Abstract:
A printed circuit board, containing thermal pads, is adhered to a rigidizer plate whereupon the entire unit can then be bent over itself to create a compact assembly which can be substantially smaller, but contain the same number of traces and electrical components, as an unbent printed circuit board of the same surface area. Further, a complete housing assembly is formed which is sealed on each edge of the rigidizer by inserting the edge into a panel with a groove. This assembly provides a secure fit that provides great stability with a relatively low weight and volume. The assembly also provides a better RF non-mechanical connection and much better thermal performance.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to automotive electronic control assemblies, and more particularly to electronic control assemblies containing printed circuit boards. 
     BACKGROUND 
     Automotive electronic control assemblies are well known in the art, however, they contain several distinct disadvantages. Most often the assemblies contain a relatively small printed circuit board heavily populated with electrical traces and components. Furthermore, these assemblies must be contained within a thick housing that increases the overall mass of the assembly. Moreover, the housings are then required to be filled with a potting material to ensure that the components and connections are kept clean of various materials that may infect the assembly. In addition, the printed circuit board has an RF ground that is typically mechanically attached to a ground member. This type of assembly lacks in that it is heavy and has poor heat sink capabilities. 
     Some assemblies have attempted to solve some of these problems by bending the assembly upon itself, such assemblies are disclosed in U.S. Pat. Nos. 5,998,738; 5,103,375; 5,170,326. These assemblies attempt to reduce the volume and weight of the assembly overall. However, the disclosures also contain areas of needed improvement. Most of these assemblies disclose a mandrel about which the assembly is bent and this mandrel is not removed. This internal mandrel unit increases the weight of the assembly. Furthermore, these disclosures require either a non-continuous printed circuit board or a unique and labor intensive adhesive process. All of these are disadvantages of the previous art. 
     SUMMARY OF THE INVENTION 
     According to the present invention a printed circuit board is adhered to a rigidizer plate whereupon the entire unit can then be bent over itself to create a compact assembly which can be substantially smaller, but contain the same number of traces and electrical components as a flat printed circuit board. Further, the present invention discloses a complete assembly which is sealed on each edge of the rigidizer by inserting the edge into a panel with a groove. The panel can be a plate, casting or an extrusion. This assembly provides a secure fit that provides great stability with a reduction in weight and volume. 
     Furthermore, according to the present invention a thermal pad design is provided to heat sink thermal energy, created by components on a printed circuit board, to a rigidizer/heatsink plate. First, the printed circuit board is adhered to the rigidizer plate with an adherent that is thermally conductive and compliant. Moreover, the printed circuit board contains pads with thermal vias to dissipate heat from components to the rigidizer plate. 
     Additionally, the rigidizer plate is formed so as to minimize stresses placed upon the printed circuit board during bending and after being bent. The present invention, while reducing stresses, provides for a printed circuit board adhered to the rigidizer plate and openings in the rigidizer plate to minimize stresses during bending. The bending area can also be removed with the use of a breakaway piece. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood however that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a front-top perspective view of the assembly in its completed form. 
     FIG. 2 is a back-top perspective view of the assembly in its completed form. 
     FIG. 3 is a plan view of a side rail according to the principles of the present invention. 
     FIG. 4 is a top view of a side rail according to the principles of the present invention. 
     FIG. 5 is a plan view of a top core of a printed circuit board. 
     FIG. 6 is a plan view of a core of the printed circuit board containing horizontal traces. 
     FIG. 7 is a plan view of a core of the printed circuit board containing vertical traces. 
     FIG. 8 is a cross-sectional view through a thermal via with a studded component. 
     FIG. 8 a  is a cross-sectional view through a thermal via with a studded component where thermal vias are filled with adhesive. 
     FIG. 9 is a cross sectional view through a thermal via with a surface mount component. 
     FIG. 10 is a cross sectional view through the printed circuit board and rigidizer showing a ground screen addition for electrical continuity. 
     FIG. 11 is a plan view if the rigidizer containing openings. 
     FIG. 12 is a perspective view of the bent rigidizer and printed circuit board with-out the final enclosures. 
     FIG. 13 is an exploded perspective view of the parts and process of adhering the printed circuit board to the rigidizer also showing the fixture to perform the task. 
     FIG. 14 is a plan view of the bottom fixture plate. 
     FIG. 15 is a plan view of the top fixture plate. 
