Abstract:
A heat sink is provided that has a leg and a member coupled substantially perpendicular to the leg at a first end of the leg. The leg is surface mountable to a first surface of a printed circuit board at a second end of the leg to receive heat from an electronic device on a second surface of the printed circuit board by a thermally conductive via through the printed circuit board.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This Application is a Continuation of U.S. application Ser. No. 09/841,087 filed Apr. 24, 2001 (allowed), which is a Divisional of U.S. application Ser. No. 09/597,535, filed Jun. 20, 2000 and issued as U.S. Pat. No. 6,249,434 on Jun. 19, 2001. 
     
    
     
       TECHINICAL FIELD  
         [0002]    The present invention is related to heat sinks for printed circuit boards, and more particularly to a system and method of transferring heat from electronic devices using a surface mounted conduction heat sink.  
         BACKGROUND  
         [0003]    Circuit boards are becoming more and more densely packed with electronic devices as the need for power, memory and additional capabilities increases. As a result the heat generated by these electronic devices also increases. The heat that is generated by an electronic device must be transferred away from the electronic device or the performance of the electronic device(s) will deteriorate. The problem of dissipating heat from electronic devices mounted on circuit boards gets worse when the piece of equipment employing the circuit board is mounted within a housing. For example, fiber optic transmitter/receiver modules that include circuit boards are often environmentally sealed in a housing to prevent damage from the elements. As a result, the ability to dissipate heat from the electronic devices mounted on the circuit boards becomes challenging. The environmental housing&#39;s only mode of heat transfer to the ambient is natural convection. The transmitter/receiver module&#39;s only mode of heat transfer to the environmental housing is conduction.  
           [0004]    A classic thermal management technique uses thermal vias to dissipate heat. The thermal vias are located in a copper pad of a printed circuit board, under the device that needs cooling. A heat sink pad of a device is soldered to the copper pad of the circuit board. During operation of the device, heat is transferred from the device&#39;s heat sink pad to the copper pad on the primary side of the circuit board. The heat is then transferred through the thermal vias to the far (secondary) side of the circuit board and to the housing of the transmitter/receiver module. The heat is then transferred by conduction from the transmitter/receiver module to the unit&#39;s environmental housing. As the component density in an electronic module increases, it becomes difficult to use classical thermal vias to dissipate heat.  
           [0005]    For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for  
         SUMMARY  
         [0006]    The above-mentioned problems with meeting the thermal requirements of electronic devices mounted on circuit boards are addressed by the present invention and will be understood by reading and studying the following specification. Embodiments of the present invention provide heat sinks for facilitating meeting the thermal requirements of electronic devices mounted on circuit boards.  
           [0007]    More particularly, in one embodiment, a heat sink is provided that has a leg and a member coupled substantially perpendicular to the leg at a first end of the leg. The leg is surface mountable to a first surface of a printed circuit board at a second end of the leg to receive heat from an electronic device on a second surface of the printed circuit board by a thermally conductive via through the printed circuit board.  
           [0008]    In another embodiment, a heat sink is provided that has a first U-shaped portion, a second U-shaped portion, and a pair of thermally conductive vias. The first U-shaped portion is mountable on a first surface of a printed circuit board for straddling an electronic device on the first surface of the printed circuit board. Further, the first U-shaped portion has an interior surface for thermally coupling to an outer surface of the electronic device. The second U-shaped portion is mountable on a second surface of the printed circuit board. The pair of thermally conductive vias is for passing between the first and second surfaces of the printed circuit board. Moreover, each of the pair of thermally conductive vias is for respectively thermally coupling each of first and second legs of the “U” of the second U-shaped portion to the electronic device.  
           [0009]    In yet another embodiment, a method of removing heat from an electronic device is provided. The method includes transferring the heat from an electronic device mounted on a first surface of a printed circuit board to a first heat transfer pad of the electronic device. The method includes transferring the heat from the first heat transfer pad to a second heat transfer pad coupled to the first heat transfer pad, where the second heat transfer pad is on the first surface of a printed circuit board. Moreover, the method includes transferring the heat from the second heat transfer pad to a heat sink that is mounted on a second surface of the printed circuit board and that straddles a component on the second surface using a thermally conductive via that passes through the printed circuit board and that thermally couples the second heat transfer pad to the heat sink. The method also includes transferring the heat from the heat sink to an outer housing that encloses the printed circuit board.  
           [0010]    Other embodiments are described and claimed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is an illustration of one embodiment of a heat sink according to the teachings of the present invention.  
         [0012]    [0012]FIG. 2 is an illustration of another embodiment of a heat sink according to the teachings of the present invention.  
