Abstract:
An underlying recess is provided for component placement beneath Ball Grid Arrays allowing closer proximity for decoupling capacitors and other components. The underlying recess placement of components assists in minimizing reliability issues concerning surface mount components and provides closer proximity placement of components. The underlying recess placement of components is particularly useful for overcoming the problem of parasitic inductance of more distant component placements known in the art.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to Ball Grid Arrays and is particularly concerned with proximate component placement, including decoupling components. 
       BACKGROUND 
       [0002]    As electronic integrated circuit (EIC) packages such as Ball Grid Array (BGA) packages increase in density, in interface connection density, and in clock speed, the requirements for electrically decoupling a BGA device become more stringent. It is advantageous to place these decoupling capacitors as close to the BGA pads as possible. Typical placement of surface-mount decoupling capacitors is adjacent to the BGA device on the same side of the electronic circuit board or on the opposite side of the electronic circuit board, connected by vias through the circuit board. Both of these techniques can introduce parasitic inductance due to the length of the routing leads and the length of the conductive path through the via itself. 
         [0003]    One approach to providing component placement which maximizes proximity is placing the decoupling capacitors on the bottom of the BGA itself, amidst the grid array, as described in US Pat. Nos. U.S.8,806,420 “In-Grid On-Device Decoupling for BGA” and U.S.8,863,071 “De-Pop On-Device Decoupling for BGA”, the entire contents of which are incorporated herein by reference. A difficulty which arises with this approach is the possible reluctance of BGA manufactures to assume the responsibility and testing entailed with placement of the components on the BGA, as opposed to simply providing the BGA itself. 
         [0004]    Therefore, it continues to be desirable to provide alternate methods of placing small components within close proximity of Ball Grid Arrays. 
       SUMMARY 
       [0005]    A brief summary of various exemplary embodiments is presented below. Some simplifications and omissions may be made in the following summary, which is intended to highlight and introduce some aspects of the various exemplary embodiments, but not to limit the scope of the invention. Detailed descriptions of a preferred exemplary embodiment adequate to allow those of ordinary skill in the art to make and use the inventive concepts will follow in later sections. 
         [0006]    According to an aspect of the invention there is provided a through-hole printed circuit board (PCB) having a ball grid array (BGA) of BGA pads on one side of the PCB, arranged in a tight-pitch grid pattern; a milled recess on the same side of the PCB in the PCB adjacent and between a first and a second of the BGA pads of the ball grid array of BGA pads; the recess sized to contain a Surface Mount Component (SMC); a first pair of separated conductive pads at the bottom of the milled recess, each pad respectively conductively coupled to the first and second BGA pads. 
         [0007]    In some embodiments of this aspect of the invention a second pair of separated conductive pads surround the opening of the milled recess, each pad respectively conductively coupled to the first and second BGA pads. 
         [0008]    In some embodiments of this aspect of the invention the tight-pitch grid pattern has a 1 mm pitch. In some of these embodiments the SMC has a industry nominal  0201  size; and the first and second BGA pads are diagonally situated relative to the grid pattern. In others of these embodiments the SMC has a industry nominal  01005  size; and the first and second BGA pads are diagonally situated relative to the grid pattern. 
         [0009]    In some embodiments of this aspect of the invention at least one BGA pad of the ball grid array of BGA pads has been removed; and the milled recess is located in the array grid where the at least one BGA has been removed. In some of these embodiments the SMC has an industry nominal  01005  size, in some an industry nominal  0201  size, in some an industry nominal  0402  size, and in some an industry nominal  0603  size. 
         [0010]    In some embodiments of this aspect of the invention the milled recess was milled by a laser. 
         [0011]    According to another aspect of the invention there is provided a method of manufacturing a multilayer PCB wherein the PCB has a ball grid array (BGA) of BGA pads on one side of the PCB arranged in a grid pattern; the method having the steps of: milling a recess on the same side of the PCB in the PCB adjacent and between a first and a second of the BGA pads of the ball grid array of BGA pads, the bottom of the recess having arranging a first pair of separated conductive pads at the bottom of the milled recess, each pad respectively conductively coupled to the first and second BGA pads; sizing the recess to contain a Surface Mount Component (SMC); placing solder paste in the first pair of separated conductive pads; placing solder paste on the ball grid array (BGA) of BGA pads; placing an SMC within the recess; placing a BGA component over the SMC; reflow soldering the SMC component and the BGA component. 
