Abstract:
A wrap-around cooling arrangement for a printed circuit board is disclosed. Such an arrangement comprises: a printed circuit board (“PCB”) having a first side and a second side opposite to said first side; a heat sink arranged on said first side of said PCB; a first to-be-cooled component coupled to said second side of said PCB; and a thermal jumper to thermally couple said first component on said second side to said heat sink on said first side, said jumper being configured to extend physically around a side edge of said PCB.

Description:
FIELD OF THE INVENTION 
     The invention is generally directed to the field of arrangements for cooling components attached to a printed circuit board, and more particularly to such arrangements that wrap around an edge of the printed circuit board. 
     BACKGROUND OF THE INVENTION 
     It is known to cool components on a printed circuit board (PCB) using a heat sink. A heat sink is a device for transferring heat from the electronic components into the ambient environment. The heat sink can be a finned metal (typically aluminum) element with or without forced airflow across it. A heat sink can also be a liquid cooled jacket or thermoelectric device. Heat pipes can also be used to transfer the heat from the heat sink. Alternatively, the heat sink could be the chassis of the computer. 
     Typically, a heat sink is located on the same side of the PCB as the components that it cools. And it is common for the heat sink to sit on the components that are to be cooled in what can be described as a stacked arrangement. 
     In a circumstance in which there is relatively little clearance above the surfaces of the PCB, this stacked arrangement can be unsatisfactory. 
     SUMMARY OF THE INVENTION 
     The invention, in part, provides an arrangement for cooling components on a printed circuit board, the arrangement comprising: a printed circuit board (“PCB”) having a first side and a second side opposite to said first side; a heat sink arrangement on said first side of said PCB; a first to-be-cooled component attached to said second side of said PCB; and a thermal jumper to thermally couple said first component on said second side to said heat sink on said first side, said jumper being configured to extend physically around a side edge of said PCB. 
     The invention, also in part, provides an arrangement for cooling components on a printed circuit board, the arrangement comprising: a printed circuit board (“PCB”) having at least first and second to-be-cooled components attached to said PCB, said first component being located on a first side of said PCB and said second component being located on a second side of said PCB, said second side being opposite to said first side; a heat sink arranged on said first side of said PCB, said heat sink being thermally coupled to said first component; and a thermal jumper to thermally couple said second component on said second side to said heat sink on said first side, said jumper being configured to extend physically around a side edge of said PCB. 
     Additional features and advantages of the invention will be more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-section of a first embodiment of a wrap-around cooling arrangement for a printed circuit board according to the invention. 
         FIG. 2  is a cross-section of an alternative configuration of a thermal jumper according to the first embodiment of the invention. 
         FIG. 3  is a cross-sectional view of a second embodiment of a wraparound cooling arrangement for a printed circuit board according to the invention. 
         FIG. 4  is a cross-sectional view of a third embodiment of a wrap-around cooling arrangement for a printed circuit board according to the invention. 
         FIG. 5  is a cross-sectional view of a fourth embodiment of a wraparound cooling arrangement for a printed circuit board according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a cross-section of a first embodiment of a wrap-around cooling arrangement for a printed circuit board according to the invention. In  FIG. 1 , a printed circuit board (“PCB”)  102  is provided with a component  104  that is to be cooled. The component  104  can be located inward of a side edge  103  of the PCB  102 . 
     The arrangement  100  of  FIG. 1  is also provided with a heat sink  106 . As in the section above entitled “Background Of The Invention,” the heat sink  106  can be a finned metal (e.g., aluminum) element with or without forced airflow across it, a liquid cooled jacket and/or thermoelectric device. Heat pipes can also be used to transfer the heat from the heat sink. Alternatively, the heat sink could be (in part or in whole) the chassis of the computer in which the PCB  102  is located. 
     The heat sink  106  is depicted as abutting the component  104 . The component  104  may or may not be provided with a thermal coupling structure or compound (e.g., a thermally-conductive gap pad) between it and the heat sink  106 . Examples of the component  104  are a central processing unit (“CPU”) type of integrated circuit device, an application specific integrated circuit (“ASIC”) or a digital signal processor (“DSP”), etc. 
     Also attached to the PCB  102  are to-be-cooled components  108  and  110 , e.g., integrated circuit memory devices. Components  108  and  110  will typically be located closer to an edge of the PCB  102  than the component  104 . Inserted between the component  108  and the heat sink  106  are a thermally-conductive gap pad  111 , a planar portion  115 A of a thermal jumper  114 A, and a thermally-conductive gap pad  116 . Inserted between the component  110  and the heat sink  106  are a thermally-conductive gap pad  112  and a planar portion  115 C of the thermal jumper  114 A. Each of the gap pads  112 ,  111  and  116  is optional but preferred. 
     An example of the thermally-conductive gap pad material from which the gap pads  111 ,  112  and  116  can be made is the GAP PAD 1500 brand of thermally-conductive pad made available by THE BERGQUIST COMPANY. 