     FIG. 16 is a plan view of thermal pad according to the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 show the entire assembly of an automobile control module  10  in its complete and assembled manner, according to an embodiment of the present invention. The outer portion of the assembly  10  is comprised of: a rigidizer/heatsink plate  18 , two side rails  12   a  and  12   b,  front plate  20 , and a board connector  24 . The rigidizer plate  18  is a continuous piece of metal that is printed circuit board having bent upon itself around a bend axis  31 . This design allows for a greater surface area to fit in a much smaller volume than otherwise attainable. The printed circuit board which is adhered to the rigidizer plate  18  will be described in greater detail herein. The side rails  12   a  and  12   b  are placed on the edges of the rigidizer plate  18  after the rigidizer plate  18  is in its bent form. With continued reference to FIG.  1  and further reference to FIGS. 3 and 4, it can be seen that the side rails  12   a  and  12   b  contain grooves  34  to receive the edge of the bent rigidizer plate  18 . Furthermore, the side rails  12   a  and  12   b  contain holes  35  for receiving retaining bolts  16 . Side rails  12   a,    12   b  and front plate  20  can be combined into a casting of one piece with a similar groove. 
     Returning with further reference to FIGS. 1 and 2 the retaining bolts  16  are inserted through holes  35  in the side rails  12   a  and  12   b  and are held in place with nuts  14 . This connection provides support for the assembly  10  and seals the outside edges of the assembly  10 . The front plate  20  is comprised of two main pieces  20   a  and  20   b  connected by two minor pieces  28   a  and  28   b.  These pieces  20   a,    20   b,    28   a,    28   b  create a front enclosure that surrounds the board connector  24 . The main pieces  20   a,    20   b  are secured to the minor pieces  28   a,    28   b  and to the side rails  12   a  and  12   b  with threaded fasteners  22 . Support member  26  in the form of a gasketed cover provides further strength to the component harness area as well as sealing to the module and is mounted to the rigidizer  18  with nut and bolt assemblies  32  and to the front plate  20  with threaded fasteners  22 . 
     Particularly referencing FIG. 2 the bend radius  31  of the rigidizer plate  18  contains a plurality of elongated slots  30 . These slots  30  are diagonal to the axis of the bend radius  31  and provide a relief of stress to the rigidizer plate  18 . The slots  30  are filled with an adhesive  33  providing an additional portion of protection to prevent foreign objects from getting inside the assembly  10  and seal the module for pressurizing. Including the adhesive  33  that fills in the holes  30 , the side rails  12   a  and  12   b,  and the front plate  20  the assembly  10  is completely contained within a structure that can endure extreme conditions, including those under the hood of an automobile. 
     FIGS. 5,  6 , and  7  reference the first three layers or cores in their unbonded formation of the four layer bonded printed circuit board (PCB)  76  shown generally in FIGS. 12 and 13. FIG. 5 shows the first layer  44   a  of the PCB  76 . The first layer  44   a  indicates the general placement of components and their connections  53  (as is well known in the art) to the traces and the thermal pads  52 . Also the vias  54  for the connector harness  24  (shown in FIG. 1) are shown. FIG. 6 is a plan view of a second layer  44   b  of the PCB  76 . The second layer  44   b  generally contains horizontal traces  43  for connecting components that may be placed upon the first layer  44   a.  The second layer  44   b  also shows the grounding and some connections  54  for the connector harness  24 . FIG. 7 shows the third layer  44   c  of the circuit board, and also shows grounding and some connections for the connector. This layer generally contains the vertical traces  45  as is known in the art. The PCB  76  has four layers with  2  mil cores and is flexible and is commercially available from Photocircuits, Glen Cove, N.Y. 
     FIG. 8 is a cross-sectional detail of a thermal pad  52  on a PCB  76  bonded to a rigidizer  18 . The thermal pad  52  contains a plurality of thermal vias (or holes)  38 . The PCB  76  contains four layers  44   a,    44   b,    44   c,    44   d  bonded together, the order of which can be dictated by the design. Once the layers  44   a,    44   b,    44   c,    44   d  are bonded the thermal vias  38  are formed through all layers  44   a,    44   b,    44   c,    44   d  of the PCB  76 . The thermal vias  38  are formed in a predetermined pattern and size. Once the thermal vias  38  are drilled in the PCB  76  then a thin layer  42  is plated onto the PCB  76  in the area of the thermal pads  52  and on the walls  39  of the thermal vias  38 . The area is then plated again with the same or a different material  42   a.  The entire component is called the thermal pad  52 . The thermally conductive material  42  is preferably copper. Layer  42   a,  can be a variety of materials such as palladium, nickel, gold, immersion silver, immersion tin, or OSP, which is an organic. 
     Once the PCB  76  is bonded together it is then adhered to a rigidizer plate  18  with a thermally conductive adhesive  48   a.  In a first embodiment the adhesive  48   a  is a liquid with glass beads  46  disposed therein. The glass beads  46  are of a generally uniform diameter to help ensure a generally uniform thickness of the adhesive layer  48   a.  The beads are beneficial due to the fact that during the curing of the adhesive  48   a  pressure is applied to the PCB  76  and the rigidizer plate  18 , thus the glass beads  46  act as a spacer. The liquid adhesive is preferably Dow Corning 1-4175. 
