GB2259408A

GB2259408A - A heat dissipation device

Info

Publication number: GB2259408A
Application number: GB9119187A
Authority: GB
Inventors: Zalman Schwartzman
Original assignee: Motorola Israel Ltd
Current assignee: Motorola Solutions Israel Ltd
Priority date: 1991-09-07
Filing date: 1991-09-07
Publication date: 1993-03-10
Also published as: GB9119187D0

Abstract

A discrete heat dissipation device 20 coupled between a first point of an electrical circuit which in operation has a relatively high temperature and a second point which has a relatively low temperature. The device comprises a substrate (21 fig 2) having thermal and electrical conductive pads (22) and an electrical capacitance sufficiently low and an electrical resistance sufficiently high to have negligible electrical effect on the operation of the circuit, whereby heat is sunk through the device from the first point to the second point. Substrate materials of beryllium oxide and aluminium nitride are exemplified. <IMAGE>

Description

A Heat Dissipation Device.

Background to the Invention.

This invention relates, in general, to the field of heat dissipation and is particularly, but not exclusively, applicable to the alleviation of localised heat spots within printed circuit boards and the components situated thereon.

Summarv of the Prior Art.

The localised heating of solder joints and components within electronic equipment causes concern since reliability problems can result therein. The efficient dissipation of heat to alleviate hot spots within electronic equipment is therefore of great importance. A manifestation of this undesirable effect occurs in the application of microstrip lines in high power applications. A second corollary occurs at joint pads between high power device leads and printed circuit boards (PCBs).

Microstrip lines are commonly used in high power radio frequency (RF) circuit designs. The implementation of microstrip lines within this application is particularly attractive since the microstrip line may be easily printed onto two-sided printed circuit boards which already exist within the radio frequency circuitry.

With reference to the typical prior art printed circuit board configuration of FIG. 1, microstrip conductors 10 are formed upon an upper surface of a PCB 11. The PCB 11 is constructed from a dielectric material, as would be obvious to one skilled in the art.

Generally, these dielectric materials have good electrical but poor thermal conductive properties. Therefore, the use of microstrip conductors within high power applications is limited because of the thermal isolation of the microstrip conductor from a ground plane 12. For example, localised hot spots are known to limit reliability within high frequency power amplifiers. This reliability problem is particularly prevalent at the microstrip conductor output matching circuit of such amplifiers, at which point solder joint failures, permanent dielectric PCB material property degradation or component burn-out can result. Fortuitously, the ground plane 12 is usually cold with respect to the RF circuit, and can be easily heat-sunk without any adverse effects on the performance of the microstrip conductor 10.Unfortunately, direct methods of heat-sinking the microstrip conductor 10 degrade the RF performance thereof.

It can clearly be appreciated that there is a requirement for a device that provides an efficient thermal via, with a minimal effect on circuit performance, between a hot conductor and a ground plane Summarv of the Invention.

This invention addresses at least some of the disadvantages set out in the prior art described above. In accordance with the present invention, there is provided an electrical circuit which comprises a discrete heat dissipation device connected between a first point on the circuit which, in operation, has a relatively high temperature and a second point which has a relatively low temperature. The discrete device comprises a substrate having thermal and electrical conductive pads coupled at opposite ends thereof. Furthermore, the substrate has an electrical capacitance sufficiently low and an electrical resistance sufficiently high between said ends so as to have negligible electrical effect on the operation of the circuit, whereby heat is sunk through the heat dissipation device from the first point to the second point.

A heat dissipation device so designed and described would therefore produce the novel advantages of a substantial increase in the power handling capacity of the microstrip conductor, and a substantial improvement in the reliability and quality of solder joints used in either high power device leads or thermal cycling environments. In addition, the negligible electrical effects incurred through the implementation of this heat dissipation device allows for its immediate application within the majority of existing circuits, which experience the derogatory effects of localised heat spots, without limiting the performance capabilities thereof.

An exemplary embodiment of the invention will now be described with reference to the accompanying drawings.

Brief Description of the Drawings.

FIG. 1 illustrates the construction of a typical prior art printed circuit board.

FIG. 2 illustrates a preferred embodiment of a heat dissipation device in accordance with the present invention.

FIG. 3 illustrates an application of the device of FIG. 2 on a printed circuit board.

FIG. 4 illustrates an alternative application of the device of FIG. 2 on a printed circuit board.

