Patent Publication Number: US-2007119911-A1

Title: Method of forming a composite standoff on a circuit board

Description:
FIELD OF THE INVENTION  
      This invention relates to a circuit board having a surface mounted electronic components and more particularly to a method of forming a standoff on the circuit board to space the electronic component from the surface of the circuit board.  
     BACKGROUND OF THE INVENTION  
      An electronic component is typically mounted on a circuit board by attaching terminations on the bottom of the electronic component to contact pads on the circuit board by soldering techniques. See for instance U.S. Pat. No. 6,525,628 B1 issued to Andrew P. Ritter et al Feb. 25, 2003. In such a mounting process one or more standoffs may be formed on the circuit board to space the bottom surface of the electronic component from the surface of the circuit board prior to the soldering operation. See for instance, U.S. Pat. No. 5,056,215 entitled “Method of Providing Standoff Pillars”, granted to James A. Blanton Oct. 15, 1991 and U.S. Pat. No. 6,773,957 B2, entitled “Stereographic Method and Apparatus for Fabricating Spacers for Semiconductor Devices and Resulting Structures”, granted to Warren M. Farnworth Aug. 10, 2004.  
     SUMMARY OF THE INVENTION  
      This invention provides a method of forming a precise and controllable standoff on a circuit board while processing the circuit board that does not require any additional process steps. Forming precise and controllable standoffs are particularly important in mounting larger electronic components such as the RC arrays disclosed in the Ritter et al &#39;628 patent because the standoff height can increase the solder joint life expectancy significantly.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a flow chart for processing a circuit board for surface mounting an electronic component to the circuit board that includes forming a composite standoff according to the method of the invention;  
       FIG. 2  is a fragmentary top view of a circuit board having a composite standoff formed by the method of the invention;  
       FIG. 3  is a section taken substantially along the line  3 - 3  of  FIG. 2  looking in the direction of the arrows; and  
       FIG. 4  is a sectional view of an electronic component mounted on the circuit board shown in  FIGS. 2 and 3 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      Referring now to  FIG. 1 , a flow chart for processing a circuit board and surface mounting an electronic component on the circuit board is illustrated. Briefly, a suitable nonconductive substrate typically of plastic material, is coated with a thin layer of conductive material, usually copper or a copper alloy. The thin conductive layer is then masked with a pattern that corresponds to the shapes and locations of desired electrical circuit traces and in this instance, the pattern also includes one or more bases for desired composite standoffs. The unmasked areas of the conductive layer are then etched to leave the desired electrical circuit traces and standoff bases. The mask is then removed from the electrical circuit traces and standoff bases. At this point it should be noted that the electrical circuit traces commonly include contact pads for making electrical connections to electronic components.  
      Next a solder mask is applied to portions of the electrical circuit traces and standoff bases, for example by printing or spray coating the portions of the traces except for the contact pads that need solder. At this point it should be noted that the standoff bases are also covered with the solder mask to build up composite standoffs when the solder mask is applied to portions of the electrical circuit traces. The solder mask forms covers on the standoff bases that preferably completely cover the standoff bases.  
      Any suitable material known in the art may be used for the solder mask. For example, a suitable material for the solder mask is 77MA that is commercially available material from Vantico. This material is a generally photoimageable, modified epoxy.  
      Legend ink is then printed on the circuit board to identify the various mounting areas for the various electronic components which is a well known processing step. However, at the same time, legend ink is also printed on the covers of the composite standoffs to form shims atop the covers that provide a precise height for the composite standoffs. Any suitable legend ink known in the art may be used. Suitable legend inks are commercially available from Taiyo which is a well known company.  
      Solder paste is then printed on the unmasked contact pads. The solder paste is preferably applied after the legend ink is applied to the circuit board and the covers of the composite standoffs.  
      An electronic component is then placed on the solder paste atop the contact pads and on the composite standoffs. The solder is then reflowed and solidified to attach the electronic component to the circuit board.  
