Abstract:
A method removes heat from a densely packed electronic assemblage. Densely packed electronic assemblage has a substrate medium for supporting at least one heat generating component and means for reducing the temperature of the at least one heat generating component. A heat sink cooperates with the heat removing element for reducing heat of the heat generating component by absorbing heat from the heat generating component.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application is related to U.S. application Ser. No. ______, filed ______, by Tina P. Barcley, and titled, “Electrical Assemblage And Method For Removing Heat Locally Generated Therefrom;” U.S. application Ser. No. ______, filed ______, by Tina P. Barcley, and titled, “Remote Thermal Vias For Densely Packed Electrical Assemblage;” and U.S. application Ser. No. ______, filed ______, by Tina P. Barcley, and titled, “Densely Packed Electronic Assemblage With Heat Removing Element.” 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates generally to the field of broad electronic technology (BET), and in particular to thermal management of highly dense circuit boards. More specifically, the invention relates to a method of reducing the heat of heat-generating electronic components during service.  
         BACKGROUND OF THE INVENTION  
         [0003]    It is well known in the field of electronic technology that overheating electronic components, such as transistors, capacitors, etc., contributes to reducing the life of the component or module as well as the overall reliability while in service. As electrical assemblages or products containing such components become denser and contain components that have higher wattages per square area, component overheating becomes a larger problem as well as a limiting factor in the reliability of the electrical assemblage. Thus, eliminating or substantially reducing the heat from such components during service must be accomplished before product reliability can be greatly improved. The performance and reliability of commercial electronic products are simply limited by the inability of the products to dissipate heat generated by densely packed electrical components.  
           [0004]    Prior art attempts to address the aforementioned problem has resulted in varying degrees of success. In the area of commercial electronics, such as computer electronics, the most common solution is to utilize costly component constructions. In the automotive electronic area, costly circuit board materials are generally used to reduce component overheating.  
           [0005]    Therefore, there persists a need in the art for a densely packed electronic assemblage that operates at considerably cooler junction and board temperatures while permitting more and more components and electrical traces. Further, there is a need for a cost effective method of reducing the heat generated by hot components in electronic assemblages under high and ordinary service loads.  
         SUMMARY OF THE INVENTION  
         [0006]    It is, therefore, an object of the invention to provide a densely packed electronic assemblage that is more reliable due to lower operating temperatures.  
           [0007]    It is a feature of the invention that an electronic assemblage has a heat-removing element associated with a heat-generating element for reducing the junction temperature of the heat-generating component by means of a plurality of thermal vias.  
           [0008]    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, a method of removing heat from a densely packed electronic assemblage comprises a densely packed electronic assemblage comprising a substrate medium for supporting at least one heat generating component thereon. The heat-generating component has a characteristic junction temperature T j . A first heat removing element having a plurality of thermal vias is thermally associated with the heat generating component and reduces the junction temperature T j  of the heat generating component. The heat-removing element is spaced apart from the heat-generating component so as to produce an open space nearest to the heat-generating component for accommodating high density electrical layouts. A heat sink in fluid communications with the first heat removing element absorbs heat from the heat generating component and thereby reduces its junction temperature T j  to a temperature T 1 , wherein T 1  is less than T j .  
