Abstract:
A printed circuit board assembly employing a solder vent hole adjacent solder filled interconnect vias connecting to a conductive pallet, is disclosed. The solder vent hole allows gases to escape from an otherwise sealed cavity during solder reflow, relieving positive pressure and thereby allowing solder to flow into it. By providing an escape path for trapped air and gases generated during solder paste reflow, the out-gassing pressure and weight of the molten solder is sufficient to allow the solder paste to flow into the cavity.

Description:
RELATED APPLICATION INFORMATION  
       [0001]    This patent application claims the benefit pursuant to 35 USC §119(e) of the priority date of U.S. Provisional Patent Application Ser. No. 60/472,710, filed on May 22, 2003, the entire contents of which are hereby expressly incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to printed circuit board assemblies and methods for assembling electronic components into multi-layer assemblies. More particularly, the present invention relates to RF power amplifier circuit board assemblies and related methods of assembly.  
           [0004]    2. Description of the Prior Art and Related Background Information In high power electronics assemblies generating a relatively high amount of heat, it is typically necessary to attach a printed circuit board (PCB) containing electronic components to a metal substrate provided as a heat sink. RF power amplifiers are an example of such electronics module assemblies that incorporate heat dissipation means by design. The metal substrate for an electronics module may further be needed for grounding and electromagnetic interference (EMI) shielding. The use of nonconductive adhesives is a particular cost effective method of attaching a two sided printed circuit board to a metal substrate. The drawback to this approach is that there is no intimate metal-to-metal contact between the metal substrate and the metalized backside of the PCB. To achieve electrical and thermal contact at critical locations of this interface, it is desirable to use surface applied solder paste that is applied to a top surface of the PCB. However, trapped air and gases created during the solder reflow process create backpressure and prevent the molten solder from flowing.  
           [0005]    Accordingly, a need presently exists for a way to use cost effective nonconductive adhesive bonding and also provide effective electrical and thermal coupling to the substrate using surface applied solder reflow techniques.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    In a first aspect the present invention provides an electronics assembly comprising a printed circuit board having a via hole therethrough and an adjacent vent hole, a nonconductive adhesive layer having a first surface and a second surface, the first surface coupled to the printed circuit board, and a conductive substrate coupled to the adhesive layer second surface so that the adhesive layer is disposed between the printed circuit board and the substrate. The adhesive layer further has a void space overlapping the via hole and the vent hole. Reflowed solder is provided extending into the void space, the reflowed solder connecting the printed circuit board to the substrate.  
           [0007]    In a preferred embodiment of the electronics assembly a plurality of electronic devices are mounted on the printed circuit board. The substrate is preferably made from an electrically conductive material and the substrate provides electrical grounding for one or more of the electronic devices on the printed circuit board through the reflowed solder. A metalized pad may be provided plating and surrounding the via hole or vent hole. The substrate is also preferably made from a thermally conductive material and provides heat dissipation for the printed circuit board through the reflowed solder. For example, the substrate may be made from copper. The nonconductive adhesive layer may be composed of a thermal set epoxy.  
           [0008]    According to another aspect the present invention provides a method of interconnecting a printed circuit board, having a via hole and a vent hole, and a conductive substrate. The method comprises coupling the substrate to the printed circuit board using a nonconductive adhesive layer, the adhesive layer having a space forming a cavity between the substrate and the printed circuit board aligned to the via hole and vent hole. The method further comprises reflowing solder from a top surface of the printed circuit board through the via hole to electrically connect the printed circuit board to the conductive substrate while relieving pressure in the cavity by venting gases in the cavity through the vent hole in the printed circuit board.  
           [0009]    The method of interconnecting a printed circuit board to a substrate may further comprise plating the via hole or vent hole before the reflowing of the solder. The substrate is preferably made from a metal material and the substrate provides electrical grounding and heat dissipation for the printed circuit board through the reflowed solder. The adhesive layer is preferably an epoxy layer. For example, the epoxy layer may be a thermal set epoxy and the method comprises curing the epoxy at an elevated temperature.  
           [0010]    According to another aspect the present invention provides an RF power amplifier circuit board assembly comprising a printed circuit board having a via hole therethrough, an adjacent vent hole therethrough, and one or more RF power transistors mounted thereon. The assembly further comprises a nonconductive adhesive layer having a first surface and a second surface, the first surface coupled to the printed circuit board. An electrically conductive substrate is coupled to the adhesive layer second surface so that the adhesive layer lies between the printed circuit board and the substrate, the adhesive layer having a void space aligned to the via hole and vent hole. Reflowed solder extends through the via hole and the void space to the substrate, the reflowed solder electrically connecting the printed circuit board to the substrate. One or more conductive traces are provided on the printed circuit board electrically connecting to the reflowed solder.  
           [0011]    The RF amplifier circuit board assembly may further comprise a metalized pad plating and surrounding the via hole or the vent hole. The substrate is preferably composed of metal and the substrate provides electrical grounding and heat dissipation for the printed circuit board through the reflowed solder.  
