Abstract:
An electronic assembly includes a semiconductor device mounted on a printed circuit board, a leaf spring and a cooling plate. A plurality of fasteners pass through co-axial apertures of the leaf spring, the printed circuit board and the cooling plate, such that a contact surface of the leaf spring imparts a force on the semiconductor device to retain the semiconductor device in a thermally conductive position with respect to the cooling plate. A positive cooling effect is achieved regardless of mounting conditions including orientation of the electronic assembly, and the position of the semiconductor device on the cooling plate.

Description:
CLAIM OF PRIORITY 
     This patent application claims priority to European Application EP 03 018 046.7 filed on Aug. 7, 2003. 
     1. Field of the Invention 
     The present invention relates to the field of cooling semiconductor devices, and in particular to the field of devices for cooling semiconductor devices attached to a printed circuit board. 
     2. Related Art 
     The electric power consumed by a single semiconductor device, especially those with power transistors, has become so large that there are increasing instances where the device becomes heated to the point that reliability of the semiconductor device is decreased. The heat radiation to cool the semiconductor device relies upon heat transfer from the surface of the heated portion to the surrounding air. If the resistance of heat transfer between the surface of the heated element and the surrounding air is low, then the undesirable heat is suitably transferred. Therefore, by lowering the heat transfer resistance to the air, improved cooling of the semiconductor device is achieved. To reduce the heat transfer resistance to the air, the conventional approach has been to increase the surface area in contact with the air, as shown in  FIG. 1 . 
       FIG. 1  is a perspective view of a prior art heat radiating assembly for a semiconductor device. A radiating fin  102  is fastened to the integrated circuit (e.g., a dual-in-line package,)  104 . The radiating fin may be made of material such as copper, aluminum, or the like, having good heat conductivity. The radiating fin includes a plurality of fine grooves  102   a  on the top surface of the semiconductor package to increase the surface area in contact with the air. The radiating fin  102  is attached to the top surface of the IC  104  using, an adhesive, or nuts and bolts. The heat generated by the IC  104  is conducted to the radiating fin  102  for cooling. 
     Attaching the radiating fin  102  directly to the IC  104  appears to be effective upon initial consideration, but it involves various problems in reality. First, the direction of the grooves  102   a  to magnify the surface area of the radiating fin  102  must be in the direction of the air flow. By this arrangement to improve the radiation effect, the direction of the IC  104  to be mounted on the printed circuit board is restricted to be in one direction. As a result, the wiring pattern on the printed circuit board is also subject to restriction. 
     Second, the technique used for attaching the radiating fin  102  to the IC  104  is often problematic. The technique for attaching the radiating fin  102  to the IC  104  must endure thermal stress for a long time, so that the radiating fin  102  is held securely attached to the IC  104  even when subject to vibration or shock. However, the technique used to attach the fin  102  must be adapted such that, under all possible thermal conditions, the semiconductor device does not experience excessive contact pressure resulting from the attachment of the fin. To achieve this, U.S. Pat. No. 4,621,304 to Oogaki et al. proposes an adapted construction of a heat sink as illustrated in  FIG. 2  to adjust the height (b)  230 . 
     As indicated in  FIG. 2 , first and second heat sinks  202 ,  222  are provided. The second heat sink  222  is cylindrically shaped and includes a thread  222   a  on the periphery thereof. The second heat sink  222  also includes a slot  222   b  on the top face thereof so as to be driven by a screwdriver. The first heat sink  202  attached to a shield case  213  includes a female thread  202   a , so that the threaded heat sink  222  can be engaged with the first heat sink  202 . The threaded heat sink  222  is adapted to be threadedly engaged with the female thread  202   a  of the first heat sink  202  from above the shield case  213 , to which the heat sink  202  is attached, through a hole  213   a.    
     The threaded heat sink  222  is passed through the hole  213   a  at the top of the shield case  213  and threadedly engaged with the threaded portion  202   a  of the heat sink  202 . The heat sink  222  is pressed against an IC  204  mounted on a printed circuit board  201 . To provide smooth contact and heat conduction between the threaded heat sink  222  and an IC  204 , heat conductive rubber  221  with good heat conductivity may be previously attached to the IC  204  at the location that contacts the threaded heat sink  222 . In such a case, the threaded heat sink  222  contacts the IC  204  through the rubber  221 . Thus, any variation in the distance (a) between the electronic parts  204  and the heat sink  202  can be minutely compensated with ease by turning the threaded heat sink  222 . Meanwhile, the heat sink  202  and the threaded heat sink  222  are in threaded contact and therefore the surface of contact between them is magnified, so that the heat conducted from the electronic parts  204  through the rubber  221  to the threaded heat sink  222  is easily conducted to the heat sink  202 , and emitted to the ambient air through the shield case  213 . As described in the foregoing, no matter how varied the distances between the first heat sink  202  and the electronic parts  204  may be, sufficient contact pressure and contact surface area for good heat conductivity is obtained by virtue of the second heat sink  222 . 
