Patent Publication Number: US-2004042179-A1

Title: PCB heatsink

Description:
FIELD OF THE INVENTION  
       [0001] This invention relates to a heatsink for a printed circuit board (PCB), and more particularly to a PCB and heatsink combination wherein the heatsink extends across a plurality of heat generating components in heat conducting relation thereto.  
       BACKGROUND OF THE INVENTION  
       [0002] With the reduction in size of many electronic devices, printed circuit boards have become more and more densely populated with components. Many of these components are heat generating so that removing heat from the vicinity of the board has become increasingly important.  
       [0003] Multi-layer boards and, in particular, multi-layer power circuit boards such as power supplies, AC to DC converters, DC to DC converters and the like now employ windings buried among the board&#39;s layers and heat generating magnetic cores that penetrate the board from one exterior surface to another. These are closely spaced from the electronic circuit components on one or both of the exterior faces of the board.  
       [0004] In the past, heatsinks for various components, including integrated circuits such as processors, have been provided and these have been attached to the components in various ways, including by clamps. However, these have not served to remove heat from multiple, densely packed circuit elements on a printed circuit board. There remains, therefore, the need to relieve the heat buildup that a densely populated printed circuit board can experience.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005] In accordance with the present invention, there is provided a heatsink for a PCB that overlies a plurality of heat generating components on one face of the PCB so as to conduct heat away from those components as well as away from the face of the board. As used herein, “heat-generating components on the face of the PCB or board” means electronic components and other circuit elements mounted on the board and magnetics extending or protruding through the board to the board face. In particular, in accordance with one embodiment of the invention, the heatsink overlying a plurality of elements overlies a relatively large portion of the surface area of the board and is affixed to the board. A pliant, thermally conductive, but dielectric layer electrically insulates the heatsink from the components and conductive traces on the board surface while assuring good heat conduction away from the components to the heatsink.  
       [0006] In one particular embodiment, the heatsink is clamped to the board by resilient clamps at aligned edges of the heatsink and board. In that embodiment the pliant dielectric layer is sandwiched between the heatsink and the board surface that carries the components from which heat is conducted.  
       [0007] The heatsink surface facing the board is molded to, at least partly, fit in close proximity on and around heat generating components. In one particular embodiment, the PCB is a multilayer PCB. In an embodiment in which the PCB is a power component such as a power supply, AC to DC converter, or the like, the heatsink closely overlies magnetic cores that penetrate the board from one face to the other.  
       [0008] Preferably, the heatsink is co-extensive with the PCB to form a combined PCB and heatsink package having the same footprint as the PCB. At its edges, the PCB may have projecting dependent walls that contact the surface of the PCB, partially enclosing the PCB components and the pliant heat-conductive dielectric layer.  
       [0009] In one embodiment, the exterior surface of the heatsink facing away from the PCB is planar and in another embodiment that surface has surface area-increasing features to improve heat dissipation.  
       [0010] The above and further objects and advantages of the invention will be better understood in view of the following detailed description of at least one preferred embodiment taken in consideration with the accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011]FIG. 1 is a perspective view of a multi-layer PCB of the kind to which the heatsink of this invention is applied;  
     [0012]FIG. 2 is a top perspective view of the PCB of FIG. 1 with the heatsink of the present invention in place;  
     [0013]FIG. 3 is a bottom perspective view of the PCB of FIG. 1 with the heatsink applied and shows the face of the PCB opposite the heatsink;  
     [0014]FIG. 4 is an end view of the PCB of FIG. 1 with the heatsink in place;  
     [0015]FIG. 5 is a further end view, partially in section, showing the PCB of FIG. 1 and the cooperation of the undersurface of the heatsink with the heat generating components of the PCB;  
     [0016]FIG. 6 is a side view of the PCB of FIG. 1 with the heatsink in place and the PCB electrically connected to and mounted upon a further PCB;  
     [0017]FIG. 7 is an exploded view of a PCB like that of FIG. 1, and a further embodiment of the heatsink in accordance with the invention, having surface area-increasing features defined on the surface opposite the PCB; and  
     [0018]FIG. 8 is a further perspective view of an alternative heatsink in accordance with the invention and more clearly shows surface area-increasing features formed on the outward-facing surface of the heatsink. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT  
     [0019] Turning to FIG. 1, there is shown a multi-layer PCB  10  carrying a power supply circuit. Magnetics of the power supply circuit include magnetic cores  12 ,  14 ,  16 ,  18  and  20  penetrating the board. Various electronic components  22  and  24 , etc. are mounted on the surface  26  of the board  10 . Not shown in FIG. 1, buried windings encircle the penetrating legs of the magnetics  12 ,  14 ,  16 ,  18  and  20 . These are carried on interior faces of the one or more layers that make up the board  10 . To meet the power density requirements of, for example, today&#39;s telecommunications devices, portable computers, PDAs, etc., the board  10  is densely packed with the magnetics and the components on both faces of the board. The face  28  opposite the face  26  of the board is best seen in FIG. 3. There the magnetic cores  12 ,  14 ,  16 ,  18  and  20  are seen where they have emerged from the board and extend generally parallel to the surface  28 . Additional components  30 ,  32 ,  34 , etc. are seen mounted on this face of the board.  