           [0012]    According to another aspect the present invention provides a method of assembling and electrically coupling a metal substrate to an RF printed circuit board. The method comprises providing a printed circuit board having a via hole, a vent hole adjacent the via hole, one or more RF power transistors and one or more conductive traces. The method comprises providing a conductive metal substrate and a nonconductive layer between the printed circuit board and substrate, the nonconductive layer having an opening below the via hole and vent hole thereby forming a cavity. The method further comprises surface applying solder paste to the via hole and heating and reflowing the solder paste from the top surface of the printed circuit board through the via hole and opening in the nonconductive layer to electrically connect the conductive trace to the conductive metal substrate while relieving pressure in the cavity by venting gases in the cavity through the vent hole.  
           [0013]    In a preferred embodiment of the method of assembling and electrically coupling a conductive substrate to an RF printed circuit board, the conductive metal substrate is made from copper. The nonconductive layer preferably comprises an epoxy layer and the method comprises bonding the printed circuit board to the conductive substrate using the nonconductive epoxy layer. The conductive trace may be an RF ground connection.  
           [0014]    These and further aspects and features of the invention will be appreciated from the following detailed description.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is an exploded perspective view of an electronics assembly in accordance with a preferred embodiment of the present invention showing individual layers;  
         [0016]    [0016]FIG. 2 is a top view of an interconnect portion of the electronics assembly of FIG. 1;  
         [0017]    [0017]FIG. 3 is a cross-sectional view of the interconnect portion as seen along sectional line  3 - 3  of FIG. 2; and  
         [0018]    [0018]FIG. 4 is an exemplary RF printed circuit board assembly in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    [0019]FIG. 1 is an exploded perspective view of an electronics assembly  10  in accordance with a preferred embodiment of the present invention. In this example, assembly  10  comprises printed circuit board (PCB) layer  12 , nonconductive adhesive layer  14 , and a conductive support substrate or pallet  16 . Pallet  16  is composed of a good electrical and thermal conductor, preferably a metal such as copper. Printed circuit board layer  12  is attached to the conductive substrate  16  using the nonconductive adhesive  14  (as shown in FIG. 3). For example, a thermal set sheet epoxy  14  may be employed. The use of a thermal set nonconductive adhesive sheet (or preform) to attach a printed circuit board to a metal substrate is disclosed in U.S. Pat. No. 6,421,253 to Daniel Ash, Jr., issued Jul. 16, 2002, the disclosure of which is incorporated herein by reference in its entirety. The PCB layer  12  includes various active and passive electronic components (shown generally in FIG. 4) and in one preferred embodiment may be a two sided PCB having electronic components and/or conductive traces on both sides (as described in more detail in the &#39;253 patent). Some of these components will require electrical and/or thermal contact to the substrate. More particularly, in one preferred embodiment the assembly  10  is adapted for high power electronics applications, such as RF power amplifiers, and PCB layer  12  may further comprise a number of RF modules and discrete components. Such power amplifier components may include output couplers, power resistors, active amplifier devices (such as bipolar or LDMOS power transistors), EMI shielding (such as a ground plane in the circuit board, a shielding wall or lid), and other components well known to those skilled in the art.  
         [0020]    The assembly  10  includes layer interconnect portions  20  which provide the desired electrical and/or thermal contact of the PCB layer  12  to the pallet  16 . These interconnect portions comprise via holes  22  for electrically and/or thermally interconnecting layers  12  and  16  and adjacent vent holes  32  (interconnect portions  20  are illustrated in detail in FIGS. 2 and 3). The layers  12  and  14  are both illustrated in two sections providing respective spaces  30 ,  38  which allow mounting of certain components such as power transistors, directly on the pallet  16  for better thermal dissipation. Single piece layers  12  and  14  may also be employed, however, with the interconnect portions  20  providing the thermal coupling to pallet  16  or the layers  12  and  14  may have openings for mounting selected components directly on the pallet. The layers  12 ,  14 ,  16  may also have additional mounting holes  18  for structurally connecting the layers or coupling the pallet to a heat sink and/or to a lid. Adhesive layer  14  further includes void spaces  24  aligned with the interconnect portions  20 . As shown in FIGS. 1 through 4, void spaces  24  form a cavity in the electronics assembly  10  under both the via holes  22  and vent holes  32  through to pallet  16 .  
         [0021]    Referring to FIGS. 2 and 3 the layer interconnect portions  20  will be described in more detail. FIG. 2 is a top view of one of the layer interconnect portions  20  of electronics assembly  10 . Cavity  24  is referenced by the dashed lines as physically beneath PCB layer  12 . FIG. 3 is a cross sectional view of the interconnect portion  20 , taken along sectional line  3 - 3 .  
