Abstract:
A cooling system for electronic components and printed circuit boards (PCBs) provides close thermal contact of a bulk coolant circulating through channels formed inside or on the surface of a multi-layer PCB carrying the electronic components, such that heat produced by the components is efficiently removed to a heat sink. The circulation channels may be formed by removing portions of layers, and inserting overlaying vias, during manufacturing or assembly of the PCB.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention pertains generally to thermal management of electronic devices and, more particularly, to cooling of electronic power transistor devices and printed circuit boards.  
         BACKGROUND  
         [0002]    Electronic components typically generate some heat as electrical energy is partially converted to thermal energy due the losses in the respective components. It is normally desirable to remove the heat from these components in a rapid and efficient manner, since increased temperature can effect performance, shorten the lifetime, and cause premature failure of many types of electronic components. Power transistors, such as laterally diffused metal oxide silicon (LDMOS) field effect transistors (FETs) commonly used in RF power amplifiers, are especially susceptible to overheating. These devices typically handle the large part of the power flowing through the amplifier and consequently need to dissipate the most thermal energy. This tends to make them operate at higher temperatures than other amplifier components. In addition, amplifier efficiency tends to drop as the power transistors heat up, producing still more heat to dissipate and higher temperatures. Further, while semiconductor processing and packaging improvements have allowed designers to produce smaller electronic devices which operate at higher power, the associated increased operating temperatures have limited maximum safe power levels.  
           [0003]    One traditional method of cooling electronic power devices involves dispersing the heat generated by the device through its support structure, e.g. a metallic flange, and into a heat sink, typically a ceramic or metal material, which, in turn, dissipates the thermal energy to the environment. Of course, the device temperature depends on thermal resistance of all of the materials carrying heat away from the active components of the device, typically one or more semiconductor chips.  
           [0004]    Thermal resistance, R, is defined by the temperature drop between its endpoints and the heat flow through it:  
             T 1 −T 2 R*Q   (1)  
           [0005]    where T1 and T2 are the temperatures at the endpoints, and Q is the rate of heat flow. Thermal resistance for one dimensional heat flow in a uniform material is proportional to the length of the conducting material, and inversely proportional to the conduction area and thermal conductivity of the material, or:  
             R=L/ ( A *alpha)  (2)  
           [0006]    where alpha is the thermal conductivity, A is the cross sectional area, and L is the length of the conducting material.  
           [0007]    [0007]FIG. 1 illustrates an exemplary electronic device  100 , including a transistor die  102 , die-to-flange conductive layer  104 , mounting flange  106 , thermal grease or solder bond layer  108  and heat sink  110 . Heat produced by electrical conduction through the die  102  is transferred by passing through the die  102 , conductive layer  104 , flange  106 , thermal grease or solder bond  108 , and heat sink  110 , respectively, finally dissipating into the environment  112 . As the heat flows, the temperature of each element in the thermal path is successively lower, i.e. the temperature of die  102  is the highest, the temperature in the conductive layer  104  is less than the die  102 , and so on. The temperature of the environment  112  is the lowest.  
           [0008]    [0008]FIG. 2A is a thermal model for the system shown in FIG. 1. The power dissipated by die  102  is represented by heat source  120 . As this heat flows through thermal resistors  122 , 124 , 126 , 128  and  130 , there is a temperature drop across each.  
           [0009]    There are numerous ways to estimate the values for thermal resistances. First, second and third thermal resistances  122 ,  123 ,  124  can be estimated using equation (2). Heat sinks, such as heat sink  110 , are commercially available in many forms and well known in the art. The thermal resistances of a heat sink are usually specified by the manufacturer or can be determined from performance curves.  
