Abstract:
An RF/EMI shield has a planar conductive element and a plurality of solder spheres extending around the edges making electrical and mechanical contact with the conductive element to form a shield which can be soldered in a surface mount process directly over components needing shielding. The solder spheres have a diameter sufficient to provide the desired clearance between the shielding element and the component being shielded and a melting temperature equivalent to existing solder contacts to which the shield is bonded.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a shield against RF and EMI interference and particularly one which employs a ball grid array (BGA) mounting. 
       BACKGROUND OF THE INVENTION 
       [0002]    With the continued miniaturization of electrical circuits and circuit boards, the use of conventional radio frequency interference (RFI) and electromagnetic interference (EMI) shielding has become an increasing challenge. In the past, shields utilizing pre-tinned tabs have been placed through slots in the circuit board to cover a circuit desired to be shielded. The tabs are then twisted to pull the shield tightly against the board and subsequently wave soldered to ensure electrical contact with a shielding ground connection. 
         [0003]    Another type of shield employed is the V-groove shield which utilizes pre-tinned V-grooves inserted through holes in a circuit board. These also require the mechanical step of twisting the mounting tabs to pull the shield tightly against the board and subsequently wave soldering to make an electrical connection. As component density increases, the available real estate on a circuit board is at a premium, and designs become more constrained making the use of such conventional shielding techniques even more difficult. 
         [0004]    There exists a need, therefore, for an improved smaller shielding structure which is cost effective as compared to the pre-existing shielding techniques. 
       SUMMARY OF THE INVENTION 
       [0005]    The system of the present invention overcomes the labor intensive and cost of existing RF/EMI shielding structure and techniques by employing a shield comprising a substrate having a planar conductive element placed on one side thereof and a plurality of solder spheres extending around the edges and making electrical and mechanical contact with the conductive element to form a shield cover which can be soldered in a surface mount process directly over components needing shielding. The solder spheres have a diameter sufficient to provide the desired clearance between the shielding element and the component being shielded and a melting temperature equivalent to the existing solder contacts to which the shield is bonded. 
         [0006]    In one embodiment, the shielding element was a thin film conductive material, such as copper. In other embodiments, the shielding element may comprise a wire mesh or printed grid having a size selected to block selected high frequency interference. The spacing of the solder spheres is likewise selected to provide shielding for the gap between the conductive element and the circuit board to which the shield is mounted. 
         [0007]    In another embodiment of the invention, the shield may be divided into several sections by a plurality of lines of solder spheres separating the shield into separate areas for shielding individual components on the circuit board from adjacent components. In yet another embodiment of the invention, the shield may comprise a plurality of rows of solder spheres in staggered or aligned relationship to improve the shielding of the circuit component. 
         [0008]    Such a shield system can be employed to piggyback over existing ball-grid array circuit structure to provide the desired shielding or on other conventional circuit component mounting structures. The resultant seal is a relatively inexpensive component which is easily assembled to an existing circuit to provide the desired RF/EMI shielding with a minimum of labor. 
         [0009]    These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0011]      FIG. 1  is a perspective view of a circuit board with a ball-grid array (BGA) circuit mount overlaid by a stacked BGA shield of the present invention; 
           [0012]      FIG. 2  is a perspective underside view of the shield of the present invention; 
           [0013]      FIG. 3  is a greatly enlarged fragmentary cross-sectional view of the BGA shield and connection to the circuit board, as shown in  FIG. 1 ; 
           [0014]      FIG. 4  is a bottom perspective view of a BGA shield according to the present invention divided into two zones; 
           [0015]      FIG. 5  is a bottom perspective view of a BGA shield of the present invention, showing an alternative shape for a particular application; 
           [0016]      FIG. 6  is a bottom perspective view of a BGA shield of the present invention employing a conductive wire mesh for the shielding element; and 
           [0017]      FIG. 7  is a bottom perspective view of the BGA shield of the present invention showing the use of multiple rows of solder spheres for attachment to a circuit board. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    Referring initially to  FIG. 1 , there is shown a circuit board  10  for an electrical circuit, such as a satellite digital audio radio (SDAR) or other circuit which includes RF components which can be sensitive to RF/EMI interference. Circuit board  10  includes, in this example, a BGA circuit package  20  including soldering spheres  22  which couple the circuit components contained within package  20  to conductive pads on the circuit board for intercoupling the BGA circuit to the remaining circuit components. The components contained in the BGA circuit  20  are RF/EMI sensitive and are shielded by the BGA shield  30  of the present invention, which likewise includes a plurality of solder spheres  32  extending around the edges thereof, as best seen in the  FIG. 2  underside perspective view of the BGA shield  30  of the present invention. 
