Abstract:
A heat spreader with multiple stacked printed circuit boards (PCBS) includes top and side sections within which a first PCB is contained and bottom edges that extend to a second PCB. The heat spreader and second PCB substantially enclose the first PCB therein.

Description:
BACKGROUND 
   Electronic printed circuit boards (PCBs) commonly have a board with electronic components mounted thereon that can generate a considerable amount of heat due to electrical power consumption. The heat must be dissipated from the components and the board to ensure proper functioning of the components and to prevent damage to any part of the PCB. 
   A variety of techniques have been developed to dissipate the heat. For instance, one low-cost technique simply allows the source of the heat to transfer the heat by convection to ambient air. However, this technique has a relatively low effectiveness. To enhance the effectiveness of dissipation by convection, a fan is added to force the air to flow over the component. A heat sink or heat spreader may be attached to the component to further enhance heat dissipation by conducting the heat away from the heat source to a surface area (larger than that of the component) from which the heat may be dissipated to the air. For even greater heat dissipation enhancement, increasingly sophisticated techniques have been developed, including the use of heat pipes, active refrigeration, liquid cooled plates and spray cooling. Each such enhancement, however, is offset by an added cost. 
   It has been necessary to accept the added cost for enhanced heat dissipation techniques because a significant trend in the electronics industry has been to continually increase the processing power and power consumption of electronic components, leading to increasingly greater needs for heat dissipation. An additional trend, however, has been to decrease the size of the components, resulting in greater heat dissipation requirements in smaller volumes. In addition to decreasing the size of the individual components, such components are also often closely packed together in PCBs or circuit board modules, leaving little room between components to place the often-bulky heat dissipation apparatus. Furthermore, multiple PCBs in a single circuit board module may be closely stacked one on top of the other, leaving little or no room between the PCBs for the heat dissipation apparatus, even though the technique of flowing air between the stacked PCBs is insufficient to adequately cool the components therein. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top, front, left side perspective view of a heat spreader according to an embodiment of the present invention. 
       FIG. 2  is a bottom, front, left side perspective view of the heat spreader shown in  FIG. 1  according to an embodiment of the present invention. 
       FIG. 3  is a bottom, front, left side perspective view of the heat spreader shown in  FIG. 1  with multiple printed circuit boards according to an embodiment of the present invention. 
       FIG. 4  is a top, front, left side perspective view of a circuit board module having the heat spreader and multiple printed circuit boards shown in  FIG. 3  according to an embodiment of the present invention. 
       FIG. 5  is a cross sectional view of the circuit board module shown in  FIG. 4  according to an embodiment of the present invention. 
       FIG. 6  is a partial cross sectional view of a portion of another circuit board module according to an alternative embodiment of the present invention. 
       FIG. 7  is a bottom, front, left side perspective view of another heat spreader according to an alternative embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   An exemplary heat spreader  100 , as shown in  FIGS. 1 and 2 , incorporating an embodiment of the present invention generally includes a top section  102  and side sections  104 ,  106 , 108  and  110 . The heat spreader  100  is made of an appropriate high-thermal-conductivity material, and in some embodiments the material is also electrically conductive. The top section  102  and side sections  104 - 110  define an interior  112  of the heat spreader  100 . One or more exemplary printed circuit boards (PCBs)(e.g. PCBs  114  and  116  shown in exploded relationship with the heat spreader  100  in  FIG. 3 ) may be connected to the heat spreader  100  within and/or adjacent to the interior  112 , as described below. Although the present invention is described with respect to having two PCBs  114  and  116 , it is understood that the invention is not so limited, but may include any number of PCBs in stacked and/or side-by-side relationship (See  FIG. 6 ). According to the illustrated example of an embodiment of the present invention, the heat spreader  100  dissipates heat from the PCBs  114  and  116  and components (e.g. processors, power regulators, other integrated circuits, etc.) mounted on the PCBs  114  and  116 . In this manner, the heat generated by the components on the PCBs  114  and  116  is transferred to a single shared/common heat dissipation device (the heat spreader  100 ) for efficient transfer away from the heat sources and for enabling a denser packing of the PCBs  114  and  116  and the components on the PCBs  114  and  116  than is enabled by the techniques described in the background. If such background heat dissipation techniques were attempted to be used with the PCBs  114  and  116 , when the components are placed in relatively close arrangement and the PCBs  114  and  116  are stacked together in a relatively tight circuit board module, there would be little or no room for each individual heat dissipation device. 
   An additional benefit of some embodiments of the heat spreader  100  is that the heat spreader  100  can also serve to suppress electromagnetic interference (EMI) generated by the components during operation of the PCBs  114  and  116 , since the heat spreader  100  generally completely surrounds the PCBs  114  and  116  and at least some of the components thereon, as shown below, and since the heat spreader  100  is electrically conductive in some embodiments. In this embodiment, the heat spreader  100  is grounded to the PCBs  114  and/or  116 . The heat dissipation devices described in the background, on the other hand, are insufficient to suppress EMI, since such devices do not sufficiently surround the components and PCBs to which they are attached. 