         [0013]    [0013]FIG. 3 is an illustration of a top and a side view of an embodiment of a heat transfer pad and heat sink mounting pad according to the teachings of the present invention.  
         [0014]    [0014]FIG. 4 is an illustration of a top and a side view of an embodiment of a heat transfer pad and solder mask according to the teachings of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0015]    In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense.  
         [0016]    Since the component density is so high in the new circuit board designs, it is physically impossible to directly contact the far (secondary) side of the printed circuit board with the housing of the device modules as stated in the classic technique. This invention overcomes this obstacle by providing alternate conduction/cooling paths from the electrical device to the housing.  
         [0017]    [0017]FIG. 1 is an illustration of one embodiment of a heat sink mounted on a circuit board shown generally at  100  and constructed according to the teachings of the present invention. In this embodiment, an electronic device  110  is mounted on a printed circuit board  111 . The printed circuit board  111  has a primary surface  112  and a secondary surface  114 . In one embodiment, the electronic device  110  is mounted onto a heat transfer pad  150  on the surface of the printed circuit board  111 . In one embodiment, electronic device  110  includes a heat sink pad or heat transfer pad  161  which couples to the heat sink pad  150  on the printed circuit board  111 . The electronic device  110  is coupled to heat sink  140  via heat sinking mounting pads  154  thermally coupled to the heat sink pad  150  on the printed circuit board  111 . Heat transfer pad  150  on the surface of printed circuit board  111  and heat transfer pad  161  of electronic device  110  are illustrated as being approximately the same width and length as electronic device  110 . It is understood that the dimensions of heat transfer pads  150  and  161  are not limited to the dimensions of the electronic device such as  110  that couples to them. The heat transfer pads  150  and  161  may comprise one or more pads larger or smaller than the dimensions of the electronic device  110 .  
         [0018]    [0018]FIG. 3 is an illustration of one embodiment of a top view  300  and a side view  301  of a heat transfer pad  350  and a heat sink mounting pad  354 - 1  as discussed above. Heat transfer pad  350  and heat sink mounting pad  354 - 1  are on the top or primary surface  312  of printed circuit board  311 . In one embodiment, heat transfer pad  350  is coupled to heat sink mounting pad  354 - 1  by thermally conductive leads  352 . Leads  352  are for illustration and may comprise one or more leads of varying sizes. For example, in one embodiment, heat transfer pad  350  and heat sink mounting pad  354 - 1  are one continuous pad. In another embodiment, heat transfer pad  350  includes two heat sink mounting pads  354 - 1  and  354 - 2  having one or more interconnecting leads such as  352 . FIG. 3 illustrates that the heat sink mounting pads  354  are located on opposite sides of heat transfer pad  350 . It is understood by one skilled in the art that the heat sink mounting pads  354  are not restricted to the size, shape or location illustrated but may include any number of pads of any shape or location about the heat transfer pad  350 .  
         [0019]    [0019]FIG. 4 is another illustration of one embodiment of a top view  400  and a side view  401  of a heat transfer pad and heat sink mounting pad as a single pad  450  with a solder mask  429 - 1 . Solder mask  429 - 1  is located on top of pad  450  separating heat transfer pad portion  459  from heat sink mounting pad portion  454 - 1 . Heat transfer pad  450  and solder mask  429 - 1  are on the top or primary surface  412  of printed circuit board  411 . FIG. 4 illustrates a single pad  450  for mounting an electronic device on heat transfer pad portion  459  and mounting a heat sink on heat sink mounting pad portion  454 - 1 . As a result once soldered to pad  459  an electronic device is thermally coupled to a heat sink via single combination heat transfer pad and heat sink mounting pad  450 . In an alternate embodiment, solder masks  429 - 1  and  429 - 2  are placed on pad  450  so as to form two heat sink mounting pads  454 - 1  and  454 - 2  and a single heat transfer pad  459 . The use of solder masks in FIG. 4 is by way of illustration only and the method of separating the electronic device from the heat sink on a printed circuit board may include any suitable method such barriers, spacers or the like. The electronic devices, heat sinks, and other components are not limited to soldering but may be attached to the printed circuit board or heat sink pad via surface mounting, soldering using solder balls or the like, thermally and electrically conductive adhesives or other suitable materials having appropriate thermal and electrical properties. It is understood by one skilled in the art that the mounting pads discussed with respect to FIGS. 3 and 4, although shown on top of the printed circuit board, are not restricted to being located on top of the board. For example, the mounting pads may be part of the printed circuit board surface, below the printed board circuit surface or on top of another layer or surface on the printed circuit board.  