         [0019]    Shield  30 , as shown in  FIG. 2 , can be made of commercially available circuit board material, such as Chem 1, 2 or 3 or a FR4 material. Each of these comprise a nonconductive substrate  34  ( FIG. 3 ), which is a relatively thin substrate having a thickness of, for example, about 0.025 inches, although it can range from about 0.015 inches to about 0.040 inches in some embodiments. Substrate  34  can be paper or other suitable material to which there is bonded a conductive shielding element or layer  36 , typically of copper, which has generally the same configuration as that of the substrate. The central area  31  is covered with a solder mask, leaving bare copper edges  35 , upon which the solder spheres  32  are placed in spaced relationship along each of the edges of the BGA shield  30 . 
         [0020]    The solder spheres are made of a 63/37 eutectic solder material or SAC  305  lead-free alloy. This solder has a molten temperature of about 183° C. and 217° C., respectively, equivalent to the solder paste used to attach the spheres  32  to the edges of the shield  30 . The spheres  32  have a diameter of from about 1 mil to about 1.8 mil and can vary depending upon the size of the circuit component to which the shield is applied. In one application, they had a diameter of about 1.6 mil and were spaced apart from one another by about 1 mil. As used herein, the term “sphere” as used to describe the solder spheres  22  means a generally rounded ball-like structure that may not be perfectly spherical but can deviate, such as being ellipsoidal, as long as they function to hold the conductive element in a desired position with respect to a circuit element being shielded. 
         [0021]    The shield  30  need not be piggybacked on top of an existing BGA circuit package but can be applied to any circuit board with surface-mounted components which are sensitive to RF and EMI interference. The solder spheres  32  are placed on the edges of the substrate in contact with the conductive shield material and heated by standard techniques sufficiently to bond to the shielding element holding them in place until the shield is subsequently placed on the main circuit board  10  and heated by standard hot air methods to surface mount the shield to conductive pads or conductors surrounding the electrical component being shielded. The solder spheres will bond to contact electrodes or pads which are typically grounded to provide the desired shielding to the electrical component over which the shield is placed as seen in  FIG. 3 . The spacing between the spheres is selected to be sufficiently close to effectively block RF and EMI frequencies which could interfere with the operation of the circuit being shielded and typically has a spacing of about 1/20 of the wavelength to be blocked. For a frequency of 14.4 GHz, this would be about 1 mm. For 2.4 GHz, the spacing can be about 6 mm. 
         [0022]    The solder spheres  33  are located on shield  30  by three methods. The first method is by discrete placement using standard surface mount technology (SMT) equipment by placing the spheres in tape and reel or by using bulk feeds in the SMT equipment. The spheres are picked from the tape and reel and placed in the pre-deposited solder paste. The second method is to use an IC fabrication machine which place flux and gang pick the solder spheres for placement in the flux. The third method is to use a SMT solder printer machine to screen print the spheres onto the paste or flux on the board. 
         [0023]    There are two methods of bonding the spheres  33  to the shield  30 . The first method is to use flux such as  37  shown if  FIG. 3 . This is the method most used with the IC machines. The second method is to use solder paste (also illustrated as  37  in  FIG. 3 ) as in the SMT environment. By depositing solder paste, standard SMT processes can be employed. 
         [0024]    The shields  30  do not require wave soldering as it is a SMT process. The solder spheres are first bonded to the shield as described above during the construction of the shield. The shields  30  are then mounted to the package  20  or to a circuit board by placing them as discrete components in the SMT process. They are then processed in the SMT reflow oven, bonding the solder spheres to the main circuit board. After the placement of the solder spheres, they are held on the board by either the tackiness of the flux or the adhesion of the solder paste prior to reflowing the sphere to the circuit board pad. 