   According to various embodiments, the interior side of the top section  102  is generally shaped or formed with a variety of features, such as, for example, projections/protrusions  118 , recesses/cavities  120  and/or holes/openings  122 . The size, shape and placement of the various features  118 - 122  are at least partly dependent on the size, shape and placement of the components on the PCBs  114  and  116  to which the heat spreader  100  may be attached, as described below. Each feature  118 - 122  generally corresponds to one or more of the components on the PCBs  114  and  116 . At least some of these components generally have heat dissipation requirements that cannot be satisfied by the techniques described in the background due to physical constraints in the layout of the components and the PCBs  114  and  116 . The heat spreader  100 , however, can satisfy the heat dissipation requirements of the components on the PCBs  114  and  116  because the heat spreader  100  contacts (or is in thermal conductive relationship with) these components due to the size, shape and placement of the various features  118 - 122 . 
   The heat spreader  100  generally conducts the heat generated by these components to the outer surfaces of the sections  102 - 110  where the heat can be dissipated to ambient air or can be further transferred away by a heat sink, heat pipe, cooling plate or other appropriate heat dissipation means. According to some embodiments, therefore, such heat dissipation means may be attached to the outer surface of the top section  102  or side sections  104 - 110 . 
   Upon being attached together, as shown in  FIG. 4 , the top PCB  114  is contained inside the heat spreader  100 , and the bottom PCB  116  contacts the bottom edges of the side sections  104 - 110  of the heat spreader  100 . The combined heat spreader  100  and PCBs  114  and  116  form a circuit board module  124  having multiple stacked PCBs and that may be incorporated in a larger system, such as a computer system or other appropriate electronic device. Connector pins  125  protruding from bottom PCB  116  ( FIGS. 3 and 5 ) may connect to such a computer system. 
   The top PCB  114  generally includes a board  126  and a variety of different-height components  128 ,  129  ( FIG. 5) and 130  ( FIG. 4 ) mounted on the board  126 . Although the top PCB  114  is shown having the components  128 - 130  only on one side, it is understood that the invention is not so limited, but that the top PCB  114  may also have components on the opposite side. Some of the components  128  and  129  are hidden from view by the heat spreader  100 . Other components  130  protrude into and are visible in some of the holes  122  in the top section  102  of the heat spreader  100 , because these components  130  are too tall for the top section  102  to cover. The top section  102  may be made thicker in order to cover these components  130  and, thereby, to enhance the EMI suppression benefits of the heat spreader  100 . There may, however, be height constraints that limit the allowable thickness of the top section  102 . The components  130  that may be exposed in this manner are generally those that do not have high heat dissipation requirements (i.e. consume relatively little power) and do not need to be in thermally conductive relationship with the heat spreader  100 . Otherwise, these components  130  would also have to be covered by the heat spreader  100  or would have to be attached to an additional alternative heat dissipation device through the holes  122 . Additionally, wires or cables  132  (e.g. power or communication lines) may connect to one of the PCBs (e.g.  114 ) through the heat spreader  100  (e.g. through side section  110 ). 
   For this example, it is assumed that the components  128  ( FIG. 5 ) do not require the heat spreader  100  to dissipate heat from these components  128  and do not need to touch the interior surfaces of the recesses  120 . The components  129 , however, have heat dissipation requirements (i.e. consume a relatively large amount of power) that require the heat spreader  100  to be in thermally conductive relationship to these components  129  to efficiently transfer heat away from the components  129  to the top surface of the top section  102 . In this case, therefore, the recesses  120  and/or other features on the interior side of the top section  102  are formed to generally match up with and contact the components  128 . Where distance tolerances for mounting the components  129  on the top PCB  114  leave some uncertainty in whether the interior surfaces of the recesses  120  will actually touch the desired components  129 , a gap filler (e.g. a thermal interface material, grease, etc.) may be placed between the interior surfaces of the recesses  120  and the components  129 . Heat is thus transferred from the components  129  to the bottom surfaces of the recesses  120  and through the top section  102  of the heat spreader  100 . Additional heat is transferred from the components  128 ,  129  and  130  through the board  126  to the side sections  104 - 110  of the heat spreader  100 . 
