WO2003094587A1

WO2003094587A1 - Improvements relating to heatsinks

Info

Publication number: WO2003094587A1
Application number: PCT/NZ2003/000077
Authority: WO
Inventors: Mark William Cotton; Daniel Richard Foster; Brent Laird Drysdale; David Simmons
Original assignee: Tait Electronics Limited
Priority date: 2002-05-02
Filing date: 2003-05-02
Publication date: 2003-11-13
Also published as: AU2003228162A1

Abstract

The present invention relates to components and a method for automated assembly of a heatsink (13), component (11) and PCB (12) assembly. The heatsink (13), component (11) and/or PCB (12) are profiled to compensate for part manufacturing tolerances, so that a separation exits between a surface of the heatsink (13) and bottom surface of the component (11) upon assembly. Coupling medium is the separation is heated conform to the separation and form a bridging thermal joint between the heatsink (13) and component (11).

Description

IMPROVEMENTS RELATING TO HEATSINKS

FIELD OF THE INVENTION

This invention relates to parts and associated methods for constructing electronic assemblies adapted to dissipate heat from electronic components. In particular it relates to a PCB, component and/or heatsink adapted for assembly to dissipate heat from the component, and an automated method of assembly.

BACKGROUND TO THE INVENTION

Active components in electronic circuits generate heat during operation. In many cases it is necessary to assist dissipation of this heat away from the components and circuit, often by way of one or more heatsinks or heat spreaders. Each heatsink is thermally coupled to one or more of the heat generating components, and may also be physically coupled to the circuit board and casing as required. Often, the heatsink is coupled to the PCB on the side opposite that which the component is installed. The heatsink and component are thermally coupled by a joint extending through an aperture in the PCB. For example, US 5,646,444 illustrates one example of this type of arrangement. The drawback of such assemblies is that various aspects of the designs prevent fully automated assembly and/or reduce the yield when assembly is carried out using an automated process.

As an example, existing designs have the base of the component abutting hard up against a face of the heatsink, through the aperture. Manufacturing limitations always result in the PCB, heatsink and component dimensions varying to within a particular tolerance. Therefore, where parts vary sufficiently from their nominal dimensions, certain combinations may result in the PCB being to "thin" for the heatsink and component combination. In this case, the terminals of the component may not reach the PCB for connection, or the terminal connections may be mechanically weakened due to an upward force from the heatsink. To prevent faulty assemblies occurring as a result, manual intervention is required to ensure a good connection between the component terminals and PCB. For example, they may need to be manually soldered. If left unattended, the faulty connections will reduce yield. Various assemblies may have other design features which prevent fully automated assembly and/or result in lower yields.

SUMMARY OF INVENTION

It is an object of the invention to enable installation of heatsinking capability in an electronic circuit utilising one or more automated techniques. In general terms, the invention relates to parts and/or a process for automated installation of a heatsink in a circuit, which compensate for manufacturing tolerances so that manual intervention is not required and a good yield results.

In one aspect the present invention consists in a method for assembling an electronic component and heatsink on a PCB including: coupling a heatsink onto a first side of a PCB such that a surface of the heatsink at least partially covers an aperture in the PCB, placing an electronic component onto a second side of the PCB such that the component is suspended over or within the aperture to provide at least some separation between the bottom of the component and the surface of the heatsink, wherein at least one of the heatsink, component, and PCB is profiled to compensate for component, heatsink and/or PCB tolerances to provide the separation.

In one aspect the present invention may be said to consist in a method for assembling an electronic component and heatsink on a PCB including: coupling a first side of a heatsink with a recess onto a first side of a PCB with an aperture such that the aperture and recess are at least partially aligned, depositing a portion of coupling paste in the recess such that the paste is at least partially exposed through the aperture, soldering an electronic component onto a second side of the PCB such that the component resides aligned with or at least partially within the aperture and contacts the coupling paste, and heating the coupling paste to form a thermally conducting joint between the component and heatsink. In another aspect the present invention may be said to consist in a set of parts for an electronic assembly including: a heatsink including a first side adapted for coupling to a first side of a PCB with an aperture, a component with attachment means adapted for coupling to a second side of the PCB such that, upon coupling, the component resides aligned with or at least partially within the aperture to provide at least some separation between the bottom of the component and the surface of the heatsink, wherein at least one of the heatsink and component is profiled to compensate for, upon coupling, tolerances in the parts to provide the separation.