         [0022]    As shown in FIGS. 2 and 3, the electrical and/or thermal contact to the substrate  16  is provided by metalized pad(s)  28  with plated via holes  22  and surface applied reflowed solder  26 . The metalized pads  28  are electrically coupled to traces on the PCB layer and/or are configured adjacent to and in electrical and/or thermal contact with selected electronic components on the PCB layer  12  (as generally shown in FIG. 4). As illustrated in FIG. 3, after assembly and curing the epoxy layer  14  has a first surface  34  coupled to the printed circuit board layer  12  and a second surface  36  coupled to the substrate  16  so that the epoxy layer is disposed between the printed circuit board  12  and the substrate. As shown, spaces  24  in the nonconductive adhesive sheet  14  are provided for making contact between the printed circuit board and the metal substrate with the reflowed solder  26 . The spaces  24  in the epoxy sheet  14  would normally create air tight voids  24  as may be seen from FIG. 3, which would prevent surface applied solder  26  from flowing into the hole  22  to make adequate contact with the substrate  16  due to increased pressure of air trapped in the void as the solder is heated. Other gases and further backpressure may be created by the reflowed solder itself. To address this problem, vent hole  32  is provided in the PCB layer  12  adjacent to each via  22  and extending into void  24 . The via holes  22  and vent holes  32  may be provided in the PCB  12  through any of a variety of known techniques, including drilling, etching or during formation of the PCB material. The vent hole  32  may be provided either before or after assembly with the substrate  16 , prior to solder reflow processing, and may preferably be formed together with via hole  22  to facilitate alignment of the two holes in a closely spaced configuration. The vent hole  32  very effectively relieves gaseous backpressure in the void  24 , thereby allowing applied solder paste  26  to flow down through the vias  22  and connect the printed circuit board  12  to the substrate  16  as shown. Alternatively, the metal plating  28  may be applied to vent hole  32 . In this approach the solder paste is applied to via hole  22  as before and upon heating flows down into void  24  and flows across the void to the vent hole to contact the plating  28  and electrically and thermally connect plating  28  and substrate  16 . In this embodiment the vent hole rather than the via hole acts as the electrical and/or thermal connection point. (It will be readily appreciated that FIG. 3 equally illustrates this alternate structure with reference numeral  22  now indicating the vent hole and reference numeral  32  the via hole, although in this embodiment the solder  26  may be localized mostly in the void  24  and not extend as far up toward the top surface of PCB layer  12  as is shown in FIG. 3). In this embodiment the via hole  22  may be of larger diameter than the vent hole  32  to facilitate the solder flow and the two holes are preferably very closely spaced. Also, the gas venting assists in the solder flow and this embodiment may enhance this effect making it a preferred approach for some applications. Therefore, in either embodiment due to vent hole  32 , the desired good electrical and/or thermal contact to the substrate  16  is provided by the reflowed solder.  
         [0023]    In FIG. 4, an exemplary RF power amplifier circuit board assembly  40  of the present invention is illustrated. The illustrated assembly is highly schematic in nature as specific implementations will have a variety of layouts and components, as will be appreciated by those skilled in the art. The RF power amplifier circuit board assembly  40  comprises a printed circuit board  12  having interconnect portion(s)  20  (as illustrated in FIGS. 2 and 3). Further, RF power amplifier circuit board assembly  40  has one or more RF power transistors  44  and various other discrete components  42 ,  43  mounted thereon. The RF power amplifier circuit board also includes conductive circuit traces  46  that interconnect discrete components and also provide a ground connection, as explained herein, to the pallet  16  through interconnect portions  20 . Conductive traces  46  are shown on the top surface of PCB layer  12  in FIG. 4, for ease of illustration, but traces may also be configured on the bottom surface of PCB layer  12  as described in more detail in the &#39;253 patent. A ground plane layer may also be provided within layer  12  and this may also connect to ground traces  46  and the pallet  16  through interconnect portions  20 . Also, interconnect portions  20  may be disposed adjacent selected electronic components to provide thermal coupling to the pallet  16  for “hot spots”, even where electrical connections are not needed (as illustrated generally by component  43  and adjacent interconnect portion  20 ).  
         [0024]    The invention as illustrated herein additionally includes a method of assembling and interconnecting a PCB to a conductive substrate. The modifications to the method of assembly described in the above noted &#39;253 patent will be apparent from the above. A preferred embodiment of this method comprises assembling PCB  12  and substrate  16  using a nonconductive adhesive layer  14 , followed by reflowing solder through the via hole  22  in the PCB and the cavity  24  below the via hole  22 . More specifically the method comprises applying solder paste  26  to the via hole  22  from above using conventional techniques and heating the solder paste  26  to a temperature to provide reflow of the solder paste into the via hole. The vent holes  32  assist the reflow process by venting gases in the cavity  24  and relieving backpressure as described above. The solder paste thus flows from the upper surface of the printed circuit board  12  through the via hole  22  into cavity  24  to electrically connect to the conductive substrate  16  while substantially maintaining equalized pressure through the venting of the cavity  24 . Other aspects and features of the method of the invention may incorporate additional teachings of the &#39;253 patent. For example, details on the provision of a suitable nonconductive adhesive layer  14 , such as a thermal set epoxy perform, and bonding of the layers using the adhesive, are disclosed in the &#39;253 patent and may be employed herein, as will be readily appreciated by those skilled in the art.  
         [0025]    While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention. It is understood that such changes in the specific structure and method shown and described may be made within the scope of the claims, without departing from the spirit of the invention. Also, nothing herein is intended to limit or waive the full scope of the meaning of the claims or limit in any way the scope of equivalents of the claims under the judicial doctrine of equivalents.