Abstract:
The present invention provides a heat transfer augmentation method and device for use on a printed circuit board. The inventive device comprises one or more elements having high impedance that are attached to traces on a printed circuit board, thus permitting heat transfer without allowing the device to affect the electrical circuit on the board. The elements can be surface mount resistors that conduct heat more efficiently than does the printed circuit board insulator material, such as resistors having alumina or aluminum nitride substrates. The inventive device and method is thus compatible with standard printed circuit board assembly procedures, is inexpensive, and has a smaller foot print on the circuit board than prior art heat transfer augmentation devices.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to devices for augmenting heat transfer on printed circuit boards and, in particular, to the use of one or more surface mounted elements to increase the heat transfer between conductive traces on circuit boards while having a minimal impact on the electrical operation of the circuit.  
         BACKGROUND OF THE INVENTION  
         [0002]    The performance, reliability and lifetime of electronic circuits are affected by the temperature of the various circuit components. There are many circumstances in which the heat generated by the dissipation of electric power, usually originating in one or a few circuit components, is problematic. In addition to heating the dissipating components, heat readily flows along electrically conducting wires or conductive traces provided on a surface of the printed circuit board that are used to connect electronic components, with a resulting temperature increase of surrounding components. The management of heat flow in and from circuits is thus an important aspect of circuit design.  
           [0003]    Removal of heat from components on a printed circuit board (PCB) presents challenging design issues. In general, heat is removed from one or more electronic components and discharged at a lower temperature, preferably at a location where the temperature is not a concern. Heat removal devices can be either passive, having high thermal conductivities and large surface areas for conducting heat into the surrounding air for convective removal, or can be active thermal devices such as fans, thermionic coolers or heat pipes, that utilize electrical and/or phase change processes to improve heat transfer. In most cases, heat removal devices require space on the components requiring cooling or on the PCB surrounding the components, and require a cooling flow of sufficient volume to dissipate the removed heat. The close placement of components on a board or other, physically adjacent boards, complicates the removal of heat by increasing the temperature of the surrounding air, by inhibiting the flow of cooling air, and by generally making it more difficult to implement heat removal strategies in confined spaces.  
           [0004]    In circuits on PCBs, prior art heat removal devices draw heat away from the electric circuit and the PCB, primarily in a direction perpendicular to the PCB. One conventional, prior art heat removal device is shown in FIGS. 1A as a top view and in FIG. 1B as a sectional view  1 B- 1 B of a heat sink  120  on a portion of PCB  100  that supports a circuit  110 . For purposes of discussion of the removal of heat from a PCB, circuit  110  could be part of a power supply or power conversion circuit having a power dissipating component that might exceed a required temperature without the augmentation of heat transfer from the component or PCB.  
           [0005]    PCB  100  has a top surface  101  that includes portions of circuit  110 , and in particular has space for mounting components C, and an edge  103  and bottom side  105  for connecting the circuit to other electric circuits (not shown). Circuit  110  can be, for example a portion of a power supply including components C 1  and C 2  connected by electrically conductive traces T including traces T 1 , T 2 , and T 3  on surface  101  that carry electrical signals from external connections, for example through connectors on edge  103  or through vias to bottom side  105 , to the components, or between the various components C. PCB  100  and circuit  110  are presented to illustrate heat removal from a circuit, which may contain additional components and traces in addition to those shown in FIG. 1. In FIGS. 1A and 1B, component C 1  is a heat dissipating component to be cooled. In general, traces T are metal layers, usually copper. Dashed edges of traces T in FIG. 1A indicate the extent of the traces that are covered by portions of components C.  
           [0006]    [0006]FIGS. 1A and 1B show heat sink  120  on the top of heat generating component C 1 , with heat being discharged perpendicularly away from the PCB  100  as indicated by arrow Q in FIG. 1B. The ability of heat sink  120  to carry heat from component C 1  depends on the size of the heat sink, and the air temperature and flow rates near the heat sink. In general, heat sinks are bulky and costly devices having high thermal conductivity and large surface areas to promote convective heat transfer from the heat sink. Heat sinks are placed in thermal contact with a heat generating component to conduct heat away and have fins that then convect or radiate the heat away from the PCB. The use of a heat sink, such as heat sink  120  depends on enabling the heat sink to have access to cooling air, and thus requires some space above the heat sink that is readily accessible to such cooling air. If component C 1  is small and generates a large amount of heat, the available area on top of the component may not be sufficient to properly cool the component.  
