ASSEMBLY OF AN ELECTRICAL COMPONENT AND A HEAT SINK ON A PRINTED CIRCUIT BOARD AND METHOD OF ASSEMBLING

Abstract: This invention is a means for assembling an electrical component in a casing (28) which in use generates enough heat to require a heat sink (26) on a printed circuit board (10). The casing (28) is mounted on studs (20) swaged into the base of the heat sink (26) and having projecting heads (25) which are tinned and fit in holes (35) in the board (10) so that the exposed ends can be soldered to the printed conductive pattern to hold the heat sink and component assembled with the board.

Description: HEAT SINK MOUNTING This invention relates to heat sink mountings and one object is to provide a simple and yet efficient way of mounting on a printed circuit board an electrical component such as a semi-conductor device which generates sufficient heat in use to require to be assembled with a heat sink. A heat sink is merely a body of thermally conductive material for example, copper or aluminium, which may include fins, which is in intimate contact with the heat generating device and which provides a substantial surface area for transmission of heat to the surrounding air. Printed circuit boards tend to have a number of components assembled with them, and electrically connected to the printed circuit by soldering, and it has been usual to connect up those components which do not generate much heat by a conventional wave soldering operation. Then it has been usual to clean and trim the board and inspect it and then mount semi-conductor,devices and the like with their heat sinks in separate manual operations with separate nuts or other connections passing through the circuit board after which the leads from the components are soldered to the printed circuit and cleaning,#trimming and inspection is repeated. According to one aspect of the present invention, an electrical component and a heat sink assembly is mounted on a printed circuit board by at least one stud which is secured at one end of the printed circuit board, and serves to hold the component assembled with the heat sink. The securing is conveniently by soldering the exposed end of the stud to the printed circuit, and for that purpose that part of the stud, at least can be plated with a suitable soldering metal. The stud can be a force fit in a corresponding hole in the heat sink, or can be otherwise assembled with it, and then the exposed end of the stud can be used for assembling the component with the heat sink, perhaps by use of threads on the stud and a co-operating nut, while the one end of the stud serves to hold the assembly of heat sink stud and component to the printed circuit board. An advantage is that assembly with the board is merely by soldering the exposed end of the stud to the printed circuit, and that can be done in the same machine operation as is used.for connecting other non-heat generating electrical components to the printed circuit. The stud conveniently has a flange serving to space the heat sink from the printed circuit board, and that can as thick as the desired space and sufficient in diameter merely to engage the facing surfaces of the heat sink and the board. If a large space is required, then there may be a stand-off shank between the flange and the one end of the stud which is effectively an elongation of the flange. According to the second aspect of the invention a heat sink suitable for carrying an electrical component is assembled with at least one stud for holding the sink and the component assembled which stud has a projecting end suitable for assembly with a printed circuit board, for example by soldering in which case the exposed end at least of the stud can be plated. The/or each stud can be force fitted or swaged into the heat sink, and may have a flange with a serrated edge for that purpose. The heat sink and indeed the board can have corresponding#clearance holes through which leads from the component can pass to the printed circuit on the remote side of the board to which they can be assembled by soldering. The invention includes a method of assembling a printed circuit board in which an assembly as defined above is fitted to the board, and the projecting end of the/or each stud is soldered to the printed circuit. That soldering may be in an operation used for soldering other electrical components for the printed circuit. Assembly is simple since all the components can be electrically connected and mounted in a single machine controlled soldering operation. Also the component can be replaced quite easily by disconnecting it from the Heat sink, and then using a duplicate to replace it. The invention may be carried into practice in various ways, and two embodiments will be described by way of example with reference to the accompanying drawings; in which FIGURE 1 is an exploded view o a semi-conductor device casesa heat sink, and a printed circuit board, to which the case and heat sink are to be assembled; FIGURE 2 is an elevation of one form of mounting stud used in the assembly of FIGURE 1; and FIGURE 3 is an elevation of an alternative form of mounting stud. The printed circuit board is of conventional construction comprising a non-conducting layer with a conductive pattern printed on the underside Various components ( not shown) are to be assembled with the board and connected to the pattern on the underside. Some of those components will constitute semi-conductor devices which generate substantial heat in use so that a heat sink is necessary to dissipate some of that heat. One such semi-conductor component is fitted in the case 28 of FIGURE 1 which is a standard TO-3 header and that is to be used with a single broad base heat sink 27 The board 10 has holes 11 for the leads 12 from the semi-conductor device to pass through to the underside of the board where they can be soldered to the conducting layer. There are also a pair of holes 35 for mounting studs 20 shown to a larger scale in FIGURE 2. The stud of FIGURE 2 comprises a tin plated head 23 which is to be a snug fit in a mounting hole 35 in the circuit board 10. Above the head 23 is a radially extending flange 21 and above that a serrated base 22 which is a light fit in a mounting hole 25 in the base 26 of the heat sink. The remainder of the stud 20 is in the form of a standard threaded shaft for a lock washer 32 and a nut 31. First of all two studs 20 have their bases 22 force fitted or swaged into holes 25 in the flat base 26 of the heat sink 27, from underneath, with the flanges 21 abutting the underside of the base. That presumably may be a machine operation. The case 28 is then mounted on the heat sink with the studs 20 passing through clearance holes in a flange on the case, and the lock washers and nuts are fitted and tightened to hold the flange of the case 28 on the base 26 of the heat sink 27. The leads 12 extend through clearance holes 30 in the base of the heat sink. The complete assembly of heat sink, studs, and semi-conductor case, is then fitted to the board 10 with the leads 12 passing through the clearance holes 11 in the board, and with the stud heads 23 fitting into the holes 35, the end surfaces of the heads 23 being flush with the printed conductive pattern on the underside of the board. Then in a conventional wave soldering operation in which other non-heat generating components have their leads connected to the printed circuit, the tin plated heads 23 are also soldered to the printed circuit to retain the heat sink and case assembly secured to the printed circuit board. The leads 12 are connected into the printed circuit in the same operation using conventional printed circuit board eyelets ( not shown). It will be appreciated that a single soldering operation is sufficient to connect all the components to the printed circuit, that is to say both the heat generating components which require a heat sink, and the other components which do not. Moreover since the heat sink is held in position on the board by soldering of the stud heads 23 to the printed circuit, it is possible to replace a semi-conductor device without requiring unsoldering and resoldering but merely by removing the nuts 31 and the case 28 and then replacing it with a new semi-conductor device. The heat sink 27 may be in contact with the upper surface of the printed circuit board 10, or possibly slightly spaced from it by the thickness of the flange 21;- whereas it may be desirable to provide a more substantial space between the heat sink base 26 and the upper surface of the printed circuit board to allow for circulation of cooling air. - In such an application the studs of FIGURE 3 would be used in preferance to those of FIGURE 2. The difference between the two forms of stud is that in the stud of FIGURE 3 there is between the flange 21 and the stud head 42 ( 23 in FIGURE 2) a stand off shank 40 with a diameter between those of the flange 21wand the head 42 to define a shoulder 41 which will bear against the upper surface of the printed circuit board 10 to leave a gap with a width equal to the length of the shank 40 and the flange 21 between the board and the heat sink base 26. Although it has been suggested that only the stud head 23 or 42 is tin plated, because that is the part that is soldered to the printed circuit, it may be convenient to plate the entire stud. The case could be secured to the studs 20 by use of other mechanical expedients, such as swage clasps rather than the nut and threaded connections described. The reference to tin plating includes coating with any solder aiding or solder promoting composition such as tin, tin lead alloy, flux and so on. The advantage of the invention is that the assembly of the semi-conductor device case and the heat sink can be assembled with the printed circuit board by placing in position, and soldering in the same soldering operation used for soldering other electrical components to the printed circuit board. That saves time, avoids difficulties that may arise dut to excessive handling of the components, and avoids the need for hand soldering.

