cover.

7. Cooling system for “multi-chip” module according to claim 6, characterized in that the upper metal plate of each of the flexible metal devices with low thermal resistance is fixed to the cover, the assembly being associated with the substrate under pressure. determined by fixing means.

8. "Multi-chip" module using the cooling system according to one of the preceding claims.

9. Device, machine, computer using at least one “multi-chip” module according to claim 8.