GB2247782A

GB2247782A - A circuit board assembly

Info

Publication number: GB2247782A
Application number: GB9019338A
Authority: GB
Inventors: Ian Massey-Crosse
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1990-09-05
Filing date: 1990-09-05
Publication date: 1992-03-11
Also published as: GB9019338D0

Abstract

A circuit board assembly is provided having at least one integrated circuit package E mounted on a circuit board A either directly or via a socket G, G'. An air gap is present between the integrated circuit package and the surface of the circuit board and accommodates a heat shunt H which is connected thermally to the base of the integrated circuit package and to the surface of the printed circuit board. Heat from the integrated circuit package is conducted through the heat shunt to the circuit board and is dispersed via the power planes in the circuit board. Thermal vias I, which may be plated through holes, may be provided through the board directly beneath the heat shunt to increase thermal conduction away from the integrated circuit package. <IMAGE>

Description

A CIRCUIT BOARD ASSEMBLY The present invention relates to circuit board assembly including a heat shunt for use with integrated circuit packages which are mounted on a printed circuit board.

A problem exists when integrated circuits are mounted on printed circuit boards, in that the reliability of the integrated circuits is often reduced because of the heat dissipated within the integrated circuit.

Thermal pillars and thermal ladders are techniques already in use on conduction cooled printed circuit boards to promote heat flow from the base of an integrated circuit package. The thermal ladder is similar to a grid and is bonded to a surface of a printed circuit board. The integrated circuit straddles the rungs of the ladder, and heat dissipated by the integrated circuit is conducted by the ladder to the edge of the board which is clamped into the cold walls of a box in which the boards are housed.

Thermal ladders are used, for example, in situations where it is not reliable to cool the integrated circuits by air flow such as, in avionics, where the air may be rarified or moist.

An object of the present invention is to provide a heat shunt which reduces the operating temperature of an integrated circuit which is mounted on a convection cooled printed circuit board, and thereby improves the reliability of the integrated circuit.

A further object of the present invention is to provide a heat shunt which may be used in conjunction with an integrated circuit which is mounted within a socket.

According to the present invention there is provided a printed circuit board assembly comprising: at least one integrated circuit package mounted upon said board, such that an air gap exists between the integrated circuit package and the circuit board, characterised in that a heat shunt is accommodated within said air gap and is thermally connected to the integrated circuit package and a surface of the circuit board to transfer heat from the integrated circuit package to the circuit board.

According to yet a further aspect of the present invention, the circuit board is provided with a least one thermal via, comprising a plated-through hole for or other means for the conduction of heat from the heat shunt to copper foils within the circuit board and to the opposite surface of the circuit board.

According to a further aspect of the present invention, the integrated circuit package may be directly connected to the circuit board or may be connected thereto by means of an integrated circuit mounting socket.

The present invention therefore has the advantage of providing a means whereby heat is efficiently dissipated from the integrated circuit and thereby increases the reliability of the integrated circuit. The operating temperature of the integrated circuit may also be reduced sufficiently to allow the use of a less expensive, low temperature component.

Various embodiments of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a cross sectional view of part of a convection cooled printed circuit board assembly having a conventional integrated circuit mounted within a socket which is soldered to the board, together with its associated thermal resistance diagram, Figure 2 shows a convection cooled printed circuit board assembly having an integrated circuit mounted as shown in Figure 1 with an air gap between the integrated circuit and the printed circuit board in which is accommodated a heat shunt in accordance with the present invention, together with its associated thermal resistance diagram, and, Figure 3 shows a further embodiment of the present invention, included plated-through thermal via holes which increase thermal dissipation, together with its associated thermal resistance diagram.

Referring to Figure 1 a standard printed circuit board is shown designated A in which is soldered an integrated circuit mounting socket B, B'. The integrated circuit comprises a package C, electrical conductive leads D, D', an integrated circuit chip E connected to the electrical conductive leads D, D' by a way of wire connections F F'.

Electrical connection is made to the printed circuit board by way of the electrical connection leads D, D' andthe conductive legs G and G' of the mounting socket. The numbers shown on the diagram of the integrated circuit conform with the nodes shown on the thermal resistance diagram associated with the assembly. 1 represents the air surrounding the assembly. 2 represents the junction of the integrated circuit chip. 3 represents the area of the integrated circuit package directly beneath the integrated circuit chip. 4 represents the base of the integrated circuit package. 5 represents the surface of the printed circuit board directly beneath the base of the integrated circuit package. 6 represents the core of the printed circuit board. 7 represents the connection lead emanating from the integrated circuit package. 8 represents the lower extremity of the connection lead, and 9 represents the top surface of the integrated circuit package.

On the thermal diagram the letter Q represents the thermal dissipation of the chip, eg 600 mW.