In another aspect the present invention may be said to consist in a set of parts for an electronic assembly including: a heatsink including a first side with a recess, the first side adapted for coupling to a first side of a PCB with an aperture and adapted to enable the recess to at least partially align with the aperture upon coupling, a component with terminals adapted for coupling to a second side of the PCB such that the component resides aligned with or at least partially within the aperture upon coupling, wherein the recess is adapted to receive a deposit of coupling material which upon heating liquefies and adapts to occupy a gap which exists between the heatsink and component when both are installed in the aperture in the PCB.

In another aspect the present invention may be said to consist in a component, heatsink and PCB assembly including: a heatsink with a first surface coupled to a first side of a PCB over at least a portion of an aperture in the PCB, attachment means of a component coupled to a second side of the PCB such that the component is suspended over or within the aperture to provide at least some separation between the bottom of the component and the heatsink surface, wherein at least one of the heatsink, component and PCB is profiled to compensate for tolerances in the assembly to provide the separation

In another aspect the present invention may be said to consist in a component, heatsink and

PCB assembly including: A heatsink with a first side containing a recess, coupled to a first side of the PCB over an aperture in the PCB such that the aperture and recess are at least partially aligned, an electronic component coupled to a second side of the PCB such that the component resides aligned with or at least partially within the aperture, a thermally conducting joint between opposing surfaces of the component and recess providing thermal coupling between the component and heatsink, wherein the joint is formed by liquefying and resolidifying a coupling material wherein while liquefied the coupling material flows within the recess to bridge a gap existing between the heatsink and component.

In another aspect the present invention may be said to consist in a heatsink adapted for assembly with a component on a PCB, the heatsink including: a first side for coupling to a first side of a PCB with an aperture, a recess on the first side of the heatsink, the recess adapted for receiving a deposit of coupling material and positioned to at least partially align with the aperture when the first side is coupled to the PCB such that a surface of the component installed on a second side of the PCB can contact the coupling material deposited in the recess, wherein the recess is dimensioned to, when coupled to a PCB with an installed component, allow, upon heating, the coupling material to flow within the recess and bridge the space existing between the heatsink and component to form a thermally conducting joint.

BRIEF LIST OF FIGURES

Preferred embodiments of the invention will be described with reference to the following drawings, of which:

Figure 1 shows an isometric view of a preferred embodiment of a portion of an assembly according to the invention including a printed circuit board with an installed electronic component and associated heatsink,

Figure 2 shows an exploded isometric view of the assembly,

Figure 3 shows a plan view of the assembly,

Figure 4 shows a side elevation cross section of the assembly taken through XX, Figure 5 shows solder points for the heatsink on the underside of the PCB, Figures 6a-6e show a side elevation of a preferred method of installing the assembly, and Figures 7a and 7b show further detail of a thermally conducting joint between the component and heatsink of the assembly.

DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings it will be appreciated that the parts and methods described for providing heatsinking capabilities in accordance with the invention may be implemented in various forms. The following examples are given by way of example only. Details of surface mount population techniques are known to those skilled in the art and need not be described in detail here.

Figures 1 to 4 show a preferred embodiment of an assembly 10 according to the invention. The assembly comprises a printed circuit board (PCB) 12, (only a portion of which is shown in the Figures for clarity) on which an electronic component 11 and heatsink 13 are installed. The invention will be described in relation to one component and heatsink assembled on a portion of a PCB for illustrative purposes, however it will be appreciated that the PCB 12 may be adapted to have multiple heat generating components at various positions each of which is coupled to an associated heatsink in a similar manner. The assembly will usually form part of a larger electronic circuit with many components, some of which will not require heat dissipation. The component 11 shown is an RF power transistor in an SO 10 package however it will also be appreciated the invention could be employed in relation to various different component types in various different types of packages. Conducting tracks have been omitted from the PCB in the Figures 1-4 for clarity.