     However, the conventional apparatus shown in  FIG. 2  still exhibits some limitations as, for example, a costly manufacturing process, a relatively large number of specific parts, and unreasonable weight and height in view of the cooling effect achieved. 
     Therefore, there is a need for an improved apparatus for cooling semiconductor devices attached to a printed circuit board. 
     SUMMARY 
     An electronic assembly includes a semiconductor device mounted on a printed circuit board, a leaf spring and a cooling plate. A plurality of fasteners pass through co-axial apertures of the leaf spring, the printed circuit board and into the cooling plate, such that a contact surface of the leaf spring imparts a force on the semiconductor device to retain the semiconductor device in a thermally conductive position with respect to the cooling plate. 
     A positive cooling effect is achieved regardless of mounting conditions and the position of the semiconductor device on the cooling plate. Further, the manufacturing process is less costly, less parts are needed, and less weight and less height are required. 
     Other systems, methods, features and advantages of the invention will be, or will become, apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the following claims. 
    
    
     
       DESCRIPTION OF THE DRAWING 
       The invention can be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1  is a perspective view of a prior art heat radiator assembly for a semiconductor device; 
         FIG. 2  is a perspective view of another prior art heat radiator assembly for a semiconductor device; 
         FIG. 3  is a sectional view of a printed circuit board sandwiched between a cooling plate and a spring plate, the spring plate providing a spring force to a semiconductor device mounted on the printed circuit board; 
         FIG. 4  is a sectional view of another printed circuit board sandwiched between a cooling plate and a spring in connection with a cover plate, the spring plate providing a spring force via the printed circuit board to a semiconductor device mounted on the printed circuit board; 
         FIG. 5  is a sectional view of another printed circuit board sandwiched between a cooling plate and a spring plate, the spring plate being connected to the cooling plate by integral noses; 
         FIG. 6  is a sectional view of another printed circuit board sandwiched between a cooling plate and a spring plate, the spring plate being connected to the cooling plate by bolts; and 
         FIG. 7  is a sectional view of another printed circuit board sandwiched between a cooling plate and a spring plate, the printed circuit board having two semiconductor devices mounted thereon. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides an apparatus for cooling semiconductor devices attached to a printed circuit board. As shown in  FIG. 3 , a printed circuit board  301  is sandwiched between a cooling plate  302  and a spring plate  303  (e.g. a leaf spring). 
     The printed circuit board  301  is a plate of electrically non-conductive material (e.g., resin plate, ceramic substrate) comprising at least one structured electrically conductive layer (e.g., metallization) for connecting electrical devices such as semiconductor devices mounted on the printed circuit board. The printed circuit board  301  has first and second sides and further comprises a plurality of apertures  305  formed through the first and second sides. The first side of the printed circuit board  301  comprises a semiconductor device  304  (e.g., power amplifier, voltage regulator, or a power switch, et cetera) attached thereto (e.g., by soldering). In  FIG. 3  the printed circuit board  301  comprises a further aperture  309  arranged such that it is covered by the semiconductor device  304 . 
     The cooling plate  302  is adjacent to the first side of the printed circuit board  301  and has first and second sides itself. The second side of the cooling plate  302  has thermal contact with the semiconductor device  304 . In order to further improve the thermal contact, a heat conductive element  308  (e.g., a thermal compound) may be applied between the semiconductor device  304  and the cooling plate  302 . Additionally or alternatively an electrical insulator may be provided between the semiconductor device  304  and the cooling plate  302 . The cooling plate  302  is preferably metallic and may be provided with a plurality of fine grooves on the first side thereof to increase the surface area in contact with the air (not shown in the FIGS.). 
     The spring plate  303  may also be metallic and arranged adjacent to the second side of the printed circuit board  301 . The spring plate  303  may be wave-like embossed and it comprises a dome  307 . The dome  307  is positioned in the aperture  309  of the printed circuit board  301  and has a height relative to its ground line that exceeds the heights of all other waves or domes due to embossing such that the dome  307  is in direct contact with the semiconductor device  304  and exerting force to the semiconductor device  304  as a result of the spring force provided by the spring plate  303 . The spring plate  303  comprises apertures  306  that are operably positioned in alignment with the apertures  305  of the printed circuit board  301 . 
