Abstract:
Heat is conducted, from a heat generating electronic device that is mounted in a gap between a circuit board and a heat dissipator, along a path that includes a path segment that passes along conductive runs on the circuit board and another segment that spans the gap at a location adjacent to the device, the other path segment being spanned predominantly by a non-metallic piece that has a thermal conductivity of at least 7 W/m-°K.

Description:
BACKGROUND 
     This invention relates to heat dissipation for electronic components. 
     A DC-to-DC power converter of the kind shown in U.S. Pat. No. 5,987,740, incorporated by reference, for example, includes a heat-generating semiconductor device and a metal base plate that acts as a heat sink to dissipate heat from the device to the outside world. 
     As shown in FIG. 1, one path  8  for conducting heat from the device  10  to the base plate  12  where it can be dissipated is through silver filled epoxy  14 , a lead frame of copper conductors  16 , alumina-filled epoxy  18 , an alumina piece  20 , a copper layer  22  to which the alumina has been directly bonded, and solder  24 . The lead frame conductors are soldered to copper runs  23  on the surface of a printed circuit board  25 . The assembly is potted in a molding compound  26 . 
     The alumina piece  20  provides a relatively high thermal conductivity for good heat transfer to the base plate and a low capacitance between the copper lead frame and the base plate. A low capacitance is useful in reducing noise in the high-frequency signals that propagate in a DC-to-DC switching power converter. 
     In another approach, shown in FIG. 2, the heat-generating device  30  has conventional leads  32  that permit surface mounting on copper runs  33  of a printed circuit board  34 . Heat is conducted to the base plate  35  along a path  36  that includes the leads, the circuit runs  33  of the board  34 , metal columns  42 , and a silicone layer  44 . 
     SUMMARY 
     In general, in one aspect, the invention features apparatus that includes (1) a circuit board, (2) a heat dissipator separated from the board by a gap, (3) a heat generating electronic device mounted in the gap, and (4) a heat conducting path between the device and the dissipator. The heat-conducting path includes a path segment that spans the gap at a location adjacent to the device. The path segment includes an electrically insulating material having a thermal conductivity of at least 7 W/m-° K. 
     In general, in another aspect, the heat-conducting path including a path segment that passes along the conductive runs and another path segment that spans the gap at a location adjacent to the device. The other path segment includes by a piece of electrically insulating material that has a thermal conductivity of at least 7 W/m-° K. 
     Implementations of the invention may include one or more of the following features. The device includes a semiconductor power component, e.g., a semiconductor die and a copper base. The electrically insulating material comprises alumina that has a solderable material, such as direct bonded copper, on two opposite faces. The solderable material is soldered to the conductive runs and to the base plate. There are two parallel path segments that span the gap at two different locations adjacent to the device. The device is connected to a lead frame on the board. The device, the board, and the path segment are potted in a molding compound. The heat dissipator comprises a metal base plate. 
     In general, in another aspect, the invention features a power converter comprising (1) a heat generating power component mounted on a circuit board in a gap between the board and a heat sinking base plate, and (2) two thermally conductive and electrically insulating chimneys mounted adjacent to the device, spanning the gap, and configured to provide parallel paths for conducting heat, which has passed from the device to the board, from the board to the base plate. 
     Implementations of the invention may include one or more of the following features. Each of the chimneys includes predominantly a non-metallic piece, e.g., alumina that has direct bonded copper on two opposite faces. 
     In general, in another aspect, the invention features a method that includes conducting heat from a heat generating electronic device that is mounted in a gap between a circuit board and a heat dissipator, the heat being conducted along a path that includes a path segment that passes along conductive runs on the circuit board and another segment that spans the gap at a location adjacent to the device, the other path segment being spanned predominantly by a non-metallic piece that has a thermal conductivity of at least 7 W/m-° K. 
     Among the advantages of the invention are one or more of the following. Heat is removed effectively from the heat-generating device. The alumina thermal chimney provides good thermal conductivity, high dielectric strength and a high dielectric constant. Effective electrical isolation and low capacitance is provided between the heat sink and the circuit. The structure is mechanically rugged and inexpensive to make and is based on well-known manufacturing processes. The structure is shorter than other approaches (e.g., 0.040″ compared to 0.080″) which yields a higher power density for a given converter circuit. 
     Other advantages and features may be apparent from the following description and from the claims. 
    
    
     DESCRIPTION 
     FIGS. 1,  2 , and  3  are sectional side views of heat dissipation schemes for heat generating devices. 
     FIG. 4 is top view of the assembly of FIG. 3 before attachment of the base plate. 
     FIGS. 5,  6 , and  7  are a top view of an assembly during manufacture and a side view and a top view at a later stage of manufacture. 
     FIG. 8 is a table. 
    