The component 11 shown in Figures 1-4 has a body 16 with protruding tabs 14a and 14b which form the gate and drain terminals respectively for soldering to circuit tracks on the PCB 12. A source terminal 14c is disposed on the bottom of the package for soldering to the heatsink 13. The heatsink 13 is profiled or adapted to include a recess 18 (visible in Figures 2 and 4) and a top face 19 of the heatsink 13 is affixed to the bottom side of the PCB 12 such that the recess 18 is at least partially aligned with an aperture 17 in the PCB 12. Preferably the surface area of the recess 18 is similar to the area of the aperture 17 although this is not essential. The tabs 14a, 14b, once soldered to the PCB 12, suspend the body 16 of the component 1 1 across or within the aperture 17 such that a gap 22 or separation (visible in Figure 4) exists between the bottom surface of the component body 16 and the surface of the recess 18.

The recess 18 is machined, stamped or otherwise formed to dimensions that compensate for tolerances in the PCB 12 thickness, and less significantly tolerances in the heatsink 13 itself and the component 11. This ensures, that upon installation, there is always at least a minimal gap between the bottom of the suspended body 16 of the component 1 1 and the surface of the recess 18, irrespective of variances in dimensions due to manufacturing limitations. This tolerance compensating separation enables the heatsink 13 and component 11 to be installed using an automated process. If separation is not provided, the tolerance in the PCB 12 means that in the case of some PCBs, the bottom of the body 16 of the component 1 1 would rest on the surface of the heatsink 13 and may prevent the component terminations 14a, 14b from being coupled effectively to the top of the PCB 12 resulting in reduced yield from the automated process. The separation provided by the recessed 18 profile of the heatsink 13 ensures the component body 16 is suspended over or within the aperture, thereby enabling a good connection of the terminations 14a, 14b to the PCB 12.

The terminal 14c of the component is thermally, and also usually electrically, connected to the recessed surface 18 of the heatsink 13 through the aperture 17 via a thermally and electrically conducting joint, comprising a thermal and/or electrical coupling medium. Preferably this connection is a solder joint (visible in Figures 7a and 7b) extending across the gap 22 between the terminal 14c and the surface of the recess 18 in the heatsink 13 which provides the thermal coupling, along with electrical coupling. However it will be appreciated that electrical coupling may not always be required and therefore the joint 20 could also be a non conducting thermal connection. The distance between the body 16 and the recess 18 shown in Figure 4 can vary from assembly to assembly due to manufacturing tolerances of the component 11, heatsink 13 and more significantly the PCB 12 thickness. The recess 18 provides space for excess solder to move to enable the solder connection, during formation, to compensate for the tolerances by adapting to the distance between the recess 18 surface and the component body 16 to form a secure thermal, and preferably, electrical bond 20. The amount of coupling medium forming the joint is measured to ensure that, in combination with the recess, there is sufficient to bridge the gap 22 between the recess face and component 16. The depth of the recess and amount of coupling medium is therefore designed in accordance with known manufacturing tolerances of the PCB, component and heatsink to achieve the bridge over the gap, while still ensuring the gap is large enough to account for dimension variances to prevent the component and heatsink abutting each other. The actual amount of coupling medium and recess size will depend on the tolerances in each particular case. The PCB 12 and heatsink 13 may also include holes 15a -15d for fasteners to further secure them to each other and possibly a larger heatsink if required. The assembly 10 enables heat generated by the component to be dissipated through the heatsink via the solder joint 20.

As component tolerances are compensated for by the combination of coupling medium and the recess, the assembly 10 shown in Figures 1-4 is adapted to be put together using an automated process, for example by utilising standard apparatus and techniques commonly used for producing surface mount PCBs. As known to those skilled in the art surface mount PCBs are populated by depositing solder paste on conducting component terminal pads on the PCB, subsequently placing the required components on the pads using a "pick and place" vacuum device or the like, and then heating the PCB, using a reflow apparatus or the like, to heat and melt the paste upon which solder connections between the components and the board are formed once the melted solder paste cools.