           [0007]    What is needed is an improved heat removal device for PCBs. Such a device should provide adequate cooling and temperature control of the circuit, and should not be hindered by the close proximity of electronic components or of other nearby circuit boards or enclosure walls. The device should also be inexpensive and compatible with conventional printed circuit board assembly techniques.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention solves the above-identified problems of known heat transfer augmentation devices on PCBs by providing a thermal pathway between two traces on a PCB with an element that does not significantly affect the operation of the circuit on the PCB. In particular, the present invention provides surface mounted elements for connection between two traces that are designed such that these elements do not significantly affect operation of the electrical circuit, but do provide for improved thermal transport between the traces.  
           [0009]    It is one aspect of the present invention to provide a device that allows for increased heat transfer from heat generating components through adjoining PCB traces by use of a standard surface mounted element having electrical impedances greater than other circuit components.  
           [0010]    It is another aspect of the present invention to provide a device that shortens the thermal conductive pathway from a heat generating electrical component to a cooler region on the PCB or off of the PCB without affecting the electrical characteristics of a circuit on a PCB.  
           [0011]    It is yet another aspect of the present invention to provide an element between traces of a PCB that improves heat transport along the PCB without affecting the electrical characteristics of a circuit on the PCB,  
           [0012]    It is one aspect of the present invention to provide a heat transfer augmentation device for mounting between two positions on traces of a printed circuit board that supports an electrical circuit having a circuit impedance, where the circuit generates a first temperature difference between the two positions. The device comprises an element disposed between the two positions, having an impedance that is much larger than the circuit impedance, where the element has thermal properties that result in a temperature difference between the two positions is that is less than the first temperature difference.  
           [0013]    It is another aspect of the present invention to provide a heat transfer augmentation device for mounting between two positions on traces of a printed circuit board that supports an electrical circuit having a circuit impedance. The device comprises a surface mount resistor having an impedance that is many times greater than the circuit impedance.  
           [0014]    It is yet another aspect of the present invention to provide a method for augmenting the heat transfer between two positions on the surface on a printed circuit board that supports an electrical circuit having an impedance, where the circuit, when operating, generates a first temperature difference between the two positions. The method comprises mounting an element on the printed circuit board between the two positions. The element has an impedance that is at least 100 times the circuit impedance. The circuit, when operating with the mounted element, generates a second temperature difference between the two positions that is less than said first temperature difference.  
           [0015]    It is an aspect of the present invention to provide a method for augmenting the heat transfer between two positions on the surface on a printed circuit board that supports an electrical circuit having an impedance. The method comprises mounting a surface mount resistor between the two positions having an impedance that is at least 100 times greater than the circuit impedance.  
           [0016]    It is yet another aspect of the present invention to provide a heat transfer augmentation device that is effective, compatible with printed circuit board assembly techniques, and is less expensive than prior art devices.  
           [0017]    A further understanding of the invention can be had from the detailed discussion of the specific embodiment below. For purposes of clarity, this discussion refers to devices, methods, and concepts in terms of specific examples. However, the method of the present invention may be used to connect a wide variety of types of devices. It is therefore intended that the invention not be limited by the discussion of specific embodiments. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0018]    The foregoing aspects and the attendant advantages of the present invention will become more readily appreciated by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0019]    [0019]FIGS. 1A and 1B are views of a prior art device for removing heat from a heat generating component on a printed circuit board, where FIG. 1A is a top view and FIG. 1B is a sectional view  1 B- 1 B;  
         [0020]    [0020]FIGS. 2A and 2B are views of the heat removal device of the present invention on a printed circuit board, where FIG. 2A is a top view and FIG. 2B is a sectional view  2 B- 2 B;  
         [0021]    [0021]FIG. 3 is a perspective view of an exemplary power supply circuit having a first embodiment heat removal device;  
         [0022]    [0022]FIG. 4 is a top view of the exemplary circuit and heat removal device of FIG. 3; and  
         [0023]    [0023]FIG. 5 is a simplified circuit diagram of the exemplary circuit and heat removal device of FIG. 3. 
     
    
       [0024]    Reference symbols are used in the Figures to indicate certain components, aspects or features shown therein, with reference symbols common to more than one Figure indicating like components, aspects or features shown therein.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    To facilitate its description, the invention is described below in terms of a device for improving thermal transport from one position or area on a PCB to another such position or area. In general, the present invention is a device for providing improved thermal transport along and between nearby positions on a printed circuit board with elements having thermal conductive properties that are more favorable to conduction than the non-conducting PCB material, while having an electrical impedance high enough so that the element does not alter the electrical characteristics and operation of the circuit on the PCB.  