           [0009]    The present invention has numerous advantages over prior art developments. More particularly, the densely packed electronic assemblage of the invention operates at lower temperatures and is, therefore, more reliable. Further, the electronic assemblage can allocate space in proximity to the heat-generating element for specific electrical traces essential for the design layout. Furthermore, the electronic assemblage of the invention may contain a larger number of standard and thinner width size components. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    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:  
         [0011]    [0011]FIG. 1 is a plane view of a portion of an electronic assemblage in accordance with the invention;  
         [0012]    [0012]FIG. 2 is a cross-sectional view of an electronic assemblage in accordance with the invention;  
         [0013]    [0013]FIG. 3 is an enlarged partial sectional view of an electronic assemblage of the invention; and,  
         [0014]    [0014]FIG. 4 is a cross-sectional view of a multi-layer circuit board containing a first and a fourth heat removing element. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    Turning now to the drawings, and in particular to FIGS. 1 and 2, the densely packed electronic assemblage  10  of the invention is illustrated. Broadly defined, electronic assemblage  10  of the invention has a substrate medium  12 , or typically a circuit board, having mounted thereon at least one heat generating components  14 ,  30 . Heat generating component  14  may include any number of components that generate heat during operations or under load, such as resistors, diodes, transistors, processors, etc. Skilled artisans will appreciate that each heat generating component  14  has a characteristic junction temperature T j , i.e., the die temperature during operation. During operations, as the heat generating component  14  approaches its maximum junction temperature, the prospect for component failure, and thus reliability problems becomes eminent. It is, therefore, important to the invention that each heat generating component  14  in the electronic assemblage  10  be associated with a novel and unobvious at least first heat removing element  16  for reducing the junction temperature T j  of the heat generating component  14  to a temperature T 1 , where T 1  is considerably less than T j . This feature enables the electronic assemblage  10  to operate more reliably for a longer duration. First heat removing element  16 , comprising a plurality of patterned thermal vias  24 , forms a conduction path for carrying heat away from heat generating component  14 , as described further below. Referring to FIG. 2, a heat sink  20  is preferably associated with first heat removing element  16 . Heat sink  20  absorbs and transfers heat transported between the conduction path and the heat generating component  14 . In this way, excess heat from overheated heat generating component  14  is continually directed away from the electronic assemblage  10 , thereby preserving the reliability and performance of the electronic assemblage  10  and any heat sensitive component therein.  
         [0016]    Referring to FIG. 3, substrate medium  12  of electronic assemblage  10  may be a multi-layer circuit board, e.g., a 14-layer polyimide circuit board that uses 1 oz. copper on the inner layers  44  and 2 oz. copper on the outermost layers  46 ,  48 . Skilled artisans will appreciate that any circuit board material is contemplated by the invention, since the primary heat transfer mechanism is thermal conduction via the plurality of thermal vias  24  of heat removal element  16 .  
         [0017]    According to FIG. 1, in a preferred embodiment, substrate medium  12  uses copper as the base trace  50  with a plating of nickel gold (NiAu). Circuit boards of a variety of constructions and thickness could accomplish the same effect. Current industry standards for plating include palladium (Pd), nickel gold (NiAu), Immersion Tin, Immersion silver, and hot air solder level (HASL). Although the specific circuit board material used in the present invention is polyimide, it is of utmost importance that the thermal vias  24  of first heat removing element  16  can be drilled into the substrate medium  12 . Additional materials that can be used as substrate medium  12  with similar success include assorted resins and resin composites.  
         [0018]    Referring again to FIG. 1, it should be appreciated that practically any signal copper trace or power trace, even a “dummy” trace, may be utilized for the connecting path from heat generating component  14  to the thermal vias  24  of first heat removing element  16 . In the preferred embodiment, a particular pattern of thermal vias  24  in first heat removing element  16  is arranged remotely from the heat generating component  14 . According to FIG. 1, alternatively, a third heat removing element  32  with thermal vias  34  may also be positioned on a remote portion  18  of the substrate medium  12  away from the heat generating component  14 . Third heat removing element  32  may alternatively be connected back to the first heat removing element  16  by connecting path  28 .  
         [0019]    Referring again to FIG. 2, in the preferred embodiment, a first heat removing element  16  is positioned proximate to the heat generating component  14 . In this preferred design, first heat removing element  16  provides a specific cross-sectional area with thermal vias  24  to conduct heat from the topmost surface  46  to the bottommost surface  48  of the circuit board  12  (see FIG. 3). The final thermal transfer from the heat generating component  14  to the heat sink  20  takes place in the presence of a thermally conductive adhesive  40 . It should be appreciated that multiple remotely positioned first and third heat removing elements  16 ,  32 , respectively, may be associated with a single heat generating component  14  to improve heat transport. In addition, multiple first and third heat removing elements  16 ,  32  may be associated with multiple heat generating components, e.g., first and second heat generating components  14 ,  30  (shown in FIG. 1), to provide a dual heat transport system.  