    
     As shown in FIGS. 3 and 4, heat can be conducted from a heat generating packaged electronic device  50  to a heat sinking metal base plate  52  along a path  54 . The device  50  includes a semiconductor power component  56  attached by solder or epoxy  58  to a portion of a copper lead frame  60 . Termination pads on the top of the semiconductor device are connected to other portions of the lead frame (for example, by means of bond wires, not shown in the figure). The copper lead frame  60  is in turn soldered  62  to conductive runs  64  on a printed circuit board  66 . 
     A small gap  68  separates the top surface of the device from the bottom surface of the base plate. To each side of the device, a thermal chimney  70 ,  72  spans the larger gap  74  between the top surface of the circuit board conductive runs and the bottom surface of the base plate. Each of the chimneys includes a long rectangular alumina piece  76 ,  78  with direct bonded copper layers  80 ,  82 ,  84 ,  86  on the top and bottom faces. Solder  88 ,  90 ,  92 ,  94  connects the alumina pieces to the printed circuit runs and to the base plate. The entire assembly (except for the base plate) is potted in a molding compound  96  before attachment to the base plate. Laser etching may be used to clean away excess molding compound on the top surface of the potted assembly to expose the top surfaces of the copper layers  80 ,  84 . 
     As shown in FIG. 4 the packaged electronic device  50  and the chimneys  70 ,  72  are separate components which are individually mounted on conductive runs  64  on the printed circuit board  66 . In an alternate packaging arrangement, shown in FIGS. 5 through 7, both the semiconductor power component  56  and the chimneys  70 ,  72  are contained within a single packaged device  80 . The semiconductor device  56  is mounted on a portion of a lead frame  60  by conductive material  58  (e.g., conductive epoxy or solder). Bond wires  100  electrically connect the upper surface of the semiconductor device to other portions of the lead frame  102 ,  104 . The thermal chimneys  70 ,  72  are attached (by conductive material) to the portion of the lead frame  60  onto which the power component  56  is mounted to provide low thermal resistance paths, indicated by arrows  83 ,  85 , between the power component and the chimneys. The assembly is encapsulated in molding compound  106 . Surfaces  80 ,  84  are left exposed during the molding process or are re-exposed by a post molding process such as laser etching, for subsequent connection to a heat sink. 
     If configured for equal thermal resistances, the arrangement of FIG. 4 provides lower capacitance between the circuit etch ( 64 , FIG. 4;  33 , FIG. 2) and the base plate than the arrangement of FIG.  2 . 
     We compare the arrangement of FIG. 2 including a high thermal conductivity silicone having a thermal conductivity of 1 W/m-° K and a dielectric constant of 3.3 to the arrangement of FIG. 4 including alumina chimneys having a thermal conductivity of 25 W/m-° K and a dielectric constant of 9.9. For equal cross-sectional areas of the chimneys  76 ,  78  and the metal columns  42 , equality in thermal resistance would require that the thickness of the silicone layer (between the top of the metal column and the baseplate in FIG. 2) be approximately 4% of the length of the alumina chimney in FIG.  4 . As a result, the capacitance of FIG. 4 would be approximately 12% of the capacitance of FIG.  2 . 
     High thermal conductivity combined with low dielectric constant are desirable features in heat removal systems of the kinds shown in FIGS. 2 and 4. An increase in a figure-of-merit defined as the ratio of the thermal conductivity and the dielectric constant will result in an improved heat removal path. For the alumina chimney described above the figure-of-merit is 2.53, whereas the figure-of-merit for the system having a metal slug and silicone, also described above, is 0.30. The alumina system is superior. 
     The relative capacitances of two systems, for the same cross sectional areas and thermal resistances, is inversely proportional to the figures-of-merit of the systems; in the example just given the inverse ratio is 0.03/2.53 which corresponds to the 12% figure given above. Silicones and related materials suitable for potting electronic assemblies have thermal conductivities below 7 W/m-° K, whereas thermal conductivities of insulating ceramics used in electronic assemblies are typically above 10 W/m-° K. 
     FIG. 8 is a table showing thermal resistance calculations for the heat dissipation paths of FIGS. 3 and 4 (assuming one chimney or two) at the top of the table and for the dissipation path of FIG. 1 at the bottom of the table. With two chimneys, the cumulative thermal resistance (rise per watt) is 7.1 compared to 7.6 for the path of FIG.  1 . 
     Other implementations are within the scope of the following claims. For example, the chimneys can comprise thermally conductive and electrically insulating materials other than alumina, for example, beryllium oxide or aluminum nitride. Solderable material may be applied to the faces of the chimneys other than by direct bond copper. Alternatively, if a face of a chimney is to be connected, either to a conductive etch or to a base plate, by thermally conductive adhesive, it can be left bare.