           [0010]    Since the intermediate temperatures associated with the conductive layer, flange, thermal grease or solder bond, and heat sink are rarely important to this type of thermal analysis, the thermal model can be greatly simplified as shown in FIG. 2B. Total thermal resistance  132  is the algebraic sum of the first through fifth thermal resistances shown in FIG. 2A:  
           [0011]    The temperature of the die  122  is easily calculated from this model:  
             T _die= T _env+( Q _dissipation* R _total)  (3)  
           [0012]    The power dissipated by die  102  is usually specified, based on the application. The temperature of the environment is typically not controlled, and for devices located outdoors (e.g., in a wireless network base station), can vary widely with season, time of day, and geography. Of course, the object is to keep the temperature of the die  102  as close as possible to the temperature of the environment  112 . Towards this end, the most effective way to reduce the temperature difference is to reduce the total thermal resistance  132 .  
         SUMMARY OF THE INVENTION  
         [0013]    In accordance with one aspect of the invention, a more efficient thermal management system for cooling of electronic components, such as power transistors, is achieved by providing close thermal contact of a bulk coolant flowing through channels formed inside or on the surface of a printed circuit board (PCB) carrying electronic components.  
           [0014]    In one embodiment, flow channels are formed in the layers of a multilayer PCB in close proximity to selected electronic components, such that heat produced by the electronic components is efficiently carried by bulk motion of the coolant to a heat sink, where the heat is then delivered to the environment. In particular, as the coolant passes under the heat producing components, thermal energy is transferred from the components and into the coolant. The circulation of the coolant serves to rapidly transfer thermal energy away from the heat producing components and delivers the heat to the heat sink, which facilitates the transfer of thermal energy from the coolant to the environment.  
           [0015]    Other aspects, objects and features of the present invention will become apparent hereinafter.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The drawings illustrate both the design and utility of the preferred embodiments of the present invention, in which similar elements in different embodiments are referred to by the same reference numbers for ease in illustration, and in which:  
         [0017]    [0017]FIG. 1 is a cross-sectional diagram illustrating prior art cooling of an electronic device.  
         [0018]    [0018]FIG. 2A is a detailed thermal model for the heat flow to the environment of the device of FIG. 1.  
         [0019]    [0019]FIG. 2B is a more simplified thermal model for the heat flow to the environment of the device of FIG. 1.  
         [0020]    [0020]FIG. 3 is a cross-sectional diagram of an electronic device attached to a multilayer PCB, with a preferred thermal management system provided for cooling the electronic device, the thermal management system having a cooling channel formed in the PCB.  
         [0021]    [0021]FIG. 4 is a cross-sectional diagram of the electronic device of FIG. 3 attached to a multilayer PCB, with an alternate preferred thermal management system provided for cooling the electronic device, the thermal management system having a cooling channel formed between a layer of the PCB and a mounting flange of the electronic device.  
         [0022]    [0022]FIG. 5 is a cross section of a portion of a cooling channel in the PCB of FIG. 4.  
         [0023]    [0023]FIG. 6 is a cross-sectional diagram of a plurality of electronics devices attached to a multilayer PCB and cooled by a still further preferred thermal management system, including a cooling channel formed between layers of the PCB and respective mounting flanges of the electronic devices.  
         [0024]    [0024]FIG. 7 is a cross-sectional diagram of yet a further preferred thermal management system integrated in a PCB for cooling an attached electronic device.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    Referring to FIG. 3, in accordance with a first aspect of the invention, an electronic device  200 , includes a transistor die  202  attached to a mounting flange  206  via a conductive layer  204 . The mounting flange  206  is attached to a multilayer printed circuit board (PCB)  212  by a thermal grease or solder bond  208 . The device  200  is cooled by a thermal management system  220 , which employs a coolant  226  circulating through a closed cooling loop  230 . The cooling loop  230  comprises a flow channel  222  and a circulation pump  224 . Coolant  226  is pumped around the cooling loop  230  and passes through the heated portion of flow channel  222 , the cooled portion of flow channel  222  and the circulation pump  224 . Heat is transferred by bulk motion of the coolant  226  from the heated portion of flow channel  222  to the cooled portion of flow channel  222 . The coolant  226  may be a gas or a liquid, and preferably has a relatively low thermal resistance.  