         [0026]    The following embodiments and example illustrate the inventive device in terms of removing heat from a heat generating electronic component on a PCB. The device can be used to transport heat on or from a PCB that permits surface mounted components, and is especially useful for PCBs having high energy dissipating components in circuits such as power converters. It is understood that the inventive device can be used to affect the heat flow on a PCB by providing nearly electrically insulated, thermal contact between two trace positions on a PCB, and that the scope of the invention is not limited by the following embodiments and examples.  
         [0027]    The present invention will now be described in more detail with reference to the Figures. FIGS. 2A and 2B are views of the heat removal device of the present invention on a printed circuit board, where FIG. 2A is a top view and FIG. 2B is a sectional view  2 B- 2 B. The PCB  100  has a front surface  101 , an opposing surface  105  and edge  103 , and supports circuit  110  including components C, and traces T, as shown in the prior art drawing of FIG. 1. The inventive heat transfer augmentation device  220  is shown in FIGS. 2A and 2B as including several elements  221 , specifically elements  221   a ,  221   b , and  221   c , connecting adjacent positions on nearby traces T 1  and T 2 .  
         [0028]    Device  220  allows heat from component C 1  to be conducted from trace T 2  through elements  221  to trace T 1 , where it is either dissipated through trace T 1  into the surrounding environment or conducted off of edge  103 . Note that without elements  221 , traces T 1  and T 2  are both electrically and thermally isolated by the intervening circuit board. The presence of a thermally conductive element, such as element  221 , provides a thermally conductive path for heat transfer between T 1  and T 2 . In addition, device  220  also diverts heat away from nearby component C 2  that is thermally connected to component C 1  through trace T 2 , thus reducing the temperature of that component. In an alternative embodiment, not illustrated, PCB  100  may include conductive vias to other circuits on an opposite PCB side  105 . For multilayer PCBs, interior traces may also provide conductive paths for connecting circuit signals or voltages, for example, and also provide a path for thermal conduction of heat. In this alternative embodiment, heat can be conducted through trace T 1 , through vias or interior traces of PCB  100  and off the board.  
         [0029]    Device  220  permits heat to be conducted away from heat generating components and along a surface of PCB  100  without the drawbacks of prior art devices, such as placing a heat sink directly on a heat generating component, which require space above and to the side of the heat generating components to provide sufficient cooling. In general, inventive device  220  can be used on a printed circuit board that is single-sided, double-sided, or multi-layered. It is preferred that device  220  is connected to traces T on an exposed surface of PCB  100 . In addition to providing additional PCB surface area for the transfer of heat, PCB  100  may also include connections to other components that allow for increased heat flow. These other components include, but are not limited to, edge connectors, for example connectors on edge  103 , surface connectors, for example connectors along surfaces  101  or  105 , or other heat transfer elements on surfaces  101  or  105 , such as heat sinks, that are mounted on the trace region of the PCB and away from heat generating components at a location that is better suited to removal of heat than at the component generating heat, for example by having better cooling air flow circulation.  
         [0030]    In addition to providing a thermal bridge between traces T, elements  221  preferably do not interfere with the electrical characteristics of circuit  110 . The primary purpose of traces T is to provide reference voltages or a ground to components C, or to provide a path for voltages or signals between components C. While the connection of portions of traces T increases the heat transfer from heat generating components, device  220  is intended to provide enhanced heat transfer along a surface of the PCB  100  without altering the circuit on the PCB. Specifically, device  220  provides a thermal pathway between several of traces T without affecting the electrical characteristics of circuits on PCB  100  by connecting the traces with elements  221  having impedances that are much greater than the impedance of components C or any part of the circuit.  
         [0031]    Elements  220  are thus preferably more thermally conductive than the insulating material of PCB  100 , have an impedance that is much larger than that of components C, are inexpensive and widely available and are compatible with PCB fabrication techniques. A particularly preferred choice for element  220  is a standard surface mount electrical resistor with a substrate of either alumina or aluminum nitride having an electrical resistance that is greater than the impedance of the circuit to which it is mounted. Resistors on alumina or aluminum nitride substrates, for example those manufactured by Maruwa Co., Ltd. (Aichi, Japan), have a higher thermal conductivity than printed circuit board materials, come in a wide range of electrical impedances, are inexpensive and are compatible with PCB fabrication techniques. Thus, for example, the non-electrically conductive portion of a PCB is manufactured from a material such as glass/epoxy composite, which has a thermal conductivity on the order of 0.3 W/mK. An alumina resistor has a thermal conductivity of about 28 W/mK, and aluminum nitride resistor has a thermal conductivity of about 170 W/mK. A connected resistor having a thermal conductivity greater than that of the underlying PCB insulator will provide for improved heat transfer between the positions connected by the resistive element. Elements  220  preferably have a resistance that is much larger than the impedance of circuit  110 . It is preferred that the impedance be 100 to 1000 times the circuit impedance.  
