Abstract:
A cooling-assisted, heat-generating electrical component reduces heat generated during operation. The electrical component, typically a resistor, has a least one heat-removing element electrically associated therewith. Heat-removing element comprises a plurality of thermal vias therein and is fixedly attached to an end portion of the electrical component, typically on opposed end portions. Heat absorbed by the heat-removing element can be transported away from the electrical component during service.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application is related to U.S. application Ser. No. ______(Docket 84843), filed ______, by Tina P. Barcley, and entitled, “Insitu-Cooled Electrical Assemblage And Method Of Manufacturing Same;” and U.S. application Ser. No. ______(Docket 84844), filed ______, by Tina P. Barcley, and entitled, “Method of Cooling Heat-Generating Electrical Components.” 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to improvements in heat transfer in electronic technology. More particularly, the invention concerns electrical assemblages, such as circuit boards, which reduce heat generated in ceramic “chip” components during service.  
       BACKGROUND OF THE INVENTION  
       [0003]     It is well known that overheating of electrical components, for instance components mounted on a circuit board, contributes to reducing the life of the component. Overall component reliability is also compromised by excessive overheating during service. As circuit boards become denser and contain components that have higher power output capacity, reliability problems become magnified for solder joints of the circuit board, particularly in “chip” ceramic type parts due to CTE (Coefficient of Thermal Expansion) differences.  
         [0004]     The current trend in the industry is to either assume the reliability risk or to institute extremely expensive chip construction processes for removing excess heat. Both practices have proven undesirable from a cost management standpoint.  
         [0005]     In a typical electronic package, the largest chip ceramic component, for instance the 2512 resistor, is the least reliable component on most circuit boards and the component most likely to fracture its solder joint due to thermal cycling. The thermal cycling occurs every time the component is used. It would, therefore, be desirable to cool this component during service to dramatically improve its useful life and reliability.  
         [0006]     Therefore, a need persists in the art for a system and method of cooling electrical components, such as those mounted in circuit boards, during service that produces a component that is reliable, cost effective and has an extended useful life.  
       SUMMARY OF THE INVENTION  
       [0007]     It is, therefore, an object of the present invention to provide an electrical component protected from deterioration due to overheating during service.  
         [0008]     Another object of the invention is to provide such an electrical component that is considerably more reliable, has a longer useful life and is cost effective to build and use.  
         [0009]     The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, an electrical component has a heat-generating main portion having a junction temperature T j  and at least one mating surface. Means for reducing the junction temperature T j  of the heat-generating main portion has at least one heat-removing element. The heat-removing element comprises a plurality of arranged thermal vias and is electrically associated with the mating surface to form a thermal transport path for heat flow away from said heat-generating main portion.  
         [0010]     In another aspect of the invention, a method of manufacturing a cooling assisted, heat-generating electrical component, comprising the steps of providing a heat-generating electrical component having at least one mating portion. A heat-removing element is affixed to the mating portion of the heat-generating electrical component such that heat removed therefrom follows a path from the heat-generating electrical component through the heat-removing element.  
         [0011]     The present invention has numerous advantages over existing developments including reliability, cost effectiveness, ease of development, and increased component useful life. Moreover, the present invention has the advantage of being applicable to practically any ceramic chip, ball grid array (BGA), direct die attached components, chip-on-board, and smaller chip resistors, such as the 2010, 1206, etc. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The above and other objects, features, and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical features that are common to the figures, and wherein:  
         [0013]      FIG. 1  is an elevated plan view of an electrical component having the heat-removing element of the invention;  
         [0014]      FIG. 2  is an elevated front plan view of an electrical component having alternative heat-removing elements thereon;  
         [0015]      FIG. 3  is a cross-sectional view of a typical circuit board construction having the heat-removing element of the invention thereon; and,  
         [0016]      FIG. 4  is a plan view of a solder stencil used for automated manufacturing of heat-removing element and thermal vias. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     Turning now to the drawings, and in particular to  FIGS. 1 and 2 , the electrical component  10  of the invention is illustrated. According to  FIG. 1 , electrical component  10 , for instance a resistor or capacitor, has mating portions  12 ,  14  and a heat-removing element  16  attached to either of mating portions  12 ,  14 . Electrical component  10  is preferably made of a ceramic or ceramic-like material for suitability of die usage or affixing other electrical components (multi-chip module). Although soldering is the preferred method of attaching heat-removing element  16  to either of mating portions  12 ,  14 , skilled artisans will appreciate that other methods may be used including using electrically conductive adhesives. Mating portions  12 ,  14  are typically gold or palladium tipped for best thermal conductivity and solderability. Skilled artisans will appreciate that, alternatively, the mating portions  12 ,  14  may comprise other thermally conductive materials such as tin-lead.  
         [0018]     According to  FIG. 1 , heat-removing element  16  has a plurality of thermal vias  18  that provides the mechanism for effectively removing heat from the electrical component  10 . The thermal conductivity of thermal vias  18  is enhanced as possible with the addition of solder or some thermal compound either partially or totally filling the inner walls (not shown) of thermal vias  18 . Thermal vias  18  may have practically any size diameter. A preferred range for the diameter is about 0.022 inches to about 0.040 inches. However, a diameter in the range of about 0.022 inches to about 0.028 inches is more preferred.  