Claims: CLAIMS 1. An assembly of an electrical components (28) and a heat sink (26) mounted on a printed circuit board (10) characterised in that the assembly is mounted on the board by at least one stud (20) which is secured at one end (23) to the printed circuit board and serves to hold the component assembled with the heat sink. 2. An assembly as claimed in Claim 1 in which the stud is secured at the one end to the printed circuit by soldering. 3. An assembly as claimed in either of the preceding claims in which the stud has a flange (21) serving to space the heat sink from the printed circuit board. 4. An assembly as claimed in any of the preceding claims in which the component is held to the heat sink by a releasable device (31). 5. An assembly of-a heat sink (26) suitable for carrying an electrical component (28) and at least one stud (20) for holding the sink and the component assembled and having a projecting end (23) suitable for assembly with a printed circuit board (10). 6. An assembly as claimed in Claim 5 in which the/or each stud is force fitted or swaged into the heat sink ( at 22,25). 7. An assembly as claimed in either of Claims 5 and 6 in which the/or each stud has a flange (21) for holding the heat sink spaced from a printed circuit board when it is assembled with the board. 8. An assembly as claimed in any of Claims 5-7 including an electrical component (28) removably mounted on the heat sink by means of the/at least one stud. 9. A method of assembling a printed circuit board in which an assembly as claimed in any of Claims 5-8 is fitted to the board, and the projecting end of the/or each stud is soldered to the printed circuit. 10. A method as claimed in Claim 9 in which the projecting end is secured to the printed circuit in an operation used for soldering other electrical components to the printed circuit.

Inventors: HUNDT, ROGER C., JORDAN, WILLIAM D., PRITCHETT, JAMES D.
Applicants: THERMALLOY INCORPORATED
Application year: 1980
Main classification: H01L23
IPC codes: H01L23, H05K1, H05K7, H05K3