       FIG. 2  is a fragmentary top view of a circuit board  10  having a composite standoff formed by the method of the invention.  FIG. 3  is a section taken substantially along the line  3 - 3  of  FIG. 2  looking in the direction of the arrows. As indicated above, circuit board  10  comprises a suitable nonconductive substrate  12 . The circuit board  10  is processed to provide a desired pattern of electrical circuit traces  14  and standoff bases  16  comprising of a thin layer of copper, copper alloy or other suitable conductive material. The electrical circuit traces  14  include contact pads  18  for making electrical connections with electronic components. In this particular example, circuit board  10  has two sets of eight contact pads  18  for making electric electrical connections with two electronic components (not shown).  
      As shown in  FIGS. 2 and 3 , the typical standoff base  16  has a cover  20  of solder mask material. Cover  20  preferably completely covers the standoff base  16 . As indicated above, the cover  20  is formed when the solder mask is applied to portions of the electrical circuit traces  14  (except for the contact pads  18  that need solder) as well as the standoff base  16 .  
      Legend ink is then printed on the circuit board  10  to identify the various mounting areas for various electronic components. For instance, the legend “ZRR1” indicated generally at  22  in  FIG. 2  is printed near the mounting area on the circuit board  10  for an electronic component as shown by the dashed line  24 . At the same time, legend ink is also printed on the cover  20  of the standoff to provide a shim  26  that finishes the composite standoff  27  providing a precise and controllable height for the composite standoff  27 . It should be noted that legend ink has a relatively high viscosity in comparison to solder and that the legend ink is applied by a printing process that can be localized. Consequently, the form, shape and height of the shim can be controlled in a precise manner by varying the printing process parameters such as number of prints, emulsion thickness, and curing process. Hence a precise and controllable height is achievable.  
      Solder paste  28  is printed on the contact pads  18  as shown in  FIG. 4 . The solder paste is preferably applied to contract pads  18  after the legend ink is applied to the circuit board  10  and the composite standoffs  27 . In this manner, the legend ink can be applied by the manufacturer of the printed circuit board while the solder paste can be applied at another manufacturing facility, for instance at the manufacturing facility of the printed circuit board customer.  
      An electronic component  30  is then placed on the solder paste  28  atop the contact pads  18  and on the composite standoff  27  that is midway between the contact pads  18 . The solder paste  28  is then reflowed and solidified to attach an electronic component  30  to circuit board  12  as shown in  FIG. 4 .  
      Thus the method of the invention forms a composite standoff of precise height for locating the electronic component  30  a precise distance above the circuit board  10  without requiring any additional steps during the processing of the circuit board  10 . With composite shim  26 , the solder standoff height of electronic components can be increased easily to increase the solder joint life expectancy.  
      While the composite standoff  27  is illustrated as having a wavy shim  26  it should be understood that the shim  26  can take many shapes and forms. For instance shim  26  could be a single strip line, a double line or a plurality of staggered dots. Moreover, while the composite standoff  27  is illustrated as a single linear standoff midway between two rows of contact pads  18 , it should be understood, that any number and/or shapes of standoffs can be formed. For instance, the four corner standoffs shown in U.S. Pat. No. 5,056,215 cited above could be formed as composite standoffs by the method of the invention.  
      In other words, it will be readily understood by those persons skilled in the art that the present invention is susceptible of broad utility and application. Many embodiments and adaptations of the present invention other than those described above, as well as many variations, modifications and equivalent arrangements, will be apparent from or reasonably suggested by the present invention and the foregoing description, without departing from the substance or scope of the present invention. Accordingly, while the present invention has been described herein in detail in relation to its preferred embodiment, it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for purposes of providing a full and enabling disclosure of the invention. The foregoing disclosure is not intended or to be construed to limit the present invention or otherwise to exclude any such other embodiments, adaptations, variations, modifications and equivalent arrangements, the present invention being limited only by the following claims and the equivalents thereof.