     [0020] As illustrated in FIGS. 2 and 3, a large plate-like heatsink  36  is affixed to the PCB  10  by four resilient metal clips  38 . The clips  38  are generally U-shaped and are dimensioned to hold the heatsink tightly to the board. The heatsink  36  is generally coextensive with the board  10  and extends generally parallel to the surface of the board  10 . The board  10  and the heatsink  36  can have aligned recesses  48  and  50  that receive the clamps  38 . As seen in FIGS. 2, 3 and  4 , projecting edge portions  40  of the heatsink  36  engage the board  10  along its edges supporting the heatsink and partially enclosing the circuit elements of the face  26  of the board. Best shown in FIG. 3, electrical signal-conducting pins  42  and electrical power pins  44  extend from the surface  28  of the board  10 . These connect the board to a circuit board  46  as shown in FIG. 6. The pins  44  supply electrical power to a circuit carried on the board  46 . The pins  42  carry electrical control signals and, as can be seen in FIG. 6, the pins  44  and  42  support the board  10  and its associated heatsink  36  above the further PCB  46 . As seen in FIG. 5, the signal and power pins that support the board and make electrical connection to the further PCB can be surface-mounting pins like the pins  43  and  45 .  
     [0021]FIG. 7 illustrates an alternative embodiment of the heatsink of this invention in which a series of surface features  152  formed across the face of the heatsink  136  enhance heat dissipation. FIG. 8 better illustrates an alternative arrangement of surface area-increasing features  252  formed on the exterior surface of yet another alternative embodiment of a heatsink  236  in accordance with the invention.  
     [0022] As best seen in FIGS. 5 and 7, each embodiment of the heatsink of the invention has on its surface facing the PCB with which it cooperates a series of raised and lowered lands. These are surface areas  56  and  58  in the embodiment of FIG. 5, and  156 ,  158  and  160  in the embodiment of FIG. 7. The contouring of this surface in this fashion tailors the surface to the particular circuitry carried by the PC board for which it is adapted. For example, the land  56  is at a level where it closely conforms to the height of the magnetics  12 ,  14  and  16  on the face  28  of the board  10 . A portion of the surface projecting outward further from the body of the heatsink  36  forms the land  58  which come closely proximate the elements  18  and  20 , elements that extend away from the surface  26  of the board  10  to a lesser extent than the magnetics  12 ,  14  and  16 . In this way the heatsink  36  is in closer proximity to the main heat-generating elements of the PCB  10 .  
     [0023] In each of the embodiments of the present invention, a pliant, rubber-like, electrically-insulating but thermally-conductive layer  60  is placed between the elements of the PCB facing the heatsink and the interior heatsink surfaces  56  and  58 . The pliant sheet  60  may be a commercially available material such as one of the Gap Pad products available from The Bergquist Company, Chanhassan, Minn.  55317 . The clips  38  hold the heatsink and thermally conductive layer tightly against the heat-generating elements on the board.  
     [0024] It will be seen, then, that by arranging a heatsink that extends over most or all of the heat-generating elements on a PCB&#39;s surface, by conforming the heatsink&#39;s interior or undersurface to the circuit for which it is designed, and by sandwiching a heat-conductive dielectric between the heatsink and the PCB, good heat transmission of heat away from the heat-generating, densely-packed elements of a PCB is achieved to dissipate such heat to atmosphere.  
     [0025] Although preferred embodiments of the invention have been described in detail, it will be readily appreciated by those skilled in the art that further modifications, alterations and additions to the invention embodiments disclosed may be made without departure from the spirit and scope of the invention as set forth in the appended claims.