Abstract:
An improved PCB bond pad ( 22, 40, 50, 60 ) having a dimensioned geometry that improves solder re-flow and facilitates outgassing of bubbles generated in solder during re-flow to reduce voiding. The improved PCB bond pad design is particularly useful to improve re-flow for RF devices that are sensitive to voiding in solder after re-flow and provides an excellent ground plane/heat sink connection. The present invention includes a printed circuit board (PCB) having a patterned bond pad defining solder channels ( 30, 42, 52,  and  62 ). During re-flow, bubbles outgas through the channels from under a contact pad ( 34 ) of an overlying IC device thereby providing nearly 100 percent solder coverage at interface of device exposed pad and PCB bond pad.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is generally related to printed circuit boards (PCBs), and more particularly to PCBs including large bond pads adapted to receive RF devices and route RF signals thereacross to a heat sink/ground plane.  
         BACKGROUND OF THE INVENTION  
         [0002]    Printed circuit boards (PCBs) typically comprise of a non-conductive layer of material having electrically conductive circuit traces extending thereacross, either as a single layer or multi-layer of traces. These traces are also patterned to define bond pads adapted to be soldered to electronic circuitry disposed thereupon and to efficiently route signals therebetween via the circuit traces. Preferably, these traces are formed using pattern (masking) and etching techniques well known in the industry. Typically, the circuit traces comprise of copper or other electrically conductive materials suited to provide low impedance signal paths between the associated electrical components.  
           [0003]    Bond pads are typically defined in designated areas in the circuitry for receiving these electrical components. These bond pads are typically comprised of electrically conductive materials well adapted to receive the initial placement of the components thereon, and also facilitate adequate re-flow of the solder paste thereacross when the PCB is processed through a re-flow process. Copper is a preferable choice for bond pads, but is subject to oxidation. Accordingly, the bond pads may be coated with hot re-flowed solder or gold plating so as to provide both high electrical and thermal conductivity, are well suited for the re-flow process, and do not easily oxidize.  
           [0004]    With respect to PCBs adapted to receive high power RF components, and route RF signals across the signal traces and large bond pads connected to an underlying heat sink/ground plane, there is a particular need that the solder coverage to the bond pad receiving the RF device after re-flow have substantially reduced voiding in the solder joint to allow effective communication of RF signals thereacross and sink heat. Currently, standard assembly processes may result in solder coverage with up to 50% voiding. A typical PCB  10  having a bond pad  12  adapted to solder to an exposed pad on the underside of an IC is shown in FIG. 1. This voiding is illustrated in FIG. 2, and is recognized as bubbling  14  which occurs within the solder  16  between the PCB bond pad  18  and an exposed solder pad on the underside of the IC during a re-flow process. While the conventional solder coverage including voiding may be adequate from a thermal conductivity stand point and provide good electrical connection for some devices, this voiding is a substantial problem particularly with respect to RF devices communicating RF signals to a ground plane as they react unpredictably to these voiding situations.  
           [0005]    There is desired an improved PCB bond pad design which is well adapted to receive RF devices and route RF signals thereacross to a heat sink/ground plane, and which reduces the voiding problems inherent in conventional bond pad designs and re-flow procedure.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention achieves technical advantages as an improved printed circuit board and bond pad design whereby the bond pad is geometrically patterned to define channels therethrough, allowing the solder to outgas bubbles generated during a re-flow process from under the device being soldered. Preferably, the bond pad is patterned, such as to define channels, grooves, parallel lines, and/or radial lines, all which provide a path allowing a solder paste to re-flow and outgas from beneath the device being soldered to substantially reduce voiding to less than 10%. Particularly with RF devices needing an excellent connection to a ground plane/heat sink, this is a significant improvement over conventional printed circuit board and bond pad designs which are prone to produce unacceptable voiding.  