   The bottom PCB  116  generally includes a board  134  and a variety of different-height components  136  and  138  ( FIG. 5 ) mounted on the board  134 . Although the bottom PCB  116  is shown having the components  136  and  138  only on one side, it is understood that the invention is not so limited, but that the bottom PCB  116  may also have components on the opposite side. In this example, it is assumed that the components  138  generally require a heat dissipation means, but that the components  136  do not necessarily require a heat dissipation means. The top PCB  114  generally has one or more holes  140  in the board  126  through which the projections  118  on the interior side of the top section  102  of the heat spreader  100  extend to the components  138  on the board  134 . In this manner, the components  138  of the bottom PCB  116  can be in thermally conductive relationship with the heat spreader  100  through the top PCB  114 . A heat dissipation means separate from the heat spreader  100 , therefore, is not needed for the components  138  of the bottom PCB  116 . Instead, most of the heat generated by the components  138  is transferred to the heat spreader  100  at the projections  118  bypassing the top PCB  114  through the holes  140  to the top section  102  of the heat spreader  100 . Additionally, where distance tolerances for mounting the components  138  on the bottom PCB  114  leave some uncertainty in whether the bottom surfaces of the projections  118  will actually touch the desired components  138 , a gap filler (e.g. a thermal interface material) may be placed between the projections  118  and the components  138  to fill any gap and enhance thermal conductivity between the projections  118  and the components  138 . Furthermore, a mechanical gap filler, such as that described in U.S. Pat. No. 6,771,507 (the disclosure of which is incorporated as if fully set forth herein) with respect to “thermally-conductive pins,” may be used when the thermal interface material alone is insufficient to fill the gap between the projections  118  and the components  138 . Additional heat is transferred from the components  136  and  138  through the board  134  to the side sections  104 - 110  of the heat spreader  100 . 
   The bottom edges of the side sections  104 - 110  of the heat spreader  100  may rest on the top surface of the board  134  of the bottom PCB  116 , as shown in  FIG. 5 . Alternatively, as illustrated by a circuit board module  124 ′ having a heat spreader  100 ′, a top PCB  114 ′ and a bottom PCB  116 ′ as shown in  FIG. 6 , the bottom edges of side sections (e.g.  106 ′) of the heat spreader  100 ′ may surround the peripheral edges of a board  134 ′ of the bottom PCB  116 ′. In this alternative, therefore, both PCBs  114 ′ and  116 ′ are contained in an interior of the heat spreader  100 ′ defined by a top section  102 ′, side section  106 ′ and other side sections (not shown).  FIG. 6  also shows another alternative embodiment which includes a third PCB  115 ′ connected in a stacked configuration with and between the top and bottom PCBs  114 ′ and  116 ′ and contained within the interior of the heat snreader  100 ′. In each embodiment, since the heat spreader  100  (or  100 ′) surrounds the components  128 ,  129 ,  136  and  138  of the PCBs  114  and  116  (or  114 ′.  115 ′ and  116 ′), the heat spreader  100  (or  100 ′) suppresses EMI from the components  128 ,  129 , 136  and  138  during operation of the PCBs  114  and  116  (or  114 ′.  115 ′ and  116 ′). 
   According to an alternative embodiment, a heat spreader  142 , as shown in  FIG. 7 , includes heat dissipation fins  144  protruding from and integral with a top section  146 . Although the heat dissipation fins  144  are shown as straight fins, the heat dissipation fins  144  may be of any appropriate type or geometry like pin fins, angled, any fin known in the art. [Mention folded, airfoil, pin, square, any geometry elsewhere.] Other elements and features of the heat spreader  142  may be similar to those of the heat spreader  100  ( FIGS. 1-5 ). Thus, the heat spreader  142  may be used with the PCBs  114  and  116  in the same manner as is the heat spreader  100 , as described above, to form a circuit board module in which the heat spreader  142  transfers heat away from the heat-generating components of the PCBs  114  and  116 . The heat dissipation fins  144  may be exposed through the holes  122 , so the heat dissipation fins  144  may be in thermally conductive relationship with the exposed components  130  ( FIG. 4 ) in order to transfer heat away from these components  130 . Alternatively, if the top section  146  were made thicker in order to cover the components  130 , then the heat dissipation fins  144  would not be exposed through the holes  122  and the EMI suppression benefits of the heat spreader  142  would be enhanced. However, minimum height constraints for the heat dissipation fins  144  and maximum available space in the overall computer system may constrain the thickness of the top section  146 , making the holes  122  necessary. As a single integral device, the heat spreader  142  has an enhanced thermal performance over the heat spreader  100 , which may have a separate heat sink attached to the top section  102  ( FIGS. 1-5 ). The heat spreader  100 , however, has a greater flexibility of usage, since any appropriate heat dissipation means (e.g. a heat sink, a heat pipe, a liquid cooled plate, etc.) may be attached to the top section  102 . 
   The heat spreaders  100  and  142  may be formed in any appropriate manner. For example, the heat spreaders  100  and  142  may be cast, molded, cold forged, stamped, extruded, etc. Additionally, some of the features, such as recesses, hollowed out areas, openings or holes, on the heat spreaders  100  and  142  may be machined out of a single block of material having an initial size of approximately the general outer rectangular dimensions of the heat spreaders  100  and  142 . Other features, such as projections, extensions or mechanical gap filler devices, may be attached to or mounted onto surfaces of the heat spreaders  100  and  142 .