         [0020]    Referring to FIGS. 1 and 2, first heat removing element  16  may contain any number or size of thermal vias  24  to provide a conduction path between the outermost surfaces  46 ,  48  of the substrate medium or circuit board  12  to facilitate and improve the thermal management. As depicted in FIG. 1, in a preferred embodiment of the invention, heat-removing element  16  has a single group of thermal vias  24  remotely spaced about 1.0 inch away from the heat generating component  14  on substrate medium  12 . It should be appreciated that a remote first heat removing element  16  having thermal vias  24  may be arranged in any number of patches or arrays and may be situated practically any distance away from the heat generating component  14 , although closer is generally better. A skilled artisan will recognize that any layer may be used for the conduction path to the remotely positioned first heat removing element  16 . Thermal conduction paths contemplated by the invention include electrical connections, non-electrical thermal connections, a fluid material connection between the heat generating component and first heat removing element, and surface bus wires, etc.  
         [0021]    Referring again to FIG. 1, in the preferred embodiment, first heat removing element  16  is comprised of a plurality of regularly spaced thermal vias  24  formed in the substrate medium  12 . Thermal vias  24  are generally round shaped and have a diameter of about 0.022 inches. Preferably, each one of the thermal vias  24  is spaced about 0.040 inches apart for optimum effectiveness in transporting heat. According to FIG. 2, in the preferred embodiment, the thermal vias  24  are filled with a thermally conductive material  22 . Preferred materials include a material selected from the group consisting of: tin-lead solder; silver solder; thermally conductive liquid silicon adhesive; and thermally conductive liquid epoxy adhesive. Additionally, the same or another thermally conductive adhesive  40  may be disposed in the open space  42  produced between the heat generating component  14  and the substrate medium  12  thereby thermally connecting the heat generating component  14  and the second heat removing element  17  with thermal vias  36 .  
         [0022]    Turning again to FIG. 2, in a preferred embodiment of the invention, an aluminum heat sink  20  about 0.090 inches thick is associated with the substrate medium or circuit board  12 . Heat sink  20  is attached to substrate medium or circuit board  12  with a thermally conductive material  38 , preferably a Dow Corning silicone thermally conductive, electrically isolative adhesive (1-4174™). This particular adhesive material comprises 7-mil glass beads to facilitate the finished bonding spacing between the circuit board  12  and the heat sink  20 . Of course, practically any metal heat sink  20  having a range of thickness may be used with substantially similar results.  
         [0023]    Further, practically any thermally conductive adhesive (including epoxies and sheet films) may be used to bond the heat sink  20  and substrate medium  12  with similar results. Generally, any material that can bond any circuit board to any rigidizer with even a nominal thermal conductivity may be used since the heat has the entire circuit card area to transfer through the adhesive. Even if substrate medium or circuit board  12 , and heat-sink  20  are somehow clamped together to provide a thermal path, the first heat removing element  16  of the invention could still be used to reduce the heat of the heat generating components  14  thereon.  
         [0024]    Skilled artisans will again appreciate that a wide range of thermally and electrically conductive adhesives is within the contemplation of the invention. In the preferred embodiment, an electrically isolative adhesive is used due to trace population on the bottom layer of the circuit board  12 . Otherwise a shorting path to the heat sink  20  will exist. However, an electrically homogeneous layer (or partial layer) with the same signal as the thermal vias  24  may also be used. According to FIG. 4, in this latter design, the thermal vias  54  of fourth heat removing element  52  would pass through the circuit board  12  and electrical vias would terminate at some layer above bottommost layer  48 . It should be appreciated that this latter design is a more expensive board to manufacture, but one that enables the use of higher conductive (electrically or thermally) material since adhesives that are electrically conductive can be more thermally conductive.  