         [0012]    In some embodiments of this aspect of the invention the milling is performed by a laser. 
         [0013]    In some embodiments of this aspect of the invention there are the further steps of the multilayer PCB having a second pair of separated conductive pads surrounding the opening of the milled recess, each pad respectively conductively coupled to the same first and second BGA pads as the respective conductive pads at the same end of the recess; and prior to the step of placing an SMC within the recess, placing solder paste on the second pair of separated conductive pads. 
         [0014]    In some embodiments of this aspect of the invention the SMC is a capacitor. 
         [0015]    In some embodiments of this aspect of the invention the first and second BGA pads are diagonally situated relative to the grid pattern 
         [0016]    In some embodiments of this aspect of the invention at least one BGA pad of the ball grid array of BGA pads has been removed; and the milling of the recess is located in the array grid where the at least one BGA has been removed. 
         [0017]    According to yet another aspect of the invention there is provided a computer aided design tool implemented on a computing device for accommodating a multilayer printed circuit board (PCB) wherein the PCB has a ball grid array (BGA) of BGA pads on one side of the PCB arranged in a grid pattern having: a design tool mode configured to select two adjacent BGA pads on the printed circuit board (PCB) for connection to a two-lead component; a design tool mode configured to identify a placement of a recess between the two BGA pads for containing a Surface Mount Component (SMC); a design tool mode configured to identify a placement of separated component pads on an inner layer of the PCB so as to define a bottom of the recess; and a design tool mode configured to conductively connect the separated component pads to a respective BGA pad of the two BGA pads. 
         [0018]    In some embodiments of this aspect of the invention the design tool further has a design tool mode configured to identify a placement of a second pair of separated conductive pads surrounding the opening of the milled recess; and a design tool mode configured to conductively connect the second pair respectively to the same first and second BGA pads as the respective conductive pads at the same end of the recess. 
         [0019]    In some embodiments of this aspect of the invention the design tool further has a design tool mode configured to identify remove at least one BGA pad from the ball grid array of BGA pads and situate the recess in the array grid where the BGA pad has been removed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    In order to better understand various exemplary embodiments, reference is made to the accompanying drawings, wherein: 
           [0021]      FIG. 1 a    shows a cross-sectional view of a portion of a fine-pitch through-hole multilayer circuit board according to an embodiment of the invention; 
           [0022]      FIG. 1 b    shows a cross-sectional view of a laser etching a portion of the of the fine-pitch through-hole multilayer circuit board of  FIG. 1 ; 
           [0023]      FIG. 1 c    shows a cross-sectional view of a laser etching a further portion of the of the fine-pitch through-hole multilayer circuit board of  FIG. 1 ; 
           [0024]      FIG. 2 a    shows a cross-sectional view of solder paste applied within and adjacent to the laser etched recess of the of the fine-pitch through-hole multilayer circuit board of  FIG. 1 ; 
           [0025]      FIG. 2 b    shows a cross-sectional view of a surface mount component placed within the laser etched recess of the of the fine-pitch through-hole multilayer circuit board of  FIG. 2   a;    
           [0026]      FIG. 2 c    shows a cross-sectional view of a BGA component placed over the laser etched recess of the of the fine-pitch through-hole multilayer circuit board of  FIG. 2   b;    
           [0027]      FIG. 3 a    shows a top view of a copper land pattern on the top layer of a fine-pitch through-hole multilayer circuit board having a surface mount component mounted thereon according to an embodiment of the invention; 
           [0028]      FIG. 3 b    shows a top view of a copper structure pattern on an inner layer of a fine-pitch through-hole multilayer circuit board according to an embodiment of the invention; 
           [0029]      FIG. 3 c    shows a top view of a copper land pattern on the top layer of a fine-pitch through-hole multilayer circuit board relative to a pair of BGA landing pads according to an embodiment of the invention; 
           [0030]      FIG. 4  shows a top view of a copper land pattern on the top layer of a fine-pitch through-hole multilayer circuit board relative to a grid of BGA landing pads according to an embodiment of the invention; and 
           [0031]      FIG. 5  shows a flowchart of a series of method steps according to an embodiment of the invention. 