         [0020]    [0020]FIG. 3 illustrates heat sink mounting pads  354  located on opposite sides of heat transfer pad  350  and FIG. 4 illustrates solder masks  454  for separating heat sinks mounted on opposite sides of heat transfer pad portion  459 . It is understood by one skilled in the art that the heat sink mounting pads  354  and masks  454  are not restricted to the size, shape or location illustrated but may include any number of pads of any shape or location adjacent to and thermally coupled to heat transfer pads  350  and  450 . It is also understood that the heat transfer pads  350  and  450  are not restricted to one single pad, in alternate embodiments, pads  350  and  450  comprise two or more smaller pads that are thermally interconnected to provide a path to draw heat from the electronic device.  
         [0021]    Heat sink mounting pads  354  and heat transfer pad  350  and  450  are made of a thermally conductive material such as copper. For an alternate embodiment of the present invention, the pads and interconnects are formed from other suitable materials having appropriate thermal and electrical properties such as, for example, aluminum, gold, silver, or other metal. In addition, to improve the thermal conductive properties of the pads  350 ,  354  and  450 , the pads may include a thermal insert layer such as T-PLI™, T-FORM™, CHO-THERM™, or thermally and electrically conductive adhesives or pastes or other suitable materials having appropriate thermal and electrical properties. T-PLI™ and T-FORM™ are thermal interface materials manufactured by Thermagon, Inc.,  4707  Detroit St., Cleveland, Ohio 44109. CHO-THERMTM is a thermal interface material manufactured by Chomerics, a division of Parker Hannifin,  77  Dragon Court, Woburn, Mass. 01888.  
         [0022]    In operation, an electronic device such as  110  of FIG. 1, heats up. The heat is transferred to a heat transfer pad such as  350  and to heat sink  140  via heat sink mounting pads such as  354 . In another embodiment, the heat is transferred to heat sink  140  via a heat transfer pad such as  450 . Heat sink  140  conducts the heat away from electronic device  110 . In one embodiment, the printed circuit board  111  is mounted into a housing  130  and heat sink  140  is thermally coupled to housing  130 . In this embodiment, heat sink  140  conducts the heat to housing  130  that in turn disperses the heat to the ambient air. In another embodiment, heat sink  140  straddles the electronic device  110  and is thermally coupled with the top of the electronic device  110  and draws heat from the surface of the electronic device  110 . In one embodiment, heat sink  140  includes a thermally conductive interface layer  120  between the electronic device  110  and the heat sink  140 . The thermal interface layer  120  consists of a thermal interface material such as T-PLI™, T-FORM™, CHO-THERM™, or the like. In another embodiment, the heat sink  140  is coupled to an outer housing  130  that includes an inner layer of a thermally conductive material  122  such as T-PLI™, T-FORM™, CHO-THERM™, or the like. In an alternate embodiment, heat sink  140  includes a layer of thermally conductive material such as T-PLI™, T-FORM™, CHO-THERM™, or the like.  
         [0023]    In one embodiment, printed circuit board  111  is part of a fiber optic transmitter/receiver module within housing  130 . In another embodiment, the fiber optic transmitter/receiver module is environmentally sealed within a second housing  155 . In one embodiment, the transmitter/receiver module housing  130  is thermally coupled to environmentally sealed housing  155 . In another embodiment, environmentally sealed housing  155  includes an inner layer of a thermally conductive material between the transmitter/receiver module housing  130  and the environmentally sealed housing  155 . The thermally conductive material comprises a material such as T-PLI™, T-FORM™, CHO-THERM™, or the like.  
         [0024]    [0024]FIG. 2 is an illustration of another embodiment of a heat sink mounted on a circuit board, shown generally at  200 , and constructed according to the teachings of the present invention. A circuit board  211  has a primary surface  212  and a secondary surface  214 . In this embodiment, an electronic device  210  is mounted to the primary surface  212  of circuit board  211 . In one embodiment, electronic device  210  includes a heat transfer pad  261  coupled to the bottom of electronic device  210 . The heat transfer pad  261  draws heat from the electronic device  210 . In one embodiment, electronic device  210  is mounted to a heat transfer pad  250  that is on the primary surface  212  of the circuit board  211  and aids in drawing heat from electronic device  210 . A component  260  is mounted to the secondary surface  214  of circuit board  211 . Component  260  may be an electronic device, an enclosure for an electronic device, a spacer or any other component mounted on the printed circuit board. A heat sink  240  is coupled to the secondary surface  214  of circuit board  211 . In one embodiment, circuit board  211  includes thermal vias  280  which are formed through circuit board  211  and aid in conducting heat from electronic device  210  on the primary side  212  through the printed circuit  211  to heat sink  240  on the secondary side  214 . Electronic device  210  on the primary side is thermally coupled to heat sink  240  on the secondary side by vias  280 . Although only two vias  280  are illustrated with respect to FIG. 2, it is understood that any number of vias as appropriate may be utilized to aid in conducting heat from electronic devices such as  210  mounted on the circuit board  211 . In one embodiment, heat sink  240  includes thermally conductive heat sink mounting pads  264  that aid in conducting heat from electronic device  210  through vias  280 .  