A preferred method by which the assembly is constructed will now be described in detail with reference to Figures 5 to 7b. The preferred order of installation is described, although it will be appreciated that the component and heatsink may be installed on the PCB in a different order. Figure 5 shows a solder pad 21 on the underside of the PCB 12 for enabling connection of the heatsink 13 to the PCB by way of a soldering process and preferably a surface mount soldering process. The dotted lines show the positions of the solder pad 21, heatsink 13 and recess 18 in relation to the aperture 17 when the heatsink is installed. Referring to Figure 6a, solder paste is deposited at various positions 23 on the solder pad 21 using a screen application method. A screen 24 may be placed over the PCB with apertures that coincide with positions 23 on the solder pad 21. A suitable amount of solder paste is applied to the screen and wiped over to leave solder past depositions at the aperture positions 23. The heatsink 13 then is placed on the solder pad 21 using standard surface mount "pick and place" equipment 25 as shown in Figure 6b.

As illustrated in Figure 6c the PCB 12 is then heated, for example by an infrared heat or convection oven source 26, to commence the reflow process whereby the solder paste 23 melts and flows to occupy substantially the entire pad 21. The pad is bounded by PCB resist that prevents the flow of solder beyond the boundary of the pad 21. As the solder paste 23 melts, the fluid dynamics of the solder assists positioning of the heatsink on the pad 21, and consequent alignment of the recess 18 over the aperture 17. In particular, the surface tension effect at the perimeter of the solder pad 21 causes a capillary action, or suction effect, of the solder between the heatsink and the PCB to overcome the weight of the heat sink while the assembly goes through the reflow process. This capillary action prevents the heatsink 13 falling away from the PCB, and also aligns the heatsink 13 to the pad. The melted solder paste 23 then cools and resolidifies to form solder joints between the heatsink 13 and solder pads 21 to secure the heatsink in place. The capillary action obviates the need for physical locator means, for example protrusions on the heatsink, recesses in the PCB or the like.

The PCB 12 and heatsink 13 assembly is then turned over so that the recess 18 is visible through the aperture 17 as shown in Figure 6d. A measured amount of solder paste 27b (which is determined in conjunction with the recess size and manufacturing tolerances as mentioned above) is deposited on the surface of the recess so it is visible in the aperture 17, and solder paste 27a, 27c is also deposited on terminal solder pads (not visible in Figure 6d) on the top side of the PCB 12 to enable connection to the component 1 1 gate and drain terminals 14a, 14b. Again the solder paste can be applied by way of a screen application process as described earlier. It will be appreciated that it is not essential for the deposit 27b on the recess to be solder paste. It could instead be any suitable thermal coupling material or medium, such as thermal coupling paste, which can form a thermal joint or connection between the component 11 and heatsink 13. The component 11 is placed on the PCB 12 with a "pick and place" device 25 as shown in Figure 6e so that the tabs 14a, 14b reside on their respective solder pads. In this position the body 16 of the component 11 sits over or within, at least partially, the aperture 17 so that the bottom surface of its body 16 sits on the solder paste 27b deposited in the recess 18. The PCB 12 is then subjected to an infrared or conventional heat source 25 as shown in Figure 6f to commence the reflow process. The terminals 14a, 14b are thereby soldered to the terminal pads on the top side of the PCB 12. When the PCB 12 is inverted and heated to install the component 11 the solder joints 23 holding the heatsink 13 in place will also remelt. A capillary action takes place between the solder pads 21 and the heatsink 13 to retain the heatsink 13 in place and affixed to the PCB 12.

During the heating process shown in Figure 6f the solder paste 27b in the recess 18 also melts, flows within the recess 18 and then cools to form a thermally conducting solder joint (and usually also electrically conducting, depending upon the coupling material) 20 between the bottom surface of the component body (in this case the source 14c) and the heatsink 13. The component 11, PCB 12 and heatsink 13 are manufactured to prescribed tolerances and as a result the gap 22 between the bottom surface of the component body 16 and the recess 18 can vary between assemblies. The recess thereby enables the melted solder paste 27b to reposition depending on the gap between the bottom surface of the component body and heatsink and compensate for any variances in the gap 22. For example, Figure 7a shows a joint 20 in which the gap 22 is relatively large and therefore the solder 20 has not spread out in the recess 18 as much as in, for example, the joint 20 in Figure 7b which is formed between a much smaller gap 22. The ability of the solder 20 to adapt in this manner to compensate for part tolerances enables a robust electrical connection 14a, 14b (and 14c if required) and thermal connection 20 to take place in an automated process. Further it reduces the stress on the terminals 14a, 14b and the surrounding PCB 12 which would otherwise have to compensate for a gap 22 size which was too small or too big. The size of the recess 18 can be changed for a particular assembly design along with the amount of solder paste 27b deposited in the recess 18. The amount of solder paste is chosen to ensure that, upon melting, there is sufficient to flow and occupy the gap between the component and heatsink to bridge the gap and form a joint.