Detailed Description of the Preferred Embodiment.

FIG. 2 illustrates a heat dissipation device 20 in accordance with a preferred embodiment of the invention. A ceramic or single-phase polycrystalline substrate 21, is constructed from a material of high thermal conductivity, X, such as, for example, beryllium oxide (BeO = 219.66 W m-l K-1 at 373.16K; 1987 CRC Handbook of Chemistry and Physics). The thermal conductivity, X, of such a ceramic or polycrystalline, single-phase compound is of an order of magnitude greater than that for an aluminium oxide based prior art resistor. Metalization 22 is applied to opposite ends of substrate 21 in order to form an electrical and thermal bonding area.The substrate 21 and metalization 22 may be in a regular form, such as the rectangular block illustrated, with typical dimensions comparable with either a resistor or chip capacitor i.e. -5mm3. Such a configuration lends itself to implementation through surface mount technology. The device 20 is further characterised in that it has a very low electrical capacitance (-0.14pF = O.l4B-l2 Farads) and very high electrical resistance (-10 0Mfl) between its pads. In addition, empirical result indicate that the device 20 has a thermal resistance of approximately 7.6 K W-1; this being substantially higher than that for a comparable resistor.

By way of example, a first method of implementation for the device 20 is illustrated in FIG. 3. A two-sided PCB 11 comprises microstrip conductors 10 and an upper ground plate 30 deposited on an upper surface thereof. A lower ground plate 12 is deposited on an underside of the PCB 11. This lower ground plate acts as, or is coupled to, a heat-sink (not shown). A thermal via 31 is provided between the upper 30 and lower 12 ground plates by means of a core of solder fused therebetween. Clearly, it should be appreciated by one skilled in the art that the thermal via 31 is essentially any good thermal conducting medium e.g. an aluminium wire. The heat dissipation device is then coupled between the upper ground plate 30 and the microstrip conductor 10 using surface mount techniques 32.

With reference to FIG. 4, an alternative implementation for the heat dissipation device 20 is demonstrated. In this instance, the device 20 is coupled between a lower ground plate 12 and the lead 41 of a high power device, such as a power transistor or a power resistor. The device provides heat-sinking for solder joints 42 which couple the lead 41 of the high power device to an electrical circuit (not shown).

Experimentation has shown that the preferred embodiment of the invention can achieve a reduction in the steady state temperature of a localised hot-spot by more than 42K, without a degradation in circuit performance. In the majority of cases, the application of this device can be considered as only relating to the thermal considerations within the circuit. An exception arises in the application of the device to extremely high frequency devices, such as a 1GHz power amplifier. Minor circuit trimming may be required in order to achieve optimum component and circuit operation in extremely high frequency applications.However, the near negligible electrical effect incurred through the implementation of this heat dissipation device 20 is such that it can be dealt with at the design stage of the high frequency circuit without limiting the performance capabilities thereof e.g. by selectively trimming a microstrip conductor utilised therein. Furthermore, test result indicate that the recommended physical parameters for the substrate (21 of FIG. 2) and the device 20 are an electrical capacitance less than 0.2pF, a minimum resistance of lOMQ and a thermal resistance of approximately 7 K W-l; the combination of which is unobtainable in a prior art chip capacitor or resistor.

An invention so designed and described would therefore achieve the novel advantages of a substantial increase in the power handling capacity of the microstrip conductor, and a substantial improvement in the reliability and quality of solder joints used in either high power device leads or thermal cycling environments.

Typically, these high power applications might include implementation within high frequency power amplifier design. In addition, the characteristic properties of the device allow for its immediate implementation within the majority of existing circuits which experience the derogatory effects of localised heat spots.

Furthermore, the general size and configuration of the preferred embodiment gives rise to the benefit of ease of application within both new and existing circuits in which there is only a limited space available for such heat dissipation devices.

It will, of course, be understood that the above description has been given by way of example only, and that modifications of detail, such as the the variation in the form and shape of the invention e.g.

into a shape resembling that of a chip capacitor, can be made within the scope of the invention. Such a configuration would generate numerous benefits including the ease of device implementation without the necessity of additional and complex circuit assembly techniques. In addition, the nature of the substrate allows the device to be supplied in a reel or tape form; ideal for surface mounting techniques within electrical circuits. An alternative embodiment, obvious to one skilled in the art, would allow for coupling leads (not shown) to be attached to the metalization to facilitate coupling to an electrical component or PCB. Furthermore, it should be appreciated by one skilled in the art that the beryllium oxide substrate 21 could be substituted for an alternative material or ceramic with substantially similar physical properties, such as aluminium nitride.