         [0019]     As shown in  FIG. 3 , skilled artisans will appreciate that thermal vias  18  may be electrically connected to a high density copper plane  20  to increase the thermal conductivity of the vias  18 . Therefore, practically any high density copper circuit or collection of signals can be used to increase the thermal conductivity of the vias  18 . The maximum effect on thermal conductivity is realized when the thermal vias  18  are connected to a large copper area, such as high-density copper plane  20 . Typically, large copper areas are generally used for grounds and power circuits. This construction would also include a bonded heat sink  40 , as described below. Use of a thermal conductive bonding adhesive or thermally conductive compound will effectively maximize the heat transfer to the heat sink.  
         [0020]     Referring to  FIG. 3 , an electrical assemblage  30  is illustrated. According to  FIG. 3 , electrical assemblage  30  includes a main portion, or circuit board  32 , and at least one heat-generating component  34 , each with the heat-removing elements  16  of the invention. Heat-removing element  16 , as described above, has a plurality of thermal vias  18  for transporting heat away from heat-generating component  34 . Heat-generating component  34  has a junction temperature T j . To reduce the junction temperature T j  of the heat-generating component  34 , the thermal vias  18  transport the heat generated by component  34  to the bottom outer layer  38  of circuit board  32 . The heat is then transported from the bottom outer layer  38  to the adjoining heat sink  40 . Heat sink  40  may be adjoined to bottom outer layer  38  with, for instance, a thermally conductive adhesive material. In this way, the junction temperature T j  of the heat-generating component  34  is reduced to a temperature T 1 , wherein T 1  is less than T j .  
         [0021]     As an example, a 10-layer polyimide circuit board  32  may comprise ½ oz. copper on the inner layers  35  and 1 oz. copper on the outer layers  37  (top),  38  (bottom). The heat-removing element  16  of the invention can be used with any material circuit board since the primary heat transfer mechanism is conduction through the thermal vias  18 . Moreover, heat-removing element  16  can be used on any number of layers or copper weight available from any circuit board manufacturer. Current industry standards for circuit board plating include Pd, NiAu, Immersion Tin, Immersion Ag, and HASL for plating types. The specific board material was polyimide, but any board material where vias can be drilled could use this invention (FR2, FR4, CEM, Rogers, etc.). For the electrical assemblage  30  of the invention, a particular pattern of thermal vias  18  is used on the heat-removing element  16 . The thermal vias  18  are preferably about 0.025 inches in diameter with a grid spacing of about 0.040 inches center-to-center for the hole pattern (see for instance,  FIGS. 1 and 2 ). This hole pattern provides a specific cross-sectional area of copper in the circumference of the thermal vias  18  to conduct the heat from the top outer layer  37  of the circuit board  32  to the bottom outer layer  38 . The copper of heat-removing element  16  is used to transport heat across the surface of the circuit board  32 . Additionally, the similar heat-removing elements on the bottom outer layer  38  of the circuit board  32  enhance the transport of the heat from the thermal vias  18  to the heat sink  40 .  
         [0022]     Referring again to  FIGS. 1 and 2 , heat-removing element  16  may have practically any geometric configuration. In  FIG. 1 , heat-removing element  16  is substantially rectangular shaped. As shown in  FIG. 2 , alternatively, heat-removing element  16  may be substantially T-shaped. In this latter design the T-shaped pads are used to separate the thermal vias  18  from mating surfaces  12 ,  14  to improve solderability and manufacturability. Typically one would stay away from the actual solder joint (not shown), i.e., the interface of the mating surface  12 ,  14  and the heat-removing element  16  of the electrical component  10  in order to maintain solder joint integrity. Separation of thermal vias  18  from the mating surfaces  12 ,  14  may be accomplished by use of webs  22 , typically comprising copper, on circuit board  32 . Any number of webs  22  may be used depending on the application.  
         [0023]     Referring to  FIG. 3 , electrical assemblage  30  of the invention has an aluminum heat sink  40  that is 0.090 inches thick. The heat sink  40  was attached via adhesive layer  39  to the circuit board  32  using an Arlon Thermobond® silicone, thermally conductive, electrically isolative adhesive film. Any metal heat sink  40  could have been used of any thickness, preferably between 0.060 inches and 0.090 inches. Additionally, any thermally conductive compound or adhesive (including epoxies and liquid silicones) could be used with substantially similar results. Those skilled in the art will appreciate that thermally conductive, electrically conductive adhesives may be used as a substitute for thermally conductive compounds.  
         [0024]     Turning now to  FIG. 4 , to construct electrical assemblage  30  of the invention, a solder stencil  50  may be used to automatically apply solder paste to the circuit board  32  in high volume electronic applications. Solder stencil  50  has at least one cut-out portion  52  (two are shown) which provides a path for the solder paste to be applied to the heat-removing elements  16  of circuit board  32 . Although this process may be accomplished manually, solder stenciling has the advantage of consistency and ease of use. The solder stencil  50  has standard support features that reduces sagging in the vicinity of the cut-out portions  52 . The amount of solder paste applied to the heat-removing elements  16  can be increased by increasing the size of the cut-out portions  52 . The increased amount of solder paste associated with the increased sizes of the cut-out portions  52  will accommodate the volume of paste that eventually fills the thermal vias  18 .  
         [0025]     The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention.  
       PARTS LIST  
       [0000]    
       
           10  electrical component  
           12 ,  14  mating portion of  10   
           16  heat-removing element  
           18  thermal vias  
           20  high density copper plane  
           22  webs of heat-removing element  16   
           30  electrical assemblage  
           32  circuit board  
           34  heat-generating component  
           35  inner layers of circuit board  32   
           37  top outer layer of circuit board  32   
           38  bottom outer layer of circuit board  32   
           39  adhesive layer  
           40  heat sink  
           50  solder stencil  
           52  cut-out portion of solder stencil  50