     To connect the spring plate  303  to the cooling plate  302 , securing members are provided that extend through the apertures  305  of the printed circuit board  301  and the apertures  306  of the spring plate  303 . As shown in  FIG. 3 , the securing members may be bolts  310  in connection with integral noses  311  formed in a single piece from the cooling plate  302 . The integral noses  311  extend partly into the apertures  305  while the bolts  310  extend through the apertures  305  and  306  into tap holes  312  arranged in the integral noses  311 . A cover plate  313  may be arranged adjacent to the spring plate  302  as an outside surface opposite to the outside surface established by the cooling plate  302 . 
     The apparatus shown in  FIG. 3  can be easily mounted. Starting with the cooling plate  302  as a base plate, the printed circuit board  301  with the semiconductor device  304  mounted thereon is arranged on the base plate with the semiconductor device  304  face down. The height of the semiconductor device  304  exceeds all other elements arranged on this side of the printed circuit board  301 . The leaf spring  303  is operably positioned such that the apertures are aligned. The next step is to screw the bolts  310  into the tap holes  312 . In the apparatus of  FIG. 3 , the spring force provided by the spring plate  303  depends essentially on the material used, its structure, its thickness, and the size of the dome  307 . 
     The spring plate  303  provides a spring force forcing the semiconductor device  304  against the cooling plate  302 , thus providing good thermal contact between the semiconductor device  304  and the cooling plate  302 . The cooling plate  302  has a lower heat transfer resistance to the air due to a larger surface in contact with the air. 
       FIG. 4  is a sectional view of a printed circuit board  401  sandwiched between a cooling plate  402  and a spring associated with a cover plate to provide a spring plate  403 . The spring plate  403  provides a force against the printed circuit board at the backside of where the semiconductor is mounted. In the apparatus shown in  FIG. 4 , again the printed circuit board  401  is sandwiched between the cooling plate  402  and the spring plate  403 . 
     Similar to the printed circuit board illustrated in  FIG. 3 , the printed circuit board  401  is a plate of electrically non-conductive material comprising at least one structured electrically conductive layer. The printed circuit board  401  has first and second sides and includes a plurality of apertures  405  formed through the first and second sides. A semiconductor device  404  is arranged on the first side of the printed circuit board  401 . 
     The cooling plate  402  made of heat conducting material is adjacent to the first side of the printed circuit board  401  and has first and second sides itself. The second side of the cooling plate  402  is in direct thermal contact with the semiconductor device  404 . 
     The spring plate  403  is arranged adjacent to the second side of the printed circuit board  402  and may be wave-like embossed, such that it comprises at least one dome  407 . The dome  407  is preferably located in that area of the printed circuit board  401  where on the other side the semiconductor device  404  is attached to the printed circuit board, and is preferably higher than all other waves or domes of the spring plate  403 . Thus, the dome  407  forces the semiconductor device  404  against the cooling plate  402  via the printed circuit board  401 . The spring plate  403  includes apertures  406  corresponding to the apertures  405  of the printed circuit board  401 , and has on its outer side a cover member established by a cover plate  413  attached thereto. 
     To connect the cover plate  413  to the spring plate  403  and both to the cooling plate  402 , bolts  410  extend through apertures  414  in the cover plate  413 , spacer leeves  423 , the apertures  405  of the printed circuit board  401 , and the apertures  406  of the spring plate  403 . The bolts are screwed into tap holes  412  arranged in integral noses  411  formed in a single piece from the cooling plate  402 . The height of the semiconductor device  404  exceeds all other elements arranged on this side of the printed circuit board  401 . 
     As shown in  FIG. 5 , in yet another embodiment a printed circuit board  501  is sandwiched between a cooling plate  502  and a spring plate  503 . The printed circuit board  501 , the cooling plate  502 , and the spring plate  503  are substantially the same as those illustrated in  FIGS. 3 and 4 . 
     The printed circuit board  501  has first and second sides and includes a plurality of apertures  505  formed through the first and second sides. On the first side of the printed circuit board  501  a semiconductor device  504  is arranged. The cooling plate  502  is adjacent to the first side of the printed circuit board  501  and has first and second sides itself. The second side of the cooling plate  502  has thermal contact with the semiconductor device  504 , the thermal contact is improved by a heat conductive device  508  (e.g., a thermal compound) located between the semiconductor device  504  and the cooling plate  502 . 