         [0026]    In particular, heat flows from the transistor die  202  and to the coolant  226  as the coolant  226  passes through the flow channel  222 . Heat flows from the coolant  226  and into the environment  112  as the coolant  226  passes through the cooled portion of the flow channel  222 , which is thermally coupled to the heat sink  210 . Heat sink  210  facilitates the flow of heat from the coolant  226  into the environment  112 .  
         [0027]    The heated portion of flow channel  222  is fully contained by the PCB  212  and formed in close proximity to the flange  206  in order to minimize the thermal resistance between the flange  206  and the heated portion of the flow channel  222 . This, in turn, minimizes the temperature drop from the flange  206  to the coolant  226  as it passes through the heated portion of the flow channel  222 , which maximizes the heat flow from the transistor die  202  into the coolant  226 .  
         [0028]    Carrying heat by bulk motion of the coolant  226  from the heated portion to the cooled portion of the flow channel  222  tends to minimize the temperature difference between the PCB  212  in the vicinity of device  200  and the heat sink  210 . This, in turn, tends to minimize the temperature difference between transistor die  202  and the environment  112 , thus lowering the temperature of the die  202 .  
         [0029]    In accordance with a further aspect of the invention, and referring to the electronic device  200 , it may be advantageous in some applications to eliminate the mounting flange  206  and attach the transistor die  202  directly to the PCB  212  by other means. For example, the transistor die  202  may be attached directly to an integral conductive (e.g., gold or copper) foil layer on the surface of the PCB  212 . In this case, the die  202  is preferably attached to the foil via a die-to-foil conductive layer, which provides good electrical conductivity as well as good thermal conductivity. The thermal resistance associated with the flange  206  and die-to-flange conductive layer  208  are thus eliminated and replaced by (much lower) thermal resistances associated with the foil layer and the die-to-foil conductive layer. A practitioner, skilled in the art, will appreciate that heat produced by the transistor die  202  can be transferred to the coolant  226  more efficiently in some applications using this alternative means for attaching the die  202  to the PCB  212 .  
         [0030]    Referring to FIG. 4, in accordance with a further aspect of the invention, the electronic device  200  is cooled by an alternate thermal management system  320 , which includes a coolant  326  circulating through a closed cooling loop  330 . System  320  is similar to system  220  of FIG. 3, but includes direct contact of the coolant  326  with the device  200 , which is attached to a multilayer PCB  312 .  
         [0031]    The cooling loop  330  comprises a flow channel  322  and a circulation pump  324 . Coolant  326  is pumped through the cooling loop  330 , passing through the heated portion of flow channel  322 , the cooled portion of flow channel  322 , and the circulation pump  324 , respectively. Heat is transferred by bulk motion of the coolant  326  from the heated portion of flow channel  322  to the cooled portion of flow channel  322 .  
         [0032]    In particular, heat flows from the transistor die  202  to the coolant  326  as the coolant  326  passes through the flow channel  322 . Heat flows from the coolant  326  and into the environment  112  as the coolant  326  passes through the cooled portion of the flow channel  322 , which is thermally coupled to a heat sink  310 . Heat sink  310  facilitates the flow of heat from the coolant  326  into the environment  112 .  
         [0033]    The heated portion of flow channel  322  is partially contained by the PCB  312  and partially contained by the mounting flange  206 . As coolant passes through the heated portion of flow channel  322  in the vicinity of the mounting flange  206 , there is direct contact of the coolant  326  with the mounting flange  206 , in order to minimize the temperature drop between the flange  206  and the coolant  326 . In order to prevent coolant leakage, the flange  206  is preferably sealed to the surface of the PCB  312 . Numerous materials are acceptable sealants and include polymer sealants and solder, as in solder bond  208 . The remaining portions of the heated flow channel  322  are fully contained within the PCB  312 . As the coolant  326  passes through the cooled portion of the flow channel  322 , which is thermally coupled to the heat sink  310 , heat flows from the coolant  326 , through the heat sink  310 , and into the environment  112 .  
