Abstract:
The invention relates to a method and a device for creating a thermally conductive path between a component of a printed circuit board and a heat sink using a electrical insulator material and a thermally conductive material. The electrical insulator material is adhesive, to assist in adhering surfaces against which it is applied. Typically, the circuit board is affixed to the heat sink, creating a gap between the component and the heat sink. The electrical insulator material is applied in the gap. Then, the thermally conductive material is applied within the electrical insulator material, causing the thermally conductive material to spread beyond the perimeter of the electrical insulator material. The electrical insulator material thus confines the thermally conductive material within the gap in thermally conductive contact between the electronic component and the heat sink.

Description:
FIELD OF THE INVENTION 
     The invention relates to heat dissipation and is particularly concerned with heat transfer between electronic components and heat sinks. 
     BACKGROUND OF THE INVENTION 
     In electronics systems, such as printed circuit boards carrying electronic components, heat is generated by the components in use and it is necessary to remove or dissipate heat to prevent over heating which could result in breakdown of one or more of the components and/or create a fire hazard. To remove or dissipate heat, heat sinks have been implemented. 
     To ensure that heat flows from the electronic components to the heat sink, a heat transfer medium is typically interposed between the components and the heat sink, since it would otherwise be difficult to ensure proper contact between the electronic components and the heat sink. One type of heat transfer medium used has been non-electrically conductive matrix resin. Because of the risk of a short circuit in an electronic system if the heat transfer medium is electrically conductive, non-electrically conductive media, such as that noted above, have been used. However, non-electrically conductive media are typically relatively poor thermal conductors. Accordingly, it would be desirable to achieve better thermal conductivity between electronic components and a heat sink. 
     SUMMARY OF THE INVENTION 
     According to a first aspect, the invention provides a method for creating a thermally conductive path between a component of a printed circuit board and a heat sink using an electrical insulator material and a thermally conductive material. The electrical insulator material is adhesive, to assist in adhering surfaces against which it is applied. Typically, the circuit board is affixed to the heat sink, creating a gap between the component and the heat sink. The electrical insulator material is applied in the gap. Then, the thermally conductive material is applied within the electrical insulator material, causing the thermally conductive material to spread beyond the perimeter of the electrical insulator material. The electrical insulator material confines the thermally conductive material within the gap in thermally conductive contact between the electronic component and the heat sink. 
     According to another aspect, the invention provides a method for creating a thermally conductive path between a component of a printed circuit board and a heat sink using a electrical insulator material and a thermally conductive material. The electrical insulator material is adhesive, to assist in adhering surfaces against which it is applied. The electrical insulator material is applied to either the heat sink or the electronic component (or a covering of either the heat sink or the electronic component). Then, the thermally conductive material is applied within or on top of the electrical insulator material. The printed circuit board is then affixed to the heat sink, such that the thermally conductive material creates a thermally conductive path from the component to the heat sink. The electrical insulator material confines the thermally conductive material to a volume between the electronic component and the heat sink. 
     According to another aspect, the invention provides an electronics system having a heat sink structure and a printer circuit board structure. The circuit board structure has a printed circuit board with at least one electronic component mounted thereon. The heat sink structure and the circuit board structure are affixed to each other with a gap between the electronic component and the heat sink structure. The electronics system has a means for creating a thermally conductive path from the heat sink structure to the electronic component. The means comprises both a good thermally conductive, electrically conductive material and an electrical insulator material between the electronic component and the heat sink structure with the good thermally conductive electrically conductive material being confined by the electrical insulator. 
     Advantageously, different embodiments of the present invention may permit: improved thermal conductivity or heat dissipation between electronic components on a printed circuit board and a heat sink; and/or electrical grounding of the electronic components to the heat sink. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described with reference to the attached drawings in which: 
     FIG. 1 a  is a cross-sectional view through a printed circuit board structure and a heat sink structure before either a electrical insulator material or a good thermally conductive material is applied; 
     FIG. 1 b  is the cross-sectional view of FIG. 1 a  with a electrical insulator material applied in accordance with an aspect of the present invention; 
     FIG. 1 c  is the cross-sectional view of FIG. 1 b  after a good thermally conductive material is applied; 
     FIG. 1 d  is the cross-sectional view of FIG. 1 c  after an additional portion of the electrical insulator material is applied; 
     FIG. 2 a  is a top view of an electronic component with a electrical insulator material applied in accordance with another aspect of the present invention; 
     FIG. 2 b  is the top view of FIG. 2 a  with a good thermally conductive material applied within or on top of the electrical insulator material; 
     FIG. 2 c  is a cross-sectional view of the heat sink structure being brought towards the printed circuit board structure; and 
     FIG. 3 is a cross-sectional view through a printed circuit board structure and a heat sink structure in accordance with another aspect of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In a first embodiment as shown in FIG. 1 a,  a printed circuit board structure  10  comprises a printed circuit board  12  having a plurality of electronic components  14  (only one component  14  being shown) mounted upon one side of the printed circuit board  12 . For removal of heat from the printed circuit board structure  10  during use, a heat sink structure  16  is provided. The heat sink structure  16 , in this example, comprises a planar body  18  from one side of which a plurality of parallel and spaced apart heat cooling fins  20  extend and are directed outwardly from the side of the planar body  18  remote from the printed circuit board structure  10 . The printed circuit board structure  10  and the heat sink structure  16  are maintained in a detachably fixed, spaced apart relationship by spacer columns  22  (one being shown) which are held in position by bolts or securing screws  24  extending either through the planar body  18  of the heat sink structure  16  as shown, or through the printed circuit board  12 . 
     A spatial region or gap  21  separates the components  14  from the planar body  18  of the heat sink structure  16 . The height of the gap  21  may depend upon the size of the components  14  and may vary from one component  14  to another. 
     The planar body  18  of the heat sink structure  16  optionally has formed therein one or more holes  34 . As shown in the figures, the axis of a hole  34  extends generally towards the electronic component  14  and the hole  34  is optionally generally centrally aligned with respect to the component  14 . 
     Each component  14  may be fitted with an overmold or direct lid attachment (DLA) or similar cover or device (not shown) which would be interposed between the components  14  and the gap  21 . As used hereinafter in the specification and in the claims, the expression “electronic component” means either the electronic component  14  itself where there is no overmold, DLA or other cover or the electronic component  14  in addition to the DLA (or overmold or other cover) in the case where a DLA (or overmold or other cover) is provided. 
     To thermally conductively connect the printed circuit board structure  10  to the heat sink structure  16  for removal of heat from the printed circuit board structure  10  out through the heat sink structure  16 , two compounds or materials  36 ,  42  are interposed between the components  14  and the planar body  18  as described below. 
     Then as suggested in FIG. 1 b,  an injection nozzle  38  is inserted into or disposed adjacent the opening of the hole  34  at a surface of the planar body  18  remote from the component  14 , and a mass of an electrical insulator material  36  is injected through the hole  34  so as to flow into and expand progressively within the gap  21  between the component  14  and the planar body  18 . The electrical insulator material  36  has an ability to flow at processing temperatures and may optionally be a liquid, a slurry, a paste, a viscous paste or a gel at processing temperatures. 
     Then as suggested in FIG. 1 c,  an injection nozzle  40  (which could be the same as or different from the injection nozzle  38 ) is inserted into or disposed adjacent the opening of the hole  34  and a mass of a good thermally conductive material  42  is injected through the hole  34  so as to flow into and expand progressively within the gap  21  to be substantially surrounded by and contained in a volume of the gap  21  as defined by the component  14  (or the overmold or DLA (not shown)), the planar body  18  and the electrical insulator material  36 . 
     Then as suggested in FIG. 1 d,  the injection nozzle  38  is inserted into or disposed adjacent the opening of the hole  34  and an additional mass of the electrical insulator material  36  is injected into the hole  34  to confine the good thermally conductive material within the gap  21  by blocking off the opening of the hole  34 . The additional electrical insulator material  36  may optionally be injected so as to substantially push all of the good thermally conductive material  42  out of the hole  34  and into the gap  21 . However, ideally, as shown in FIG. 1 d,  the additional electrical insulator material  36  is not injected into the gap  21 . 
     As shown in FIGS. 1 c  and  1   d,  for example, the good thermally conductive material  42  is positioned so as to be in direct contact with both a surface of the component  14  and a surface of the planar body  18 . 
     The electrical insulator material  36  is a suitable material being a poor electrical conductor and being able to intimately contact the surfaces against which it has been injected. More specifically, the material  36  has viscous properties and adhesive properties to assist in adhering the surfaces against which it has been injected and to confine the thermally conductive material  42 . In other words, the electrical insulator material  36  has good flow control at processing temperatures before setting or curing at room temperature and ideally, upon setting, acts as a compliant seal having mechanical integrity. However, the electrical insulator material  36  could be designed to cure at any temperature. The electrical insulator material  36  is optionally a dielectric or electrically isolating overfill material such as, for example, silicone rubber, silicone gel, fluoro silicone rubber, acrylates, epoxy, natural rubbers or any family of electrical insulator materials can be used if they are able to maintain mechanical integrity over their intended lifetime and the operating temperatures to which they will be exposed. 
     The good thermally conductive material  42  has relatively high thermal conductivity, and optionally has good “wetting” ability to adjacent surfaces. The good thermally conductive material  42  should flow well at processing temperatures, which are typically around 20-30 degrees Celsius, and is optionally a slurry or a paste at those temperatures. Suitable good thermally conductive materials include, for example, pure metals such as gallium, mercury and woods metal. Such good thermally conductive materials typically have good electrical conductivity as well. 
     For the good thermally conductive material  42 , metal alloys could be used instead of pure metals. The alloys or the pure metals could also optionally include fillers to increase viscosity and possibly to change the melting temperature. Optionally, the good thermally conductive material  42  is a liquid when applied and during operation of the electronic system, because the volume it occupies may change if it is frozen or solidified. 
     Pure gallium, for example, may be used as the good thermally conductive material  42 . Alternatively, alloys such as gallium/tin or gallium/indium or gallium/indium/tin could also be used. 
     As also noted above, fillers could also optionally be used, such as powders, gels or fibers, such as refractory, (including ceramics such as aluminum oxide, boron-nitride, aluminum-nitride, etc.), metal powders (such as silver, copper, nickel, aluminum, palladium tungsten, gold, molybdenum, etc.) or elements from Group IV of the Periodic Table (such as carbon, silium, germanium, etc.). The dispersion of a filler in a metal or alloy typically requires both heat and mechanical agitation. The decision as to which filler to use and the relative amount, will depend upon the particular application, the type of metal or alloy used, the desired melting temperature of the resulting mixture and the desired viscosity. 
     The amounts of the electrical insulator material  36  and the good thermally conductive material  42  used in a particular situation depend upon surrounding circumstances including the nature of the materials  36 ,  42 , the temperature, the volume of the gap  21  and the area of the surface of the component  14  (or DLA or overmold (not shown)) facing the gap  21 . 
     When the electrical insulator material  36  is initially injected into the gap  21 , as shown in FIG. 1 b,  a sufficient amount should be injected so that it will ultimately be able to confine the good thermally conductive material  42  between the component  14  and the planar body  18 . However, ideally, the electrical insulator material should not extend beyond the edges of the component  14 . 
     There are several methods for determining how much of the electrical insulator material  36  to inject, which would be known to those skilled in the art. For example, if the height of the gap  21  is known (ie: the distance between the component  14  and the planar surface  18 ), then the volume of the electrical insulator material  36  to be injected can be calculated. If the height of the gap  21  is not known, then there are several methods for determining the height, such as, for example, inserting a dip stick (not shown) into the hole  34 . If the height of the hole  34  is known, then the height of the gap  21  is calculated by subtracting the height of the hole  34  from the dip stick measurement. 
     The amount of the good thermally conductive material  42  to inject depends upon the particular circumstances. There should be a sufficient amount of the good thermally conductive material  42  to bridge the gap  21  between the component  14  and the planar surface  18 . However, there should not be so much good thermally conductive material  42  such that there would be insufficient electrical insulator material  36  to contain it between the component  14  and the planar surface  18 . Similarly, there should not be so much good thermally conductive material  42  such that the injection of the good thermally conductive material  42  would push the electrical insulator material  36  beyond the edges of the component  14 . 
     According to one aspect of an embodiment, there is approximately a ratio of three to seven of the good thermally conductive material  42  to the electrical insulator material  36 . However, an infinite number of other ratios could also be used. 
     Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practised otherwise than as specifically described herein. 