         [0025]    Referring once again to FIG. 1, thermal vias  24  of first heat removing element  16  may be filled with a solder material  22  (as described above) to increase the effectiveness of the thermal vias  24  in removing heat from the heat generating component  14 . Alternatively, the thermal vias  24  of first heat removing element  16  may remain unfilled or may be filled with a different thermally conductive material. In the case of filling the thermal vias  24  of first heat removing element  16  with solder, a hand application or a specially designed solder paste stencil may be used. The specially designed solder paste stencil would have increased cut-out size to allow for the extra volume of solder required to fill the holes during processing. The actual increase in size can be calculated by adding the volume of the thermal vias  24  of first heat removing element  16  to the normal volume of solder paste desired.  
         [0026]    Referring again to FIGS. 1 and 2, with the remote thermal vias  24  of first heat removing element  16 , the substrate medium or circuit boards  12 , can be made of any material and used for any industry. The components specifically addressed are large plastic or ceramic components with lead frames, such as quad flat packs or dips, but it is additionally suited to even higher I/O component packages such as BGAs of any type. For extremely hot components, this method of providing remote vias  24  for cooling will dramatically reduce the junction temperatures of the heat generating components which will improve the life and reliability of the component and thus the board and module. This method is especially useful where there is minimal area underneath proximate to the component for thermal vias due to critical electrical routing requirements. This is a common problem in all industries due to the ever-increasing number of I/O pins required on components. This arrangement of first heat removing element  16  remotely located relative to the heat generating component  14  will help any package, but is generally used on these more densely packed electrical components that are routing space critical. The thermal vias  24  of first heat reducing element  16  are made as “thermally conductive” as possible with the addition of solder inside the through-opening, as previously described. Any diameter size via can be used, but the optimum size (copper area vs. cost of drilling smaller holes and more of them) is currently around 0.022 inches in diameter (finished size). Additionally any “circuit” or “net” can be used, but the optimum for most designs is the “ground” nets. This effectively maximizes the transfer to the heat sink  20 , which is typically “grounded.” This is typical of a bonded board assembly, but can additionally be used in a wedge-lock or direct fastener to chassis configuration where the heat transport is restricted to the mounting to the frame, card guide, or chassis. Additionally, a thermal conductive, electrically isolative silicone adhesive was used directly under the component to maximize heat transfer from the case of the component to the underlying copper ground paths. For BGA type components, the term “lead” would be replaced with the word “ball.” Conceptually, it is the same—using the grounding copper nets on the board to transfer heat to a remote area where the thermal vias can be placed. The extra ground nets have an additional benefit in making the circuit board less “noisy” electrically, which minimizes cross-talk issues.  
         [0027]    The invention has been described with reference to a preferred embodiment thereof. 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  
       [0028]    [0028] 10  electronic assemblage  
         [0029]    [0029] 12  substrate medium or circuit board  
         [0030]    [0030] 14  first heat generating element  
         [0031]    [0031] 16  first heat removing element  
         [0032]    [0032] 17  second heat removing element  
         [0033]    [0033] 18  remote portion of substrate medium  12   
         [0034]    [0034] 20  heat sink  
         [0035]    [0035] 22  thermally conductive material  
         [0036]    [0036] 24  thermal vias in first heat removing element  16   
         [0037]    [0037] 28  connecting path from first heat removing element  16  to third heat removing element  32   
         [0038]    [0038] 30  second heat generating element  
         [0039]    [0039] 32  third heat removing element  
         [0040]    [0040] 34  thermal vias in third heat removing element  32   
         [0041]    [0041] 36  thermal vias in the second heat removing element  17   
         [0042]    [0042] 38  thermally conductive material between the substrate  12  and the heat sink  20   
         [0043]    [0043] 40  thermally conductive adhesive between first heat generating element  14  and second heat removing element  17   
         [0044]    [0044] 42  open space (described in description of part number  40 )  
         [0045]    [0045] 44  inner layers of substrate medium  12   
         [0046]    [0046] 46  topmost layer of substrate medium  12   
         [0047]    [0047] 48  bottommost layer of substrate medium  12   
         [0048]    [0048] 50  base trace connecting first and third heat removing elements  
         [0049]    [0049] 52  fourth heat removing element  
         [0050]    [0050] 54  thermal vias in fourth heat removing element