         [0032]    One example of the inventive heat removal device is shown in FIGS. 3-5. Specifically FIGS. 3-4 illustrate the mechanical and electrical features of the inventive heat transfer device, where FIG. 3 is a perspective view of a power supply circuit having a first embodiment of a heat transfer device and FIG. 4 is a top view of the circuit and heat transfer device of FIG. 3, and FIG. 5 is a simplified circuit diagram of the circuit and heat transfer device of FIG. 3. FIGS. 3 and 4 show a PCB  300  having a front surface  301  and a back surface  305  and supporting a circuit  310 . Circuit  310  is a power converter circuit that includes components  311  and traces  313  formed on front surface  301 . An inventive heat transfer device  320  includes several elements  321 , shown as  321   a ,  321   b ,  321   c , and  321   d  disposed on PCB  300 .  
         [0033]    Components  311  include components  311 - 1  to  311 - 7 , a listing of which is provided in Table I. Components  311  are in contact with traces  313 , including trace  313 - 1 , which is at ground trace  313 - 4 , which provides an input voltage VIN, trace  313 - 2 , which is at an output voltage V0, and traces  313 - 3  and  313 - 5  which provide signal conduction paths between selected components  311 . In addition, circuit  310  is connected to other circuits though connectors  330  that are used to electrically connect and mechanically mount PCB  300  to another part of an exemplary electrical circuit.  
                             TABLE I                           Component listing for circuit 310                Label   Component Type                       311-1   Capacitor           311-2   Capacitor           311-3   MOSFET           311-4   MOSFET           311-5   Inductor           311-6   Capacitor           311-7   Capacitor                      
 
         [0034]    Each of elements  321  is an alumina or aluminum nitride resistive element placed about component  311 - 4 , which is a heat generating MOSFET, to direct the heat flow away from component  311 - 5 . In order to not electrically participate or interfere with the circuit  310 , the resistances of elements  321  are selected to have a resistance that is greater than the circuit impedance, as discussed subsequently.  
         [0035]    A simplified electrical circuit diagram of circuit  310  is shown in FIG. 5, with the values of the various electrical components presented in Table II. As listed in Table II, Q1 and Q2 represent the conduction impedance of MOSFET  311 - 3  and  311 - 4 , respectively. C1IN and C2 OUT represent the capacitance of capacitors  311 - 1  and  311 - 12  and  311 - 6  and  311 - 7 , respectively. L is the inductance of inductor  311 - 5 . R2 is the resistance of an external load (not shown in FIGS. 3 and 4), and VIN and VO are the supplied voltages at traces  313 - 4  and  313 - 2 , respective. R1 is the equivalent resistance of elements  320  of heat transfer device  320 .  
                                 TABLE II                           Values of voltages and component parameters of circuit 310                Label on   Corresponding Components   Electrical               of FIGS. 3 and 4   parameter value                       Q1   311-3   0.0145 Ω           Q2   311-4   0.0080 Ω           C1 IN   311-1 and 311-2   20 μF           C2 OUT   311-6 and 311-7   20 μF           L1   311-5   2.8 μH           R2 LOAD   (not shown)   0.1 to 0.45 Ω           VIN   313-4   1.8 to 13 V           VO   313-2   0.9 to 6 V           R1   321-a   250 kΩ                      
 
         [0036]    The circuit of FIG. 5 and Table II results in a circuit impedance of less than 1 kΩ. The impedance of circuit  310  without resistor R1 (that is, without elements  321 ) is less than 1 kΩ. The resistance of each of elements  321  should be greater than 100 kΩ, a value selected to be 100 times greater than the 1 kΩ maximum circuit impedance. The resistance of each element of  321  having values of from 400 kΩ to 1000 kΩ were found to be particularly useful. It is thus seen that the use of heat transfer device  320  provides improved heat transfer characteristics without affecting the electrical performance of the circuit on which it is used.  
         [0037]    The invention has now been explained with regard to specific embodiments. Variations on these embodiments and other embodiments may be apparent to those of skill in the art. It is therefore intended that the invention not be limited by the discussion of specific embodiments. It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims