Patent Publication Number: US-2006002092-A1

Title: Board mounted heat sink using edge plating

Description:
TECHNICAL FIELD OF THE INVENTION  
      The present invention is directed, in general, to a printed wiring board (PWB) having a heat sink mounted thereon, and more specifically, to a PWB having a heat sink connected to its internal circuits by use of a conductive interconnect.  
     BACKGROUND OF THE INVENTION  
      It is well known that electronic and electrical components or devices mounted on a PWB generate considerable operating heat. In addition, however, internal circuits or traces in the PWB generate a considerable amount of heat within the internal portions of the PWB. Heat build-up within and on these PWBs has been exacerbated by increased device density, which results in more devices and internal interconnects, both of which generate more heat than ever before, and at the same time, makes less space available on the board for conventional heat sink devices. As is well known by those in the industry, unless this heat is properly dissipated, it can result in temperature related circuit or component failure. Therefore, it is highly desirable that as much of this heat as possible is removed.  
      The generally preferred method to effectuate heat dissipation is to use a metallic heat transfer device, such as a heat sink or heat plate, to transport heat from a component to the surrounding ambient air. Heat transfer devices can be made of any material with favorable heat transfer characteristics, such as copper or aluminum. In most cases, the heat transfer device and the related heat generating surface mounted components are placed in close proximity with one another and coupled with a thermal interface material in order to provide more efficient cooling of the component. This permits the heat sink to absorb component heat directly and transfer it to the surrounding ambient air by conduction or convection.  
      While, these types of heat sinks are able to dissipate heat from top surface devices, they are ineffective in removing heat generated by internal circuit traces. The reason that they are ineffective is that the heat has to travel a rather long and arduous distance to ultimately reach the heat sink. For example, a trace located in the internal portions of the PWB must conduct through several insulating layers before finally reaching the externally mounted device and the heat sink which is in contact with the mounted device. These insulating layers do not have a high thermal conductivity coefficient, and as a result, the heat cannot be dissipated rapidly enough to prevent an excessive build-up of heat within the PWB, given the amount of heat that is generated by the components and the internal circuits themselves.  
      Accordingly, what is needed is a heat sink that is capable of removing heat not only from the external components on the outer portions of the PWB, but is capable of efficiently removing heat from the internal circuits as well.  
     SUMMARY OF THE INVENTION  
      To address the above-discussed deficiencies of the prior art, the present invention provides a PWB having a heat sink connected to the internal circuits of the PWB that allows for heat dissipation from those internal circuits. In one embodiment, the PWB includes at least two insulating layers that are coupled together and that have a conductive layer located therebetween. A conductive interconnect is thermally coupled to the conductive layer, and a heat sink is thermally coupled to the conductive interconnect.  
      In another embodiment, the present invention provides an electronic circuit module that includes a PWB that has heat generating components located thereon. The PWB has a plurality of insulating layers coupled together with a conductive layer located between each pair of the plurality of insulating layers. This embodiment further includes a conductive interconnect that is thermally coupled to the conductive layer, which is connected to ground. A heat sink is thermally coupled to the conductive interconnect.  
      In another embodiment, there is provided a method of manufacturing a PWB. The method includes providing at least two insulating layers coupled together that have a conductive layer located therebetween, forming a conductive interconnect that is thermally coupled to the conductive layer, and thermally coupling a heat sink to the conductive interconnect.  
      The foregoing has outlined preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a more complete understanding of the present invention, reference is now made to the following detailed description taken in conjunction with the accompanying FIGUREs. It is emphasized that various features may not be drawn to scale. In fact, the dimensions of various features may be arbitrarily increased or reduced for clarity of discussion. Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  is an enlarged, partial sectional view of one embodiment where the heat sink is connected to the conductive layer by an edge plating interconnect;  
       FIG. 2  is an enlarged partial sectional view of another embodiment of the device illustrated in  FIG. 1  wherein the edge plating interconnect is divided into multiple interconnects on the edge of the PWB;  
       FIG. 3  is an enlarged partial cross-sectional view of another embodiment wherein the conductive interconnect is a via formed through the PWB, and the heat sink is connected to the conductive layers by the via;  
       FIG. 4A  is a perspective view of opposing sides of an electronic circuit module implementing a heat sink in accordance with the principles of the present invention.  