     For example, the embodiments noted above describe injecting the materials  36 ,  42  into the gap  21  between the circuit board structure  10  and the heat sink component  16 . However, it is possible to apply the materials  36 ,  42  other than by injecting. For example, the materials could be applied to either a surface of the component  14  (or a DLA or an overmold (not shown)) or the planar surface  18  before the printed circuit board structure  10  and the heat sink structure  16  are affixed to one another. 
     As an example, an amount of the electrical insulator material  36  could be applied to the surface of the component  14  remote from the printed circuit board  12  by any method, such as that shown in FIG. 2 a.  Then a frozen or a relatively solid disc, or other shape, of the good thermally conductive material  42  is applied in the centre of the electrical insulator material  36 , as shown in FIG. 2 b.  Alternatively, the frozen disc, or other shape, of the good thermally conductive material  42  could be applied first, and then electrical insulator material  36  could be applied around the perimeter of the disc. The circuit board structure  10  and the heat sink structure  16  are then brought towards each other, as suggested in the cross-sectional view of FIG. 2 c.  Then, the circuit board structure  10  is affixed to the heat sink structure  16  in a manner such as that shown in FIG. 1 a . The result (not shown) is that the good thermally conductive material  42  is confined between the component  14  and the planar surface  18  by the electrical insulator material  36 . 
     The good thermally conductive material  42  need not be solid or frozen as described above when applied. However, because the good thermally conductive material  42  may be harmful to circuits or other components (not shown) of the printed circuit board structure  10  or human beings if the good thermally conductive material  42  is splattered, it is optionally applied in solid or frozen form in this embodiment. 
     It is possible that after some period of use, a user may wish to separate the printed circuit board structure  10  from the heat sink structure  16  (for repairs, cleaning, modification, etc). Accordingly, it would be desirable if the electrical insulator material  36  and the good thermally conductive material  42  were not positioned in the gap  21  directly against the planar body  18 . For example, as shown in FIG. 3, according to another embodiment, a thermally conductive pedestal structure  60 , has a generally horizontal portion  62  and a stem portion  64  extending perpendicularly therefrom. The stem portion  64  is adapted to fit within the hole  34  of the heat sink structure  16 . The pedestal structure  60  is affixed to the heat sink structure  16  by means of a nut  66  engaging an external thread on the stem portion  64 . A bore  68  extends through the stem portion  64  and the horizontal portion  62  allowing the materials  36 ,  42  to be injected through the bore  68 . In this embodiment, the materials  36 ,  42  are interposed between the component  14  and the horizontal portion  62  of the pedestal structure  60 . 
     Other devices or means (not shown) could be used to separate the materials  36 ,  42  from the planar body  18 , such as thermally conductive tape, for example. 
     If it is desired to separate the printed circuit board structure  10  from the heat sink structure  16 , then it is also likely desirable that the good thermally conductive material  42  not be in a liquid state at the time of separation, to avoid the possibility of the good thermally conductive material  42  splattering, contaminating components, etc. Accordingly, prior to separating the circuit board structure  10  from the heat sink structure  16 , the combination of all or part of the printed circuit board structure  10  and the heat sink structure  16  could be placed in a cold environment to freeze the good thermally conductive material  42 . 
     As another potential problem, liquid metals often dissolve and amalgamate with many common materials used to fabricate printed circuit board structures  10 , heat sink structures  16  and electronic components  14 , which could destroy or compromise the integrity of these structures or components  10 ,  14  and  16 . 
     To help overcome this problem, the good thermally conductive material  42  (a liquid metal such as gallium and/or its alloys, for example) may be dispersed as a filler material in a matrix gel which may be an organic substance such as soft acrylic, for example, or an inorganic substance such as silicone gel. The resulting material  42  (not shown) would be an encapsulation of a liquid metal by a dielectric substance which would limit the liquid metal from direct contact with electrical components  14  or structures  10  and  16 . 
     As an alternative or additional means for protecting structures  10  and  16  and components  14  from direct contact with the liquid metal, the structures and components  10 ,  14  and  16  could include a film or barrier layer (not shown) to limit contact of the liquid metal with the components or structures  10 ,  14  and  16 . For example, for an aluminum heat sink structure  16 , a barrier layer may consist of tin metal plated over nickel over the base aluminum. As another option, a ceramic aluminum barrier could be used. Many other barriers could also be used.