     Once the PCB  76  is adhered to the rigidizer plate  18 , components  36  may be reflow or hand-soldered onto the thermal pad  52 . Large components  36  may contain a metal stud to which the solder  40  adheres. Then, while the solder  40  is flowing it fills the thermal vias  38  and comes into contact with the adhesive  48   a.  To ensure that a sufficient amount of solder  40  is placed below the component  36  a larger area of solder paste is laid on the thermal pad  52  area. The solder paste extends approximately 0.005 to 0.050 inches on each side of the component  36  surface area. In this way it is assured that a sufficient amount of solder  40  is placed on the thermal pad  52  to fill the thermal vias  38 . Through filling the thermal vias  38  and contacting both the stud in the component  36  and the adhesive  48   a  there is created a direct thermal path from the component  36  to the rigidizer plate  18 , which acts as a heatsink. A standard 7 or 8 mil stencil is used (industry standard); however, the squeegee does 2 passes across the board to better press solder paste into the thermal via hole. 
     FIG. 8 a  is a second embodiment as to how to fill the thermal vias  38  created in the thermal pad  52 . In this embodiment the thermal vias  38  are left unmasked during the curing process of the liquid adhesive  48   a.  Leaving the thermal vias  38  unmasked allows the liquid adhesive  48   a  to fill the thermal vias  38 . Then when the component  36  is reflow soldered onto the thermal pad  52  the thermal vias  38  are already filled thus the solder  40  remains only on the surface of the thermal pad  52 . To keep the adhesive from flowing out of the holes, Kapton tape is placed over the holes as a block. When removed, some of the adhesive is pulled out of the hole. For soldering set-up a 1:1 stencil is used still with two passes of the squeegee to ensure material is pressed into the upper portion of the thermal via. 
     FIG. 9 shows an alternative to the direct thermal pathway described in FIG.  8 . Again, once the cores  44   a,    44   b,    44   c,    44   d  are bonded together, vias  38  are drilled through the PCB  76 . The vias  38  and the pad areas  52  are then typically plated with a thermally conductive material  42 . The PCB  76  is then adhered to the rigidizer  18  using the thermally conductive adhesive  48   a,  in this case a liquid containing spacer beads  46 . Here the component  35  does not contain a metal stud to be soldered to the thermal pad  52 , however, the component  35  is reflow soldered at its distal terminals  47 . The solder  41  at the distal terminals  47  acts to fix the component  35  to the PCB  76  and to make electrical contact, however, it does not act as a thermal conductor within the thermal vias  38 . In this case the heat from the component  35  is radiated to the thermal pad  52  and the thermal vias  38  rather than traveling through a direct path. This method, however, allows for a more efficient thermal path than if no thermal pad  52  were present and allows a thermal path for components that have no stud. The holes can also have the alternative method described with regard to FIG.  82  and can be filled or partially filled with liquid adhesive  482 . 
     Referring to FIG. 10 the PCB  76  is shown in cross-section adhered to the rigidizer plate  18  with the thermally conductive adhesive  48   a.  Glass beads  46  are disposed in the adhesive to ensure a generally uniform thickness of the adhesive throughout. Furthermore, a metallic screen  90  is disposed in the adhesive  48   a  and between the PCB  76  and the rigidizer  18 . The screen  90  acts an electronic connector between the bottom core layer  44   d  of the PCB  76  and the rigidizer plate  18 . This connection is to ground the PCB  76  to the rigidizer plate  18 . The screen  90  is a weave of metal fibers with fibers running perpendicular to one another and on top  90   a  and below  90   b  each other. The screen  90  which in the form of a small circular pad ensures that there are always a plurality of actual point contacts between the bottom layer  44   d  and the rigidizer plate  18  through the adhesive  48   a.  Furthermore, the screen  90  provides a connector that is relatively clean and does not interfere with the other components. Screen  90  can be in any shape for individual location. Screen  90  can be formed or cut into a strip format or gasket format as well to aid manufacturing. The screen strip, gasket or pad (rectangular or circular) can be tacked down by any industry standard adhesive just to hold it in place for processing. A minimal amount of adhesive is used toward the edge or corners. 
     FIG. 11 shows a plan view of the rigidizer plate  18  alone. A plurality of openings  30  are disposed diagonally along the bend axis  31  and provide a way to relieve stress, from the bending, for both the rigidizer plate and any printed circuit board that is placed over the bend. Additional openings  86  are provided at one end of the rigidizer to receive the board connector  24  (shown in FIG.  1 ). Openings  76  are provided as guides for being placed in the curing fixture (shown in FIG.  13 ). The opening guides can be placed further apart for better tolerance in the alignment of the board to the rigidizer. 