     In addition to the planar portion  115 A and  115 C, the thermal jumper  114 A includes a portion  115 B that wraps around the side edge  103  of the PCB  102 . In cross-section, the wrap-around portion  115 B has a substantially square-U shape.  FIG. 2  depicts an alternative configuration to that of thermal jumper  114 A, namely thermal jumper  114 B. Jumper  114 B has the same planar portion  115 A and  115 C. But jumper  114 B differs by the configuration of the wrap-around portion  130 , which in cross-section is substantially semi-circular. It should be noted that the configuration in cross-section of the wrap-around portion of the jumper is not critical. The wraparound portion should clear the end  103  of the PCB  102 . 
     To ensure good thermal contact between the surfaces in the stack including planar portion  115 C, gap pad  112  and to-be-cooled device  110 , as well as in the stack including to-be-cooled device  108 , gap pad  111 , planar portion  115 A and gap pad  116 , an optional (but preferred) compression arrangement  124  is provided. The arrangement  124  can include a threaded bolt  126  that has a corresponding threaded hole in the heat sink  106  such that the bolt  126  passes through the planar surface  115 C, the gap pad  112 , the to-be-cooled device  110 , the PCB  102 , the to-be-cooled device  108 , the gap pad  111 , the planar portion  115 A and the gap pad  116 . Optionally, a washer  128  can be provided between the head of the bolt  126  and the planar portion  115 C. Alternatively, instead of a threaded hole in the heat sink  106 , the bolt  126  can be of sufficient length to pass through the heat sink  106  where it can couple to a nut (not depicted) and optionally a lock washer (not depicted) and/or flat washer (not depicted). 
     In the circumstance where the to-be-cooled devices  108  and  110  are the same type of device, then probably it will be desired for these devices to run at substantially the same temperature. If so, then both should be provided with an equivalent thermal resistance. 
     Thermal resistance is, in part, a function of a number of factors including the distance between the device that is to be cooled and the heat sink, cross-sectional area of the thermal connector and thermal conductivity. Inspection of  FIG. 1  reveals that the thermal path to the heat sink  106  from the device  108  includes the gap pad  116  and the planar portion  115 A. In contrast, the thermal path between the heat sink  106  and the device  110  includes the planar portion  115 C, the wraparound portion  115 B, planar portion  115 A and the gap pad  116 . By having fewer segments, the thermal path to dissipate heat from the device  108  could be shorter, i.e., could be more efficient, than for the device  110 . If devices  108  and  110  are to face equivalent thermal resistance, then a compensation factor must be added to the thermal path for the device  108 . This can be accomplished by appropriate selection of the thicknesses and thermal conductivities of the gap pads, e.g., by making the gap pads  111  and  112  be of different thickness (assuming the same thermal conductivity). 
     According to the first embodiment of the invention depicted in  FIG. 1  (which assumes devices  108  and  110  are the same device), the gap pad  111  is much thicker than the gap pad  112 . The greater thickness of the gap  111  decreases the thermal efficiency of the thermally conductive path between the device  108  and the heat sink  106 . In one example embodiment, the gap pad  112  has a thickness of 0.03 inch while the thickness of the gap pad  111  is 0.08 inch. 
     An advantage to the wrap-around thermal jumper according to the invention is that it permits devices on a second side of a PCB  102  to effectively be cooled by a heat sink on a first side of a PCB  102 . This can be important in situations where there is very little clearance between the PCB  102  and an adjacent structure. An example of such a circumstance is where the PCB  102  is a daughter card to a mother card, denoted as  118  in FIG.  1 . There, the gap  120  between the PCB  102  and the PCB  118  is relatively small. Use of the wrap-around thermal jumper permits the device  110  to be cooled via the heat sink  106  and yet preserve a gap  122  between the components of the PCB  102  and the PCB  118 . An example of the size of gap  122  is one millimeter. 
     The wrap-around sections  115 B and  130  have been depicted as significantly thinner in cross section than the planar portions  115 A and  115 C. Example thicknesses for the thermal jumper  114 A are 0.04 inch for the planar portions  115 A and  115 C versus 0.01 inch thickness for the wrap-around portion  115 B. 
     The significantly thinner wrap-around sections  115 B and  130  permit the thermal jumper  114 A/ 114 B to exhibit flexibility in the distance  123  between the planar portions  115 A and  115 C. This confers an advantage during manufacture. 
     It has been found that the surfaces of the gap pads  111  and  112  have a significant coefficient of friction. It is difficult to slide the planar portions  115 A and  115 C across the surfaces of the gap pads  111  and  112 , respectively, due to the pads&#39; coefficient of friction. If thermal jumper  114 A/ 114 B were extremely rigid, then this would be the only way to bring the planar portions  115 A and  115 C into contact with the gap pads  111  and  112 . 
