Abstract:
A low thermal impedance printed circuit board assembly compatible with surface mount assembly reflow solder processes is presented. The low thermal impedance printed circuit board assembly may have filled vias soldered directly to solder balls of the surface mount assembly.

Description:
BACKGROUND 
       [0001]    In today&#39;s super high density printed circuit board assemblies containing numerous surface mount semiconductor devices, the heat generated by these semiconductor devices has become problematic and is quite a challenge to remove effectively. For example,  FIG. 1  illustrates a side cut-away view of part of a printed circuit assembly  10  with a device package  20  attached thereto, showing the heat path  55  from the device package  20  on the top side  12  of the printed circuit board  10  to the back side  14  of the circuit board  10 , where heat dissipation means generally reside (not shown). 
         [0002]    Many packages  20  have heat slugs (not shown) mounted inside the package  20  to help spread the heat generated by the semiconductor die (not shown). Then, the surface mount device  20  is soldered via solder balls  40  directly to solder pads  60  located on the printed circuit board  10 . Attached to each solder pad  60  is a small, thin surface trace  70  referred to as a pin escape. These pin escape traces  70  are then connected to via holes  30  in the printed circuit board  10 . Most device packages  20  allow for a multitude of solder pads  60 , a multitude of pin escape traces  70 , and a multitude of via holes  30 . The back side  14  of the printed circuit board assembly  10  usually contains fans for air cooling, a metal block filled with liquid for water cooling, or other means to remove the heat (not shown). 
         [0003]    One disadvantage of this approach to removing heat from a device package  20 , is that the heat travels through a package solder ball  40 , a solder pad  60 , a pin escape trace  70  and a via hole  30 , which significantly effects the path  55  of the heat flow. The solder pad  60  is not placed directly over the via hole  30 , because when the solder reflows  50 , it would potentially flow down the via hole  30  and starve the solder joint  45 . The pin escape trace  70  and the solder pad  60  constitute major components of the thermal resistance for the thermal path  55 . It is desirable to reduce the thermal impedance of the thermal path of high density surface mount printed circuit board assemblies. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    An understanding of the present teachings can be gained from the following detailed description, taken in conjunction with the accompanying drawings of which: 
           [0005]      FIG. 1  illustrates the thermal path from a surface mount package mounted on a top side of a printed circuit board assembly to the back side of the printed circuit board assembly according to the prior art. 
           [0006]      FIG. 2  illustrates the thermal path from a surface mount package mounted on a top side of a printed circuit board assembly to the back side of the printed circuit board assembly according to an embodiment of the present invention. 
           [0007]      FIG. 3  illustrates a process for manufacturing a printed circuit board assembly with high density surface mount device packages according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    As shown in  FIG. 2 , the present invention removes the solder pad and the pin escape trace from the thermal path of the heat transferring from the top side of the printed circuit board assembly to the bottom side of the printed circuit board assembly. This is accomplished by filling the via holes  130  in the printed circuit board assembly  100  with copper epoxy  180 . This epoxy may completely fill the via holes  130  and should deter solder from wicking down the via hole  130  during solder reflow when the surface mount package  120  is mounted to the printed circuit board assembly  100 . 
         [0009]    As shown in  FIG. 2 , the thermal path  155  from the surface mounted device package  120  on the top side  114  of the printed circuit board assembly  100  to the back side  112  of the printed circuit board assembly  100 , where thermal dissipation means may reside, is significantly reduced. The thermal impedance of the thermal path  155  between the surface mount device package  120  and the back side of the printed circuit board assembly  100  is greatly reduced over the thermal impedance of the assembly in  FIG. 1 . 
         [0010]    The epoxy mixture  180 , may be copper epoxy or silver epoxy, or any known thermally conductive epoxy. The epoxy may also be non-thermally conductive. The via hole  130  structure may be filled with printed circuit board resin. Even thought the via hole is filled with non-thermally conductive material, the via hole structure will be significantly improved in terms of thermal impedance, due to the fact that the land pad or solder pad and pin escape traces are removed from the path between the solder ball and the via, since the solder ball can now be soldered directly to the via hole  130 . As shown in the flow chart of  FIG. 3 , the via holes  130  of a provided ( 210 ) printed circuit board assembly are filled ( 220 ) with epoxy. 
         [0011]    The epoxy  180  may be planarized ( 230 ) with the surface of the printed circuit board assembly  100  with any known planarizing process, such as a chemical etch process, followed by a mild, quick sanding operation. The surface  182  of the epoxy  180  may be plated ( 235 ) with copper nickel, and/or gold, which is known as capping the via, or the via may be left as is, depending on the application and desired thermal and electrical performance. The via hole  130  may then be attached ( 240 ) directly to the solder ball  140  of the surface mount device package  120  using a surface mount reflow process ( 250 ). 
         [0012]    As will be readily appreciated, attaching device solder balls  140  directly to solid filled via holes  130  completely eliminates the pin escape traces and solder pads of the prior art, and also reduces the overall thermal path and thermal impedance of the printed circuit board assembly. This also permits the via hole  130  to be drilled to a larger diameter than in the past. The larger via hole  130  contains more copper due to the larger circumference of the via hole  130 . This will improve the thermal impedance of the via hole  130 . 
         [0013]    It will be appreciated that other methods and materials may be used within the spirit of the present invention. For example, the vias may be filled with copper epoxy, silver epoxy, or non-thermally conductive printed circuit board resin. The planarization process may be omitted if the solder stencil is thick enough to compensate for the surface roughness of the filled via  130 . If the planarization is done, this may be done by a process called nub removal, which involves a simple sanding process followed by a quick chemical polish. The vias  130  may be capped with copper, nickel, gold or with copper, pladium, gold. The nickel deposition may be electroplated or electroless deposition. The gold may be pure immersion gold or hard electroplated gold. The pladium may be either electroplated or electroless.