         [0025]      FIG. 3  is an enlarged cross-sectional view of the shield  30  of the present invention, which includes the substrate  34 , the conductive shielding element  36  bonded thereto, and a solder sphere  32  which is soldered to the shielding element  36  by a solder joint  33  and to the circuit conductor  26  of the circuit board  21  to which the shield  30  is attached, again, by a solder joint  33 . Areas of the BGA shield  30  and circuit board  21 , which do not have an exposed conductive surface, are covered by a solder mask  38  and  28 , respectively, as seen in  FIG. 3 . 
         [0026]    As seen in  FIG. 4 , a zoned BGA shield  130  is shown which also includes a substrate  134  with a conductive shielding element  136  overlying and bonded to the substrate  134 . Shield  130  is fabricated in the same manner as shield  30  but is divided into sections  140  and  141  by a row  137  of spheres  132 . Solder spheres  132  are soldered to the exposed conductive edges  135  of the shield along each of the edges and an exposed conductive strip  135  for the row  137  of such spheres which bisect the shield  130  into two generally rectangular sections  140  and  141 . These sections are located to overlie separate circuit components on a circuit board, such as board  10  shown in  FIG. 1 , which has adjacent components, such as microprocessors, which may be subject to RF/EMI interference. Thus, shield  130 , shown in  FIG. 4 , can be employed for isolating circuit components from one another on the same circuit board. The geometry of a multiple section shield such as shield  130  can be selected to include any number of a plurality of sections such as  140  and  141  and of any desired shape depending upon the circuit components being shielded and their location on a circuit board. 
         [0027]      FIG. 5  is an alternative embodiment of the invention showing a shield  230  which likewise includes a substrate  234  and shielding element  236  made of a conductive material. This shield is likewise fabricated of the same material and by the same process as shields  30  and  130 . The peripheral edge of the generally L-shaped shield  230  is surrounded by solder spheres  232  of the same size, shape, spacing, and material as described in the previous embodiments. The L-shaped circuit  230  is illustrative of only one of any number of configurations that the shield may be formed into for covering RF components to be shielded. Thus, the shield can take any geometric shape as necessary for providing the shielding for a single or group of components desired to be shielded. The irregular shape of shield  230  can also be divided into sections as illustrated by the phantom line  237  representing a dividing line of spheres similar to that shown in  FIG. 4 . 
         [0028]      FIG. 6  is an alternative embodiment of the invention showing a shield  330  which includes, instead of a solid conductive shield element such as  36 , 136  and  236  in the previous embodiments, a wire mesh  338  which is bonded to a nonconductive substrate  334  and is made of a conductive material, such as copper mesh. The mesh, in one embodiment of the invention had openings of about 1 mm corresponding to the spacing of the spheres in the previous embodiments to block frequencies having a wavelength of fro example about 14.4 GHz. Again, the edges of shield  330  are surrounded by solder spheres  332  bonded to the edges of the mesh  338  to provide surface mount capability of the shield  330  to an existing circuit board with the solder spheres being aligned with grounded conductive pads or ribbon conductors on the underlying circuit board or circuit package. Instead of a mesh material  338 , the mesh pattern can be screen printed onto substrate  334  utilizing a screen printing and masking process. The mesh size can be varied according to the 1/20 relationship described above for blocking selective frequency interference. 
         [0029]    Finally,  FIG. 7  illustrates yet another embodiment of the invention in which a shield  430  is employed and includes a substrate  434 , and a conductive element, such as  436 , which can be a solid film of conductive material, such as copper as in the previous embodiments, or a wire mesh, such as mesh  338  in the embodiment shown in  FIG. 6 . Instead of a single row of solder spheres, this embodiment employs a plurality of rows  501  and  502  of solder spheres  432 . These rows may be aligned or alternately staggered to provide spacing between the spheres for blocking predetermined frequency interference from entering the circuit component being shielded. In this manner, the shielding effect of the solder spheres themselves can be adjusted and chosen to block the undesirable RF/EMI interference. 
         [0030]    It will be understood by those who practice the invention and those skilled in the art, that various modifications and improvements may be made to the invention without departing from the spirit of the disclosed concept. The scope of protection afforded is to be determined by the claims and by the breadth of interpretation allowed by law.