       
    
    
       [0032]    To facilitate understanding, similar reference numerals have been used to designate elements having substantially the same or similar structure and/or substantially the same or similar function. 
       DETAILED DESCRIPTION 
       [0033]    The description and drawings merely illustrate the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its scope. Furthermore, all examples recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Additionally, the term, “or,” as used herein, refers to a non-exclusive or (i.e., and/or), unless otherwise indicated (e.g., “or else” or “or in the alternative”). Also, the various embodiments described herein are not necessarily mutually exclusive, as some embodiments may be combined with one or more other embodiments to form new embodiments. 
         [0034]    Referring now to the drawings, in which like numerals refer to like components or steps, there are disclosed broad aspects of various exemplary embodiments. 
         [0035]    Referring to  FIG. 1A  there may be seen a cross-sectional view of a portion of a fine-pitch through-hole multilayer circuit board. The pads  101   a  and  101   b  are on the top layer of the printed circuit board and further detailed in FIG.s  3 A and  3 C. Insulating resin layers  102   a - 102   e  separate the conductive portions of the multilayer board. Conductive trace  103  represents a trace on the first signal layer, while conductive areas  105   a  and  105   b , which are further detailed in  FIG. 3B  are located on a second signal layer. The conductive areas  105   a  and  105   b  carry two roles. First they will ultimately act as soldering points for a surface mount component, and second they will act as a stop layer for a laser drill to be described in conjunction with  FIG. 1B . 
         [0036]    Referring to  FIG. 1B  there may be seen the cross-sectional view of a portion of a fine-pitch through-hole multilayer circuit board of  FIG. 1A  with a laser  116  using its beam  118  to excavate a cavity  119  in the circuit board. Conductive area  105   a , normally of copper, acts as a dimensional “stop” for the laser beam  118 , setting the depth for the excavated recess. The beam  118  vaporizes the insulating resin layers above the conductive area  105   a , leaving the area available as a component soldering pad, as will be described below. 
         [0037]    Referring to  FIG. 1C  there may be seen the cross-sectional view of the portion of a fine-pitch through-hole multilayer circuit board of  FIG. 1B  with the laser having continued further excavation. The beam  128  may be seen excavating to a further depth  129  with a conductive portion  107  acting as the “stop” for the beam. Conductive portion  107  normally would be a portion of the power or ground planes within the multilayer circuit board. The further depth portion between conductive areas  105   a  and  105   b  will act as an insulative gap between these conductive areas when the areas are later used as component soldering pads. 
         [0038]    Referring to  FIG. 2A , there may be seen the cross-sectional view of the portion of a fine-pitch through-hole multilayer circuit board of  FIG. 1C  with the laser having completed excavation. Solder paste quantities  231  have been placed on the printed circuit board on conductive pad areas  201   a  and  201   b.    
         [0039]    In  FIG. 2B  a surface mount component  243  has been inserted into the recess, distributing the solder paste onto the conductive area pads  205   a  and  205   b . In general this component will be a decoupling capacitor. Other types of surface mount components can alternatively be placed into the recess, including two-port devices such as resistors and diodes. 
         [0040]    Referring to  FIG. 2C  there may be seen a cross-sectional view Ball Grid Array placed onto the portion of a fine-pitch through-hole multilayer circuit board depicted in  FIG. 2B . The bottom portion  255  of the BGA has solder balls  259   a  and  259   b  which are lodged into respective portions of solder paste. A subsequent reflow soldering operation will secure both the BGA and surface mount component to their respective contact pads on the printed circuit board. 
         [0041]    Referring now to FIG.s  3 A to  3 C there may be seen top views of conductive land patterns, typically copper, which correspond to the previous Figures. In  FIG. 3A  may be seen the generally U-shaped conductive pads  301   a  and  301   b  on the top layer of the multilayer circuit board. These pads correspond to the conductive cross-sections  101   a  and  101   b  respectively of  FIG. 1A  and surround the recess in which the surface mount component  343  is placed. 