           [0007]    The present invention requires only a simple modification to conventional printed circuit board fabrication techniques, providing that the bond pads have patterns defining channels not conductive to solder during re-flow to allow outgassing. This design realizes a bond pad having between about 70 and 90% surface area coverage with the exposed pad of the overlying device mounted thereon, which achieves the technical advantages of improving solder re-flow behind the exposed pad into these recesses to facilitate outgassing of bubbles generated during re-flow to reduce voiding. With regards to RF devices, this is a significant improvement.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a prior art PCB design with a large bond pad adapted to solder to an exposed pad of an overlying IC;  
         [0009]    [0009]FIG. 2 is an illustration of a prior art bond pad design soldered to an exposed pad of an overlying IC with voiding existing in the solder after a re-flow process;  
         [0010]    [0010]FIG. 3 illustrates a PCB according to the present invention with an improved channeled bond pad facilitating outgassing during re-flow;  
         [0011]    [0011]FIG. 4 depicts an enlarged view of the bond pad embodiment of FIG. 3 whereby the recesses are defined in a radial pattern;  
         [0012]    [0012]FIG. 5 is a top view of a solder pad/bond pad combination according to the present invention providing recesses/channels between portions of the bond pad to improve solder re-flow and outgassing during re-flow, whereby this design is embodied as a series of grooves defined in a lattice arrangement;  
         [0013]    [0013]FIG. 6 depicts another embodiment including radial lines terminating at alternative distances from a focal point and with an increased number of radial lines as compared to the embodiment of FIG. 4;  
         [0014]    [0014]FIG. 7 is an embodiment depicting a series of parallel grooves defined through the bond pad;  
         [0015]    [0015]FIG. 8 depicts the embodiment of FIG. 4 after a re-flow processes, depicting the uniform solder coverage on the bond pad with minimal voiding as represented by the bubbles;  
         [0016]    [0016]FIG. 9 depicts the embodiment of FIG. 5 after a re-flow process also depicting uniform solder coverage on the bond pad with minimal voiding after re-flow;  
         [0017]    [0017]FIG. 10 depicts the bond pad embodiment corresponding to FIG. 6 after a re-flow process;  
         [0018]    [0018]FIG. 11 depicts the bond pad embodiment of FIG. 7 after a re-flow process;  
         [0019]    [0019]FIGS. 12 and 13 depict diagrams of thermal efficiency for the various designs for solder and palladium plated devices, respectively; and  
         [0020]    [0020]FIG. 14 depicts the methodology for fabricating the PCB with the improved bond pad design minimizing voiding. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    Referring now to FIG. 3, there is depicted an improved PCB  20  with an improved bond pad arrangement designed generally at  22 . Bond pad  22  is seen to form a portion of the printed circuit board which is in electrical and thermal contact with the larger heat sink/ground plane  24 , which may include vias/feed-throughs  26  extending to another layer forming a heat sink and ground plane. Advantageously, as shown in FIG. 4, the PCB bond pad  22  is patterned and/or partitioned by channels  30  to facilitate the re-flow of solder paste during a re-flow process and facilitate outgassing of bubbles generated in the solder during re-flow to substantially reduce voiding therein. The channels  30  defined in the bond pad  22  render the bond pad  22  non-planar. The outline of an overlying exposed solder pad  34  of an overlying IC illustrates the orientation of the solder pad  34  over the significantly larger bond pad  22 . Advantageously, rather than conventionally having one large continuous planar bond pad, solder paste flow channels  30  are defined which permit the solder paste to flow across the bond pad, inducing outgassing of bubbles in the channels, and achieving a good electrical and thermal interface.  
         [0022]    The grooves  30  terminate at, but are spaced slightly from, a focal point  28  defined generally beneath the center of the overlying solder pad  34 . This bond pad design  22  also facilitates the solder paste to flow through the corresponding grooves  30  during a re-flow process, facilitating outgassing of bubbles generated in the solder during re-flow to minimize voiding in the solder as previously described. In the embodiment shown in FIG. 4, the portion of the underlying bond pad  22  that is directly below the overlaying solder pad  34  may have a surface area of 75%. These solder pad flow channels, embodied here as radial channels  30 , radially direct any bubbles outwards from beneath the center of the overlying solder pad  34 .  
         [0023]    With regards to the embodiment of FIG. 5, another embodiment of a bond pad is shown as bond pad  40  seen to be partitioned by a plurality of horizontally and vertically extending channels  42  forming a lattice configuration. These channels define a plurality of rectangular bond pad portions  44  which collectively comprise the bond pad  40 .  
         [0024]    As appreciated in FIG. 5, the bond pad  40  area is significantly larger than the overlying corresponding solder pad  34 , and may comprise an area 2× that of the corresponding solder pad  34 . This large bond pad allows an adequate portion of solder paste to be applied prior to re-flow and accounts for shrinkage of solder paste during re-flow. As depicted in FIG. 5, the underlying bond pad portions  40  may have a contact area of approximately 69% of the corresponding overlying solder pad  34 . However, for RF circuits components, a continuous solder connection is achieved with minimal voiding to achieve maximum RF operating characteristics, as is shown in FIG. 9 to be discussed shortly.  