       FIG. 4B  is an alternative embodiment of the electronic module circuit module of  FIG. 4A  showing interconnects that can be used to connect to another PWB board; and  
       FIG. 4C  illustrates the electronic circuit module of  FIG. 4B  attached to another PWB board, which can function as a heat sink to dissipate internal heat within the electronic circuit module.  
    
    
     DETAILED DESCRIPTION  
      The present invention recognizes the advantages associated with providing a heat sink that is connected to internal conductive layers of a PWB through a conductive interconnect. Because the heat sink is connected to the internal conductive layers, it provides a thermal path for heat that is generated by the internal circuits located between the insulative layers of the PWB. Thus, internal heat is more easily dissipated than conventional heat sink configurations.  
      Turning initially to  FIG. 1 , there is illustrated an enlarged, partial sectional view of a PWB  100  showing multiple insulating layers  110  that have conductive layers  115  therebetween, only two of which, in each instance, have been designated for simplicity. The PWB  100  further includes a conductive interconnect  120 , which, in this exemplary embodiment, is an edge plate interconnect and is discussed in more detail below. The conductive interconnect  120  may be formed on an edge of the PWB  100  and is connected to conductive layers  115 . The conductive layers  115  extend to a via  125 , which in one embodiment, is connected to ground. The conductive layers  115  are thermally conductive, and as such, provide a thermal path for heat generated within the interior portions of the PWB  100 . Heat generating components  130  are located on a surface of the PWB  100 , and they may be of any type of heat generating components typically found on a PWB. For example, they may be processors, capacitors, inductors, transformers, memory devices, switches or resistors.  
      A heat sink  135 , which is also shown in this embodiment, is located over the PWB  100  and over the heat generating component  130 . The heat sink  135  has a first end  135   a  that is in contact with the conductive layers  115  by way of the conductive interconnect  120  and a second end  135   b  that is in contact with the via  125 , which in one embodiment, may be connected to ground. In one embodiment, the heat sink  135  may also include an edge  135   c  that laps over and contacts the PWB  100  as shown. Because the heat sink  135  is in contact with the conductive layers  135 , the heat can conduct along the conductive layers  135  to the heat sink  135  and be dissipated, thereby allowing internal heat within the PWB  100  to be more efficiently removed from the PWB  100 . Further, since the heat sink  135  can be placed in close proximity to the heat generating components  130 , it is able to remove heat from the surface of the PWB  100 , as well. The heat sink  135  may be coupled to the PWB  100  in a number of ways. For example, the first end  135   a  and the edge  135   c  may be soldered onto the PWB  100 , or alternatively, they may form a spring clip that allows the heat sink  135  to be clipped onto the PWB  100 . Other mechanical means known to those skilled in the art for attaching the heat sink  135  to the PWB  100  are within the scope of the present invention.  
      With an overview of one device having now been discussed, attention will now be turned to other embodiments illustrating different examples of the types of conductive interconnects that can be effectively used in conjunction with a heat sink to remove heat from internal portions of the PWB.  
      Turning now to  FIG. 2  with continued reference to  FIG. 1 , there is illustrated an enlarged, partial sectional view of an edge of the PWB  100 , as illustrated in  FIG. 1 . This figure illustrates one exemplary embodiment of the conductive interconnect  120  to which the heat sink  135  of  FIG. 1  may be connected. In this embodiment, the conductive interconnect  120  is an edge plate interconnect  210 . The edge plate interconnect  210  may be separated into multiple and electrically separate plates on a given edge of the PWB  200 , as shown in  FIG. 2 , or it may be a single plate as illustrated in  FIG. 1 . In the embodiment where the edge plate interconnect  210  is separated into multiple plates  210   a ,  210   b , the heat sink  135  may contact either one or both of the plates  210   a ,  210   b . In the illustration, the first end  135   a  of the heat sink  135  contacts only plate  210   a . In this embodiment, a group of conductive layers  115   a ,  115   b , terminate at and contact the edge plate interconnect  210  of the PWB  100 , while conductive layer  115   c  does not terminate at the interconnect  210 , as shown.  