The parts of the assembly, including the component, PCB with aperture and heatsink may be supplied separately or as set. Any such set may included multiple components and/or heatsinks all for installation on one PCB. The assembly could also be supplied fully installed. It will be appreciated that the actual order in which parts of the assembly are installed as shown in Figures 6a-6f will depend on various manufacturing factors. For example, the installation order could be reversed. That is, the component placed on the PCB first, solder paste deposited in the recess of the heatsink, and the heatsink then placed on the other side of the PCB. If several of the assemblies require installation on one PCB then for practicality reasons all the heatsinks may be installed first before all the components are installed, or vice versa. In any circuit, constructing the assembly according to the invention will occur as part of the overall PCB population process in which many components will be installed on the PCB, not all of which will require heatsinking. Again this may have a bearing on the population procedure which will be determined by the eventual production line requirements.

In an alternative embodiment the separation may be provided by adapting or profiling the component 1 1, and more particularly the placement of the component terminations 14a, 14b. The component 11 can be manufactured so that the terminations 14a, 14b protrude from a lower down position on the sides of the component 11. Alternatively the legs of the component could be bent downwards. This component profile has the effect of suspending the component body 16 higher up when the component 11 is installed on the PCB 12. This has the same effect as the recess 18, namely providing a tolerance compensating separation between the bottom of the component body 16 and the surface of an unrecessed heatsink. In yet a further embodiment, the PCB 12 can be profiled to have a larger thickness to provide the separation and therefore compensate for tolerance.

Claims

Claims:

1. A method of assembling an electronic component and a heatsink on a PCB using an automated process, including: coupling a heatsink with a recess and a bottom surface of a component through an aperture in a PCB by placing the heatsink and component over and/or within the aperture on opposite sides of the PCB with a coupling medium deposited on one or both of the heatsink recess and bottom surface of the component such that both contact the coupling medium, and heating the coupling medium to form a thermally conducting joint between the component and heatsink, wherein the depth of the recess is dimensioned to provide at least some separation between the surface of the recess and the bottom surface of the component to limit upward pressure on the component, and the quantity of coupling medium deposited on the recess and/or bottom surface of the component is controlled to ensure that, upon heating, the coupling medium bridges the separation to form a thermally conducting joint by conforming to the separation to compensate for tolerances in the dimensions of the PCB, heatsink and/or component.

2. A method according to claim 1 wherein the heatsink has a front face that is placed flush onto one side of the PCB.

3. A method according to claim 1 wherein the heatsink is placed onto one or more solder pads of the PCB by way of solder paste and is coupled to the PCB by a reflow process, wherein capillary action of the liquid solder during reflow retains the heatsink on and aligns the heatsink with the pad.

4. A method according to claim 1 wherein the heatsink and component are placed using a surface mount pick and place apparatus, the coupling medium is deposited using a screen deposition process, and the coupling medium is heated using a reflow process.

5. A method of assembling an electronic component and a heatsink on a PCB using an automated process, including: coupling a heatsink and a bottom surface of a component through an aperture in a PCB by placing the heatsink and component over and/or within the aperture on opposite sides of the PCB with a coupling medium deposited on one or both of the heatsink and bottom surface of the component such that both contact the coupling medium, and heating the coupling medium to form a thermally conducting joint between the component and heatsink, wherein at least one of the heatsink, component, and PCB is profiled to provide at least some separation between a surface of the heatsink and the bottom surface of the component to limit upward pressure on the component, and the quantity of coupling medium deposited on the recess and/or bottom surface of the component is controlled to ensure that, upon heating, the coupling medium bridges the separation to form a thermally conducting joint by conforming to the separation to compensate for tolerances in the dimensions of the PCB, heatsink and/or component.