Claims

Claims.

1. An electrical circuit comprising: a discrete heat dissipation device (20) connected between a first point on the circuit which, in operation, has a relatively high temperature and a second point which has a relatively low temperature comprising: a substrate (21) having thermal and electrical conductive pads (22) coupled at opposite ends thereof, and an electrical capacitance sufficiently low and an electrical resistance sufficiently high between said ends so as to have negligible electrical effect on the operation of the circuit, whereby heat is sunk through the heat dissipation device (20) from the first point to the second point.

2. An electrical circuit comprising: i) a printed circuit board (11) having an upper and lower surface; ii) a conducting track (10), deposited on said upper surface, having localised heat spots generated therein as a result of its power consumption within the electrical circuit; iii) an upper conducting ground plane (30) deposited on said upper surface; iv) a lower conducting ground plane (12) deposited on said lower surface for heat-sinking heat; v) means (31) for electrically and thermally coupling the upper and lower ground planes together, therein establishing equilibrium therebetween; and vi) a discrete heat dissipation device (20) of claim 1, wherein the thermal and electrical conductive pads (22) of the device (20) are coupled between the conducting track (10) and the upper ground plane (30), whereupon heat, generated at the localised heat spots within the conducting track (10), is sunk to the lower conducting ground plane (12) through the heat dissipation device (20) and the means for electrical and thermal coupling (31).

3. An electrical circuit comprising: i) a printed circuit board (11) having an upper and lower surface; ii) high power electrical components (41) which generate heat as a result of the power consumption thereof; iii) means (42) deposited on said upper and lower surfaces through which electrical components (41) are coupled to the electrical circuit; iv) a lower conducting ground plane (12) deposited on said lower surface for heat-sinking heat generated by the electrical circuit and components; and v) a discrete heat dissipation device (20) of claim 1, wherein the heat dissipation device (20) is coupled between said means (42) and the lower conducting ground plane (12), whereupon heat, generated at said means (42) by the coupling of the high power electrical component (41) thereto, is sunk to the lower conducting ground plane (12) via the the heat dissipation device (20).

4. A device (20) in accordance with any one of the preceding claims, wherein said thermal resistance is at least approximately 7 Kelvin per Watt.

5. A device (20) in accordance with claim 4, wherein said electrical capacitance is less than 0.2x10-12 Farads.

6. A device (20) in accordance with claim 5, wherein said electrical resistance is greater than lOMQ.

7. A device (20) in accordance with any one of the preceding claims, wherein the volume of the device (20) is approximately 5mm3.

8. A device (20) in accordance with any preceding claim, wherein the substrate (21) is constructed from a ceramic.

9. A device (20) in accordance with any preceding claim, wherein the substrate (21) is beryllium oxide.

10 A device (20) in accordance with any one of claims 1 to 8, wherein the substrate (21) is aluminium nitride.

11. A discrete heat dissipation device (20) for use in an electrical circuit comprising: a substrate (21), having thermal and electrical conductive pads (22) coupled at opposite ends thereof, with an electrical capacitance less than 0.2pF and an electrical resistance greater than lOMQ between said ends; characterised in that the substrate (21) has a thermal resistance greater than 7 K W-1.

12. A method of dissipating heat generated within an electrical circuit comprising the steps of: i) identifying a first point within a circuit which, in operation, has a relatively high temperature; ii) identifying a second nearby point within the circuit which, in operation, has a relatively low temperature; iii) providing a discrete heat dissipation device (20) comprising: a substrate (21) having thermal and electrical conductive pads (22) coupled at opposite ends thereof, wherein the substrate (21) has a sufficiently low electrical capacitance and a sufficiently high electrical resistance between said ends such that the device (20) has negligible electrical effect on the operation of the circuit; and iv) coupling said device (20) between said two points so as to sink heat through the device (20) from the first point to the second point.

13. A heat dissipation device (20) in accordance with any preceding claim, wherein coupling of the device (20) is achieved through a surface mounting technique.

14. A heat dissipation device (20) as substantially described herein and with reference to Figures 2 to 4 of the accompanying drawings.