         [0025]    Referring now to FIG. 6, there is shown at  50  yet another embodiment similar to the bond pad arrangement  22  of FIG. 4, but including shorter radial channels  52  alternatingly disposed between the longer radial channels  53  as shown. These radial channels  52  provide additional solder re-flow channels beneath the corresponding solder pad  34  to facilitate the outgassing of bubbles from thereunder. These alternately disposed radial channels  52  extend towards the focal point  56 , but terminate at a point more distant from focal point  56  than the distal ends of the longer channels  53 , as shown. Each of these flow channels  52  &amp;  53  are separated from one another by portions of the bond pad  50  exposing the PCB board to facilitate the outgassing of bubbles from beneath the solder pad  34 .  
         [0026]    Referring now to FIG. 7, there is depicted at  60  yet another embodiment of a bond pad with these solder flow channels depicted as a plurality of parallel channels  62  extending through the corresponding bond pad  60 . In this embodiment, roughly 77% of the solder pad  34  overlies a corresponding portion of the bond pad  60 , yet an excellent thermal and RF solder contact is achieved.  
         [0027]    As previously described, the channels extending through the corresponding bond pad are defined during the mask and etching process which defines the electrical circuit traces on the printed circuit board as well as the corresponding bond pads. Thus, the conductive patterned portions of the bond pad are disposed above the printed circuit board base material, including those PCB portions defining channels between corresponding portions of the bond pads. A solder mask is applied to the bond pads after this mask and etching process. A solderable protective coating such as hot re-flowed solder or gold plating is applied and thus is non-planar due to the underlying non-planar bond pad structure.  
         [0028]    Turning now to FIGS.  8 - 11 , there is depicted the bond pad structures corresponding to the embodiments of FIGS.  4 - 7 , depicting solder uniformly flowed across the upper surface of the bond pad, and also fills the corresponding channels. Importantly, there is depicted the substantial reduction of voiding in the solder after re-flow due to the induced and permitted outgassing of bubbles generated in the solder during re-flow. Test results have achieved voiding in the range of no greater than 10%, which is a significant improvement over the nearly 50% voiding which may occur in some conventional designs. Additional solder paste is provided as compared to conventional designs, which has nominal cost and process concerns, and which ultimately flows under the corresponding solder pad  34  into the corresponding solder flow channels, as depicted by the shading.  
         [0029]    Referring now to FIG. 12 there is depicted a graph of the thermal performance of the various designs using solder plated devices. Line  70  illustrates the thermal performance of the arrangement  22  depicted in FIG. 4, line  72  depicts the performance of the arrangement  40  shown in FIG. 5, line  74  depicts the thermal performance of the arrangement  50  shown in FIG. 6, and line  76  depicts the thermal performance of the design  60  shown in FIG. 7. All four designs are seen to have excellent thermal performance. As a reference, devices were assembled on to PCB&#39;s using Alpha 606 solder paste; a 0.006 inch thick metal stencil with an aperture equal to the exposed pad on the bottom of the device and processed though a Heller 1500 infrared solder re-flow oven operating at a belt speed of 50 centimeters per minutes. The thermal impedance measurement was performed at 2 watts.  
         [0030]    Referring now to FIG. 13, there is depicted the same thermal performance information whereby the device leads and exposed pad had a palladium finish instead of solder plate. The lines  80 ,  82 ,  84  and  86  all correspond to the designs  22 ,  40 ,  50  and  60  depicted in FIGS.  4 - 7 , respectively.  
         [0031]    Referring now to FIG. 14, there is shown a flow diagram of a printed circuit board fabrication methodology  90  which may be used to fabricate the bond pad arrangements previously discussed. At step  92 , the printed circuit board core material is prepared and shaped according to the particular design desired.  
         [0032]    At step  94 , vias are drilled through the PCB core material at locations according to the PCB design, and these vias are plated as is conventional in the art.  
         [0033]    At step  96 , both the electrical circuit traces and the bond pads are defined using a conventional pattern and etching process. Importantly, it is during this step  96  that this process is modified from conventional approaches, whereby the bond pads are patterned to define solder flow channels therethrough as previously described. This pattern and etching is well known in the industry, however, the partitioning and shaping of the bond pad is unique.  
         [0034]    At step  98 , the solder mask is applied to define and expose the bond pads.  
         [0035]    Finally, at step  100 , hot re-flow solder may be applied to coat the exposed copper pads and vias.  
         [0036]    Though the invention has been described with respect to a specific preferred embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present application. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.