Typical thermal resistance values are shown for each element on the thermal diagram. The units used are OC per Watt, and the values shown are typical for a high density printed circuit board with forced air cooling. For example, 196 C per Watt is a typical value for the thermal resistance from the core of the printed circuit board 6 to the air 1. Similarly 99 OC per watt is shown as a typical value for the thermal resistance from the top surface of the integrated circuit package 9 to the air 1. It will be noted that a value of 6.3 exists between nodes 2 and 3. A value of 23 exists between nodes 3 and 7. A value of 5.6 exists between nodes 7 and 9. A value of 3.9 exists between nodes 3 and 4. A value of 40.6 exists between nodes 5 and 6. A value of 35 exists between nodes 7 and 8.A value of 12.6 and 25.7 which together equal 38.3 exists between nodes 8 and 6. A value of 520 exists between node 4 and node 5 which represent the air gap between the base of the integrated circuit package and the surface of the printed circuit board, and it is this value which is greatly reduced by the present invention.

The references Rx and Ry have values of 64.3 and 19.2 respectively and represent the thermal resistance of the conduction paths through the plane of the printed circuit board in the X plane and the Y plane away from this particular location, where the direction of air flow is parallel to the Y plane.

Referring to Figure 2, an integrated circuit package is shown mounted in a connection arrangement mounted on a printed circuit board. The parts identical to those shown in Figure 1 are designated the same. However it will be evident that a heat shunt H is now positioned in the air gap between the base of the integrated circuit package C and the surface 5 of the printed circuit board A. The heat shunt may be made of any material which provides the required improvements in heat flow. For best results a high thermal conductivity material is used. The heat shunt should be in good thermal contact with both the surface of the printed circuit board and the base of the integrated circuit package. The heat shunt may be bonded to the printed circuit board with an adhesive, which may be an epoxy with a filler material to improve the thermal conductivity. An adhesive or heat sink compound may be used to fill the gap between the integrated circuit package and the heat shunt.

As can be seen from the associated thermal diagram, the thermal resistance between nodes 4 and 5 is greatly reduced from a value of 520 as shown in Figure 1, to a value of 14 as shown in Figure 2. A reduction in the Rx value is also apparent. It will therefore be appreciated that the object of the present invention has been satisfied, the heat shunt providing a very much reduced thermal resistance than the air gap in which it is accommodated.

Referring to Figure 3, a further embodiment of the present invention is shown. The drawing shows exactly the same components as shown in Figures 1 and 2 and like components are given like designations.

It is evident that a thermal via I is provided in the board; this may conveniently be a plated through hole. In practice any number of thermal vias could be positioned in the board beneath the length of the integrated circuit package. The thermal vias provide an enhanced thermal conduction path from the surface of the printed circuit board to the thermal conduction planes within the board and to the opposite surface of the board, and greatly enhance the dissipation of heat.

The thermally conductive planes in the board may conveniently be zero power level planes or the power planes of the board.

Referring now to the associated thermal diagram it is evident that by using thermal vias the thermal resistance between nodes 5 and 6 is reduced. The thermal resistance between nodes 6 and 8 is reduced due to the presence of additional thermal conduction planes within the printed circuit board, and there is an associated reduction in the Rx and Ry values of the thermal resistance through the plane of the printed circuit board.

The use of the thermal vias and additional thermal conduction planes within the printed circuit baord does not improve the reduction of the thermal resistance of the air gap but it evident that a greater overall reduction in operating temperature of the integrated circuit is achieved when such enhancements are used, and therefore once again the object of the present invention is satisfied.

It will readily be appreciated by those skilled in the art that alternative embodiments for implementing the present invention are possible which fall within the spirit and scope of the present invention. Although the integrated circuit packages described in the example are designed for mounting in holes in the printed circuit board, the present invention is equally suitable for insertion under integrated circuit packages which are mounted on the surface of the printed circuit board. It will also be appreciated that while the above embodiments are described with respect to the shunting of heat from an integrated circuit package to a printed circuit board, the invention is equally applicable in those cases where heat is required to be transferred from a printed circuit board to an electrical component to provide optimum operation of that component.

Claims

1. A circuit board assembly comprising at least one integrated circuit package mounted upon said board such that an air gap exists between the integrated circuit package and the circuit board, characterised in that a heat shunt is accommodated within said air gap and is thermally connected to the integrated circuit package and a surface of the circuit board to transfer heat from the integrated circuit package to the circuit board.

2. A circuit board assembly as claimed in claim 1, wherein as least one thermal via is provided through the circuit board directly under the heat shunt, said thermal via being a plated through hole or other means to provide thermal conductivity through the circuit board, to connect thermally conductive planes.

3. A circuit board assembly as claimed in claim 1 or 2, wherein the integrated circuit package is mounted directly to the circuit board or mounted in an integrated circuit socket.

4. A circuit board assembly as claimed in claim 2 or 3, wherein the planes are power planes comprising copper conductive paths by way of which the heat from the integrated circuit package is dissipated.

5. A circuit board assembly substantially as herein before described.

6. A circuit board assembly substantially as herein before described with reference to Figure 2 and 3 of the accompanying drawings.