         [0034]    Having direct contact between the coolant  326  and the flange  206  eliminates the thermal resistance of the PCB  312 , which lowers the temperature difference between the flange  206  and the coolant  326 . This, in turn, lowers the temperature of the transistor die  202  even more efficiently that the thermal management system  220 , although the efficiency advantage is somewhat offset by the increased complexity and extra cost associated with properly sealing the flange  206  to the PCB  312 .  
         [0035]    [0035]FIG. 5 is a detailed drawing of a portion of the flow channel  322  in the vicinity of device  200  which illustrates formation of the flow channel  322  among the layers  1 -N of the PCB  312 . In particular, the flow channel  322  formed in the PCB  312  is a series of interconnected segments which allow coolant  326  to flow. The portions of the flow channel  322  illustrated as horizontal segments  344  may be formed by removing a volume of bulk material from the corresponding PCB layer during the manufacturing or assembly process. The portions of the flow channel  322  illustrated as vertical segments  346  may be formed by coincident layer-to-layer vias. Interconnection of a horizontal segment  344  with a vertical segment  336  can be made by having the via on one PCB layer coincide with a horizontal segment  344  on an adjacent layer.  
         [0036]    Referring to FIG. 6, in accordance with a still further aspect of the invention, multiple devices  200  are attached to a PCB  412  and cooled by a thermal management system  420 , which employs a coolant  426  and a closed cooling loop  430 . The cooling loop  430  comprises a flow channel  422  and a circulation pump  424 . Coolant is pumped around the cooling loop  430  and is heated as it passes in the vicinity of devices  200 . Heat is transferred by bulk motion of the coolant  426  from the vicinity of devices  200  to the cooled portion of the flow channel  422 .  
         [0037]    In particular, heat flows from the die  202  of each device  200  to the coolant  426  as the coolant  426  passes through the flow channel  422  in the vicinity of the respective die  202 . Heat flows from the coolant  426 , and into the environment  112 , as the coolant  426  passes through a heat sink  410 . The heat sink  410  facilitates the flow of heat from the coolant  426  into the environment  112 . Although FIG. 6 shows a flow channel  422  which has direct contact of the coolant  226  with flanges  208 , it is understood that close proximity (as illustrated in FIG. 3) is also within the scope of the invention.  
         [0038]    Cooling a plurality of devices  200  with a single cooling loop has the advantage of lower cost than multiple independent cooling loops. This advantage is offset, to some degree, by the increased complexity of the cooling loop and somewhat decreased efficiency.  
         [0039]    Referring to FIG. 7, in accordance with a further aspect of the invention, an electronic assembly  200 , including a transistor die  202  attached to a mounting flange  206  via a conductive layer  204  is attached to a multilayer PCB  512  by a thermal grease or solder bond  208 . The device  200  is cooled by a thermal management system  520 , which employs a coolant  226  and an open cooling loop  530 . The cooling loop  530  comprises a flow channel  522  and a circulation pump  524 . Coolant  526  is drawn in from the surroundings by circulation pump  524  and pumped through the heated portion of flow channel  522  where the coolant is expelled into the surroundings. Heat is transferred by bulk motion of the coolant  526  from the heated portion of flow channel  522  to the surroundings and, in turn, to the environment  112 .  
         [0040]    In particular, heat flows from the transistor die  202  to the coolant  526  as the coolant  526  passes through the flow channel  522 . Heat flows from the coolant  526  and into the environment  112  as the coolant  226  is expelled into the surroundings.  
         [0041]    A practitioner, skilled in the art, will appreciate that the flow channel  522  depicted in FIG. 7 could alternately be implemented in a way similar to that depicted in FIG. 4, FIG. 5, or FIG. 6.  
         [0042]    Accordingly, the invention is not to be restricted, except in light of the claims and their equivalents.