     FIG. 12 shows how the PCB  76  and the rigidizer plate  18  bent upon themselves. Openings  72  in the PCB  76  are disposed over openings  74  in the rigidizer  18  to receive guide pins on the fixture for curing. Further, the PCB  76  is adhered to the rigidizer plate  18  over generally the total area of the PCB  76 . This being the case the PCB  76  fully covers the slots  30  (shown in FIG. 11) in the rigidizer plate  18 . Thus the internal stress on the PCB  76  created by the bend is relieved by the slots  30 . 
     A pass through, straight pin, gasket connector removes the need for the gasket cover  26 . With individual holes for each pin, better structural and RF shielding is achieved. The gasket on the connector  24  allows for sealing, but is optional. The through-hole pins allow for single reglow in processing and eliminates the wave soldering process needed for the first connector  24 . 
     FIG. 13 shows an exploded view of the PCB  76  adhered to the rigidizer plate  18  in the curing fixture  60  and  62 . Adhesive masks  71  are placed over the thermal pads  52  of the PCB  76  to prevent adhesive  48   b  from entering the thermal vias  38  (shown in FIG. 8) during curing. Kapton tape  73  is placed on the under side of the PCB  76 , opposite the adhesive masks  71 , to ensure that the adhesive mask  71  does not seep onto the tooling. After the adhesive masks  71  have cured the Kapton tape  73  is removed and a layer of spray primer, preferably DOW Corning Primer 4205 is applied substantially to the thermal via areas of the PCB  76  that is to be adhered to the rigidizer plate  18 . A layer of the adhesive  48   b  is placed between the rigidizer plate  18  and the PCB  76 . The adhesive layer  48   b  shown here is a second embodiment of the adhesive. The adhesive layer  48   b  is a pre-preg film of the liquid adhesive shown in FIG. 8,  9 , and  10 . However, since the pre-preg film is already a uniform thickness glass beads are not disposed therein but may be included if the material softens during the cure cycle. Once the PCB  76 , pre-preg film adhesive  48   b  and rigidizer  18  have been put together they are placed in a fixture  60 ,  62 . Alternatively the liquid adhesive  48   a  may be placed on the rigidizer plate  18  through spraying or through a stencil. 
     The fixture comprises a top plate  60  and a bottom plate  62 . The top plate  60  contains a plurality of depressions  66  to accommodate the adhesive masks  71 . Further openings  68  are disposed in the top plate  60  as guides for the guide pins  70  on the bottom plate  62 . The bottom plate  62  contains a main depression  64 , generally the same dimensions as the PCB  76 , and rigidizer plate  18 , and guide pins  70  to ensure proper placement of the rigidizer plate  18 , PCB  76  and the top plate  60 . Pressure and heat can then be applied to the fixture  60 ,  62  to cure the adhesive  48   a  or  48   b  to produce a permanent bond between the PCB  76  and the rigidizer plate  18 . 
     FIGS. 14 and 15 show plan views of the bottom plate  62  and top plate  60  of the fixture, respectively, for curing the adhesive. In particular FIG. 14 shows a plan view of the bottom plate  62 . The bottom plate  62  contains guide pins  70  and  84  to ensure that the top plate  60  is placed correctly onto the bottom plate  62 . Further, the bottom plate  62  contains a main depression  64  that has the same general dimensions as the rigidizer plate (not shown) and top plate  60 . A second minor depression  74  is placed in the rim of the bottom plate  62  to allow the insertion of a tool to pry the top plate  60  off after curing. With reference to FIG. 15 a plan view of the top plate  60  is shown containing openings  68  to receive the guide pins  70  from the bottom plate  62 . Another opening  86  is present to receive the additional guide pin  84  from the bottom plate  62 . The inside surface of the top plate  60  is generally flat save for the plurality of depressions  66  formed in the surface. The depressions  66  are for receiving the adhesive masks  71  (shown in FIG. 13) when curing the adhesive so that additional pressure is not applied to these areas. 
     FIG. 16 shows a plan view of a thermal pad  52  with no component affixed to it. The surface of the thermal pad  52 , as well as the walls of the thermal vias  38 , have been plated with a thermally conductive material  42 . The thermal vias  38  are in a Cartesian grid array in straight lines that intersect at generally ninety degree angles. The thermal vias  38  have a preferred diameter A of approximately 0.022 inches. The spacing of both the rows B and the columns C is preferably about 0.0355 inches. The size of the pad  52 , and the number size and spacing of the thermal vias  38  may change depending upon various factors. The pad on the top and bottom may be different from each other. Better heat sinking is achieved if the bottom pad is larger than the top pad. If the top pad gets too large problems with soldering can occur (parts swim), so the top pad is sized for optimum soldering and the lower pad as large as the signal density will allow preferably 200% of the size of the top pad. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.