6. A method according to claim 5 wherein the heatsink is profiled to include a recess in the surface to provide the separation.

7. A method according to claim 5 wherein terminations of the component are profiled such that the terminations protrude laterally at a vertical position to ensure the component bottom is suspended above the heatsink surface to provide the separation.

8. A method according to claim 5 wherein the PCB is profiled to have a thickness that ensures the component bottom is suspended above the heatsink surface to provide the separation.

9. A PCB assembly assembled using an automated process, the assembly including: a heatsink with a recess, coupled to a first side of the PCB over an aperture in the PCB such that the aperture and recess are at least partially aligned, an component coupled to a second side of the PCB such that the component resides aligned with or at least partially within the aperture, a thermally conducting joint between opposing surfaces of the component and recess, the joint being formed by heating a coupling medium deposited in the recess, wherein the depth of the recess is dimensioned to provide at least some separation between the surface of the recess and the bottom surface of the component, and the quantity of coupling medium deposited to form the joint is controlled such that the joint is conformed to the separation to compensate for tolerances in the dimensions of the PCB, heatsink and/or component to bridge the separation.

10. An assembly according to claim 9 wherein the heatsink has a front face that is coupled flush onto the PCB.

11. An assembly according to claim 9 wherein the heatsink and component are coupled to the PCB using a surface mount pick and place apparatus to place the components, and the coupling medium is deposited in the recess using a screen deposition process, and the coupling medium is heated using a reflow process.

12. A PCB assembly assembled using an automated process, the assembly including: a heatsink coupled to a first side of a PCB over at least a portion of an aperture in the PCB, terminals of a component coupled to a second side of the PCB such that the component is suspended over or within the aperture to provide at least some separation between the bottom of the component and the heatsink surface, wherein at least one of the heatsink, component and PCB is profiled to compensate for tolerances in the assembly to provide the separation

13. An assembly according to claim 12 wherein the heatsink is profiled to include a recess in the surface that at least partially aligns with the aperture and the component bottom to provide the separation.

14. An assembly according to claim 12 wherein the terminals are profiled to protrude laterally at a vertical position to ensure the component bottom is suspended above the heatsink surface to provide the separation.

15. An assembly according to claim 12 wherein the PCB is profiled to have a thickness that ensures the component bottom is suspended above the heatsink surface to provide the separation.

16. A set of parts for constructing an electronic assembly using an automated process including: a heatsink with a recess adapted for coupling to a first side of a PCB with an aperture such that the recess and aperture at least partially align, a component adapted for coupling to a second side of the PCB such that the component resides aligned with or at least partially within the aperture upon coupling, wherein the depth of the recess is dimensioned to compensate for tolerances in the dimensions of the component, heatsink and a PCB to provide at least some separation between the surface of the recess and the bottom surface of the component when installed on a PCB with an aperture, to limit upward pressure on the component.

17. A set of parts according to claim 16 further including a PCB with an aperture.

18. A set of parts for constructing an electronic assembly using an automated process including: a heatsink adapted for coupling to a first side of a PCB with an aperture, a component adapted for coupling to a second side of the PCB such that, upon coupling, the component resides aligned with or at least partially within the aperture to provide at least some separation between the bottom of the component and a surface of the heatsink, wherein at least one of the heatsink and component is profiled to compensate for, upon coupling, tolerances in the parts to provide the separation.

19. A set of parts according to claim 18 further including a PCB with an aperture.

20. A set of parts according to claim 18 wherein the heatsink is profiled to include a recess that, upon coupling the heatsink and component to a PCB, at least partially aligns with the aperture and the component bottom to provide the separation.

21. A set of parts according to claim 18 wherein the component is profiled to include terminations protruding laterally at a vertical position that ensures the component bottom is suspended above the heatsink surface to provide the separation.

22. A set of parts according to claim 19 wherein the PCB is profiled to have a thickness that ensures the component bottom is suspended above the heatsink surface to provide the separation.