      Edge plate interconnects  210   a ,  210   b  respectively contact each of the groups of conductive layers  115   a ,  115   b . In those embodiments where the edge plate interconnect  210  includes multiple plates and the heat sink  135  is connected to those plates, both of the conductive layers  115   a ,  115   b  may extend across the PWB  100  and connect to a ground (not shown) such that the heat sink  135  does not emit electromagnetic interferences and needlessly draw current from the active devices of the circuit. However, in another embodiment where the heat sink  135  contacts only conductive layers  115   a , the conductive layers  115   b  may not necessarily terminate at a ground; this will be dictated by design.  
      Turning now to  FIG. 3 , with continued reference to  FIG. 1 , there is illustrated another embodiment of the PWB  100  where the conductive interconnect  120 , to which the heat sink  135  is connected, is a via  310  that is formed in or through the PWB  100 . Similar to the edge plate interconnect of  FIG. 2 , the via  310  may have edge plating deposited on an interior surface of the via  310  such that a single plated interconnect is formed, such as the one illustrated in  FIG. 1 , or multiple, separate interconnects  310   a ,  310   b , are formed. In the embodiment where the edge plate interconnect  310  is separated into multiple interconnects  310   a ,  310   b , the heat sink  135  may contact either one or both of the interconnects  310   a ,  310   b . In the illustrated embodiment, the first end  135   a  of the heat sink  135  contacts only plate  310   a . In the embodiment that is illustrated, a group of conductive layers  115   a ,  115   b , terminate at and contact the edge plate interconnect  310  of the PWB  100 , while conductive layer  115   c  does not terminate at the interconnect  310 , as shown.  
      Edge plate interconnects  310   a ,  310   b  respectively contact each of the groups of conductive layers  115   a ,  115   b . In those embodiments where the edge plate interconnect  310  includes multiple, separate plates  310   a ,  310   b , and the heat sink  135  is connected to those plates, conductive layers  115   a ,  115   b  may extend across the PWB  100  and connect to a ground (not shown) such that the heat sink  135  does not emit electromagnetic interferences and needlessly draw current from the active devices of the circuit. However, in another embodiment where the heat sink  135  contacts only conductive layers  115   a , the conductive layers  115   b  may not terminate at a ground.  
      Turning now to  FIG. 4A , there is illustrated a perspective view of opposing sides of an electronic circuit module  400  in accordance with the principles of the present invention. As shown, the electronic circuit module  400  includes a heat sink  410 . In this particular embodiment, the heat sink  410  is connected to the internal circuits of the electronic circuit module  400  by an edge plate interconnect  412 , as discussed above. While electronic design configurations my way, depending on the application, the electronic circuit module  400  may include a primary circuit  415 , including a transformer  420  and a secondary circuit  425  that includes an output inductor  430  and other components as dictated by design, which are not specifically designated.  
      Turning now to  FIGS. 4B and 4C , there is illustrated an alternative embodiment of the electronic circuit module  400  shown in  FIG. 4A . This particular embodiment includes thermally conductive interconnects  435  that are coupled, such as by solder, to the edge plating interconnect, as discussed above. In the illustrated embodiment, the conductive interconnects  435  may be copper strips that extend beyond the edge of the electronic circuit module  400 . By virtue of the conductive interconnects  435  being thermally coupled to the edge plating, the conductive interconnects  435 , in turn, are thermally coupled to the internal conductive layer or traces of the electronic circuit module  400 , as described above. Thus, the conductive interconnects  435  are capable of conducting heat from the internal portions of the electronic circuit module  400 .  
      However, in place of the conductive interconnects  435  being connected to a heat sink, as in other embodiments discussed herein, these conductive interconnects  435  can be used to thermally couple the internal conductive layers or traces of the electronic circuit module  400  to another PWB board  440 , such as a customer&#39;s board. The electronic circuit module  400  may also be electrically connected to the PWB  440  by way of conductive pins  445 . Thus, the PWB board  440  can act as a heat sink for the electronic circuit module  400  by way of the conductive interconnects  435 .  
      One who is skilled in the art, given the teachings discussed herein would understand how to construct the PWB, its interconnects, and connect the heat sink to those interconnects, including which materials to use. For example, the conductive layers may be copper trace patterns formed on the various layers of the PWB, and the conductive interconnects may comprise copper plated with a conductive solder. The conductive interconnect may be formed on an edge of the PWB as discussed above, or it may be formed on a routed slot formed in the interior of the PWB. Where the conductive interconnect is a via or some other type of opening, it may be formed by drilling a hole through the PWB.  
      Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.