     But because the thinner cross-section of the wrap-around portions  115 B/ 130  permit flexibility in the distance  123  between the planar potions  115 A and  115 C, the planar portions  115 A and  115 C can be spread apart (increasing gap  123 ) to allow positioning over the gap pad  111  and  112  by holding the planar portions  115 A and  115 C under tension. When the tension is removed, the distance  123  is restored to its nominal value (either by the inherent resilience of the jumper  114 A or by application of a compressive force), bringing planar portions  115 A and  115 C into contact with the surfaces of the gap pads  111  and  112 . It is to be noted that it is not necessary for the wrap-around portions  115 B and  130  to be thinner in cross section than the planar portions  115 A and  115 C, but it is preferred because of the flexibility in the dimension between the planar surfaces  115 A and  115 C that it confers. 
     Yet another alternative configuration for the wrap-around portion would be for it to be finned so as to also function as a heat sink. This alternative is less preferred because of the cost associated with finning the wrap-around portion. If manufacturing costs decrease, this would be a more preferable alternative. Using the jumper  114  of  FIG. 2  as an example, optional cooling fins  131  have been depicted in phantom lines. The fins  131  project from the convex side of the wraparound portion  130 . But the fins  131  instead, or additionally, could be arranged to project from the concave surface of the wrap-around portion  130 . The fins  131  should not interfere with the flexing of the jumper  114 B during the installation of the jumper. The use of fins  131  may be constrained by space and airflow considerations. The fins  131  can be placed on all other embodiments of the jumper. 
     The thermal jumper  114 A/ 114 B is preferably made of copper because copper is relatively easy to manufacture, is a good thermal conductor and is relatively cheap. Any conductor could suffice as an alternative material. Carbon fiber would be an especially good alternative but for its relatively much greater expense. 
       FIG. 3  is a cross-sectional view of a second embodiment of a wraparound cooling arrangement for a printed circuit board according to the invention. In  FIG. 3 , a wrap-around thermal jumper has been adapted so that the planar portion  115 A attaches to the upper surface of the heat sink  106 . A thermally-conductive gap pad  310  is interposed between the heat sink  106  and the planar portion  115 A. An optional (but preferred) compression arrangement  312  is provided that is similar to the compression arrangement  124 . As such, a threaded bolt  316  and an optional washer  314  are depicted. Alternatively, item  314  can be considered a nut and the bolt  126  can be of sufficient length to pass through the heat sink  106  where it can be coupled to the nut  314 . 
     Compared to the wrap-around portion  115 B of jumper  114 A in  FIG. 1 , the wrap-around portion  304  is elongated. Also depicted in  FIG. 3  are a component  306  and its corresponding gap pad  308  Component  306  and pad  308  represent an optional use of the space between the cantilevered portion of the heat sink  106  (that extends beyond the device  104 ) and the PCB  102 . 
       FIG. 4  is a cross-sectional view of a third embodiment of a wrap-around cooling arrangement for a printed circuit board according to the invention. In  FIG. 4 , a wrap-around thermal jumper  402  has been adapted so that the planar portion  404  attaches to the side end  106 A of the heat sink  106 . A thermally-conductive gap pad  414  is interposed between the heat sink  106  and the planar portion  404 . An optional (but preferred) compression arrangement  408  is provided that is similar to the compression arrangement  124 . As such, a threaded bolt  410  and an optional washer  412  are depicted. Compared to the wrap-around portion  115 B of jumper  114 A in  FIG. 1 , the wrap-around portion  406  is rounded only at one end. 
     In  FIG. 4 , the side end  106 A of the heat sink  106  has been arranged to extend beyond the side end  103  of the PCB. This is not strictly necessary, but simplifies the configuration of the wrap-around portion  406 , i.e., further articulation in the wrap-around portion  406  can be avoided. 
       FIG. 5  is a cross-sectional view of a fourth embodiment of a wraparound cooling arrangement for a printed circuit board according to the invention. In  FIG. 5 , a wrap-around thermal jumper  502  has been adapted so that the planar portion  506  attaches to the lower surface of the heat sink  106 . A thermally-conductive gap pad  514  is interposed between the heat sink  106  and the planar portion  506 . An optional (but preferred) compression arrangement  508  is provided that is similar to the compression arrangement  124 . As such, a threaded bolt  510  and an optional washer  512  are depicted. In this embodiment, the threaded bolt either screws into the heat sink  106  or is of sufficient length to pass through the heat sink  106  where it can couple to a nut (not depicted) and optionally a lock washer (not depicted) and/or flat washer (not depicted). 
     It is to be observed that the planar portion  506  has the opposite connection orientation (notch oriented outwardly) relative to the wrap-around portion  504  as does the planar portion  115 C (notch oriented inwardly). In contrast, the planar portion  115 A of  FIG. 1  can be described as having the same connection orientation (notch oriented inwardly) relative to the wrap-around portion  115 B as does the planar portion  115 C. 
     The invention may be embodied in other forms without departing from its spirit and essential characteristics. The described embodiments are to be considered only non-limiting examples of the invention. The scope of the invention is to be measured by the appended claims. All changes which come within the meaning and equivalency of the claims are to be embraced within their scope.