         [0025]    A thermal via is a thermally conductive conduit through which heat is conducted. In one embodiment, thermal vias  280  are formed by drilling holes through printed circuit board  211  and filling the hole with a thermally conductive material such as copper. In alternate embodiments, thermal vias  280  are formed by drilling, routing, punching holes or slots, or the like in the printed circuit board  211  and filling with an appropriate thermally conductive material.  
         [0026]    In operation, electronic component  210  heats up, this heat is transferred to heat sink  240  by thermal vias  280 . In one embodiment, the heat is transferred to a heat sink pad such as  350  or  450  of FIGS. 3 and 4, respectively, and then transferred to heat sink  240  by thermal vias  280 . In one embodiment, printed circuit board  211  is enclosed in a housing  230  and heat sink  240  is thermally coupled to housing  230 . The heat is conducted from heat sink  240  to enclosure  230  for dispersion to the ambient air. In an alternate embodiment, enclosure  230  includes a layer of thermally conductive material  220 , such as T-PLI™, T-FORM™, CHO-THERM™, that is thermally coupled to heat sink  240  and aids in the conduction of heat from the heat sink  240  to the housing  230 .  
         [0027]    In one embodiment, printed circuit board  211  is part of a fiber optic transmitter/receiver module within housing  230 . In another embodiment, the fiber optic transmitter/receiver module is environmentally sealed within a second housing  255 . In one embodiment, the transmitter/receiver module housing  230  is thermally coupled to environmentally sealed housing  255 . In another embodiment, environmentally sealed housing  255  includes an inner layer of a thermally conductive material between the transmitter/receiver module housing  230  and the environmentally sealed housing  255 . The thermally conductive material comprises a material such as T-PLI™, T-FORM™, CHO-THERM™, or the like.  
         [0028]    In another embodiment, both the heat sink  140  described with respect to FIG. 1 and the heat sink  240  described with respect to FIG. 2 are employed on one circuit board to draw heat from electronic devices. Heat sinks  140  and  240  may be employed in any combination for one or more electronic components. It is understood that components such as electronic device  110  and  210 , heat sink  140  and  240 , component  260  may be coupled to the printed circuit in a number of ways to include surface mounting, soldering using solder balls or the like, thermally and electrically conductive adhesives or any other suitable materials having appropriate thermal and electrical properties.  
         [0029]    In one embodiment, the heat sinks described are made of a thermally conductive material such as copper. For an alternate embodiment of the present invention, the heat sinks are formed from other suitable materials having appropriate thermal and electrical properties such as, for example, aluminum, gold, silver, or other metal.  
         [0030]    The electronic devices, heat sinks, and other components mounted on the circuit board are not limited to soldering but may be attached to the printed circuit board or heat sink pads via surface mounting, soldering using solder balls or the like, thermally and electrically conductive adhesives or other suitable materials having appropriate thermal and electrical properties.  
       CONCLUSION  
       [0031]    Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. For example, the heat sink in one embodiment is a “U” shape and straddles the electronic device. In further embodiments, the heat sink is “L” shaped and is thermally coupled to the top of the electronic device from one side. In other embodiments, the heat sink has four legs and straddles the electronic device. Each of the legs thermally couples to the electronic device. It is understood that the heat sink can be one of a variety of shapes that thermally couple to one or more electronic devices. In addition, in one embodiment the heat sink is employed in a fiber optic transmitter/receiver module that is environmentally sealed in a housing in other embodiments, the heat sink is employed in housings that are not environmentally sealed and in applications other than fiber optic transmitter/receiver modules. Further, in one embodiment the heat sink aids in drawing heat away from an electronic device, in other embodiments, the heat sink aids in drawing heat away from one or more electronic devices. For example, a heat sink is thermally coupled to one or more electronic devices, heat transfer pads and/or thermal vias or any combination of electronic devices, heat transfer pads and thermal vias to aid in drawing heat away from one or more electronic devices.