         [0042]    In  FIG. 3B  may be seen the conductive pads which the bottom of component  343  is ultimately soldered. These conductive pads correspond to the conductive cross-sections  105   a  and  105   b  respectively of  FIG. 1A  and as previously described also act as the “stop” for the laser milling process to define the bottom of the recess in those areas. 
         [0043]    Referring now to  FIG. 3C  there may be seen printed circuit board component pads  309   a  and  309   b  which comprise a portion of the grid of pads to which the BGA component is soldered. These component pads  309   a  and  309   b  are respectively conductively connected to the U-shaped conductive pads  301   a  and  301   b . As well, printed circuit board via connections also conductively connect the interior layer conductive pads  305   a  and  305   b  to component pads  309   a  and  309   b  respectively, the vias offering a degree of additional reliability of conductive connection between the component pads and the surface mount component  343  upon completion of the reflow soldering operation. 
         [0044]    Referring now to  FIG. 4 , there may be seen a top view of an embodiment of the invention within a portion of a grid of printed circuit board component pads  409   a  to  409   d  to which a BGA would be soldered. In this embodiment the BGA grid is a regular grid having a  1  mm pitch, allowing sufficient space for U-shaped conductive pads  401   a  and  401   b , a recess, and surface mount component  443  of nominal industry size “ 0201 ” in dimension. Smaller components, such as the nominal industry size “ 01005 ” could likewise be situated in a similar configuration with the appropriate dimensional adaptation of the U-shaped conductive pads and recess. 
         [0045]    According to another embodiment of the invention, larger component sizes, such as nominal industry sizes “ 0402 ” and “ 0603 ” may also be placed within the BGA grid pattern by both appropriate dimensional adaptation of the U-shaped conductive pads and recess, and by depopulating certain BGA component pads (and corresponding balls on the BGA component). As well, by depopulating certain BGA component pads and corresponding balls on the BGA component, embodiments of the invention may be implemented on regular grids of other than a 1 mm pitch, for example those having a 0.8 mm pitch. Alternatively, embodiments of the invention may also be implemented on non-regular grids, providing flexibility in component placement appropriate to the Ball Grid Array to be positioned over the components. 
         [0046]    Referring to  FIG. 5  there may be seen a flowchart  500  of the steps of a method according to an embodiment of the invention. The method commences at step  501 . At step  503  a fine-pitch through-hole multilayer circuit board is provided having component pads and conductive connections thereof appropriate to the surface mount components to be placed. At step  505  a laser milling operation carves the appropriate recesses for the surface mount components which are to be located beneath respective Ball Grid Array components. As previously described, interior conductive pads have been placed to act as “stops” for the laser milling operation. At step  507  solder paste is applied to the circuit board, and at step  509  the surface mount components are placed such that their appropriate conductive pads and balls are contacting the solder paste. At step  511  a soldering reflow operation is performed, reflowing the solder paste and conductively attaching the surface mount components. The method then terminates at step  513 . 
         [0047]    In an exemplary embodiment, a computer aided design tool allows the selection of conductive component pads on both the top layer and internal layers to be substantially automated. The computer aided design tool may automatically identify appropriate spacing and shape of the conductive pads to place standard components on within the respective BGA grid for attaching to the board within defined recesses. A computer aided design tool may also provide instructions to control a machine to manufacture the modified circuit board. Instructions may be exported to the machine or the design tool may directly control the machine. 
         [0048]    Thus what has been disclosed is a method of placing surface mount components beneath Ball Grid Arrays in respective recesses, thus providing an alternate method of placing small components within close proximity of the Ball Grid Arrays. 
         [0049]    While the figures and descriptions may depict regular circular or rectangular shapes of different elements in exemplary embodiments, it should be understood that alternative shapes may be used such as imperfect polygons and rounded forms. These alternative shapes may be substantially similar to the depicted shapes in area and outline. 
         [0050]    Although the various exemplary embodiments have been described in detail with particular reference to certain exemplary aspects thereof, it should be understood that the invention is capable of other embodiments and its details are capable of modifications in various obvious respects. As is readily apparent to those skilled in the art, variations and modifications can be effected while remaining within the spirit and scope of the invention. Accordingly, the foregoing disclosure, description, and figures are for illustrative purposes only and do not in any way limit the invention, which is defined only by the claims.