     The spring plate  503  is arranged adjacent to the second side of the printed circuit board  501  and is wave-like embossed such that it includes a dome  507 . The dome  507  is preferably located in that area of the printed circuit board  501  where on the other side the semiconductor device  504  is attached and is preferably higher than all other waves or domes of the spring plate  503 . Thus, the dome  507  forces the semiconductor device  504  against the cooling plate  502  by exerting force to the printed circuit board  501 , which exerts force to the semiconductor device  504 . The spring plate  503  comprises apertures  506  corresponding to the apertures  505  of the printed circuit board  501 . 
     To connect the spring plate  503  to the cooling plate  502 , bolts  510  extend through the apertures  505  of the printed circuit board  501 , and the apertures  506  of the spring plate  503  are screwed into tap holes  512  arranged in integral noses  511  formed in a single piece from the cooling plate  502 . The height of the semiconductor device  504  exceeds all other elements arranged on this side of the printed circuit board  501 . 
     The apparatus illustrated in  FIG. 6  is similar to the apparatus illustrated in  FIG. 5 , wherein, again, a printed circuit board  601  including a semiconductor device  604  is sandwiched between a cooling plate  602  and a spring plate  603 . However, the cooling plate  602  comprises no integral noses as the cooling plate  502  of  FIG. 5 . Instead, tap holes  612  corresponding to apertures  605  in the printed circuit board  601  and apertures  606  in the spring plate  603  are inserted into the cooling plate  602 . Bolts  610  extending through the apertures  605  and  606  are screwed into the tap holes  612 . The torque provided by the bolts  610  controls the spring force forcing the semiconductor device  604  against the cooling plate  602 . Therefore, by screwing the bolts  610  more or less into the tap holes  612 , the spring force pressing the semiconductor device  604  against the cooling plate  602  can be adjusted. In the apparatus of  FIG. 6 , as well as in the apparatus of  FIG. 5 , the respective spring plates  603 ,  503  also serve as covers. 
     The apparatus of  FIG. 7  is similar to the apparatus shown in  FIG. 5 , wherein, however, a printed circuit board  701  having two semiconductor devices  704  and  714  is sandwiched between a cooling plate  702  and a spring plate  703 . The printed circuit board  701  includes first and second sides and comprises a plurality of apertures  705  formed through the first and second sides. On the first side of the printed circuit board  701  the two semiconductor devices  704  and  714  are arranged The cooling plate  702  is adjacent to the first side of the printed circuit board  701  and has first and second sides itself. The second side of the cooling plate  702  is in thermal contact with the semiconductor devices  704  and  714 , which is improved by a heat conductive component  708  and  718 . 
     The spring plate  703  is arranged adjacent to the second side of the printed circuit board  702  and is wave-like embossed such that it comprises at least two domes  707  and  717 . The domes  707  and  717  are located in areas of the printed circuit board  701  where on the other side the semiconductor devices  704  and  714  are attached respectively. Both domes  707  and  717  have the same height and are as high as or higher than all other waves or domes of the spring plate  703 . Thus, the domes  707  and  717  force the semiconductor devices  704  and  714  against the cooling plate  702  by exerting force to the printed circuit board  701 , which exerts force to the semiconductor devices  704  and  714 . The spring plate  703  includes apertures  706  in alignment with the apertures  705  of the printed circuit board  701 . 
     To connect the spring plate  703  to the cooling plate  702 , bolts  710  serving as securing members are provided that extend through the apertures  705  of the printed circuit board  701 , and the apertures  706  of the spring plate  703  and are screwed into tap holes  712  arranged in integral noses  711  formed in a single piece from the cooling plate  702 . The semiconductor devices  704  and  714  have the same height that exceeds all other elements arranged on this side of the printed circuit board  701 . 
     Although various exemplary embodiments of the invention have been disclosed, it will be apparent to those skilled in the art that various changes and modifications can be made that achieve some of the advantages of the invention without departing from the spirit and scope of the invention. It will be obvious to those reasonably skilled in the art that other components performing the same functions may be suitably substituted. For example, rather than threaded blind holes as tap holes, threaded contact holes may be used and vice versa. The spring plate may be made of rubber or plastic instead of metal. The semiconductor devices may be integrated circuits or discrete devices in all available and possible packages, for example Dual-In-Line (DIL) packages or power packages preferably Power S 036 . 
     The illustrations have been discussed with reference to functional blocks identified as modules and components that are not intended to represent discrete structures and may be combined or further sub-divided. In addition, while various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that other embodiments and implementations are possible that are within the scope of this invention. Accordingly, the invention is not restricted except in light of the attached claims and their equivalents.