Patent Publication Number: US-2007114578-A1

Title: Layout structure of ball grid array

Description:
FIELD OF THE INVENTION  
      The present invention relates to electrical devices, and particularly to a layout structure of a ball grid array.  
     DESCRIPTION OF RELATED ART  
      A ball grid array (BGA) is one particular type of surface mount package on a circuit board. The BGA includes an array of connections on the bottom side of the connector package. In the BGA, pins extending into the circuit board are replaced by small solder balls placed on the bottom side of the connector at each contact location. The circuit board, rather than holes, has an array of solder ball pads matching the solder ball placements on the connector bottom. Connections are made by re-flowing the solder balls to mechanically and electrically engage the connector to the circuit board.  
      A conventional layout structure for a BGA is described in the following with reference to  FIGS. 3-4 .  
       FIG. 3  shows a schematic plan view of a surface layer  100  of a BGA substrate utilizing a conventional layout structure. As shown in  FIG. 3 , four uniformly aligned quadrants labeled  1 - 4  are established using the X-axis and Y-axis that intersect at the middle of the substrate and that are orthogonal to each other. The surface layer  100  includes a layout area  120  and a margin area  130 . On the layout area  120 , each one of the interconnection vias  112  and a corresponding one of the solder ball pads  100  are laid out as pairs along lines that are parallel to each other.  
       FIG. 4  shows a schematic plan view of an inner layer  200  of a BGA substrate utilizing a conventional layout structure. A metal foil  202  is laid on the inner layer  200 . The interconnection vias  112  extend through the surface layer  100  and the inner layer  200 . The interconnection vias  112  include an interconnection via  112   a  needing to be coupled to the metal foil  202 , and a majority of interconnection vias  112   b  isolated from the metal foil  202  by an isolating area  204 . If the interconnection via  112   a  is surrounded by the interconnection vias  112   b , as shown in  FIG. 4 , the interconnection via  112   a  cannot be coupled to the metal foil  202  directly. A transmission line must be traced to couple the interconnection via  112   a  with the metal foil  202 . However, it is inconvenient to trace beside the dense interconnection vias.  
      What is needed, therefore, is a layout structure of a ball grid array where the interconnection via needing to be coupled to the metal foil is not isolated from the metal foil.  
     SUMMARY OF THE INVENTION  
      A layout structure of ball grid array is provided. In a preferred embodiment, the layout structure includes: a substrate having a surface layer and an inner layer, the surface layer having a layout area and a margin area, the inner layer having a metal foil and an isolating area; a plurality of solder ball pads arrayed on the layout area of the surface layer; a plurality of interconnection vias isolated from the metal foil by the isolating area, each electrically coupled to a corresponding one of the plurality of the solder ball pads; an inner interconnection via coupled to the metal foil; and at least one of the plurality of interconnection vias laid on the margin area. It is of advantage that with the layout structure the inner interconnection via needing to be coupled to the metal foil is not isolated from the metal foil.  
      Other advantages and novel features will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which:  
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic plan view of a surface layer of a BGA substrate, in accordance with a preferred embodiment of the present invention;  
       FIG. 2  is a schematic plan view of an inner layer of the BGA substrate of  FIG. 1 ;  
       FIG. 3  is a schematic plan view of a surface layer of a BGA substrate utilizing a conventional layout structure; and  
       FIG. 4  is a schematic plan view of an inner layer of the BGA substrate of  FIG. 3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  and  FIG. 2  respectively show a schematic plan view of a surface layer  300  and an inner layer  400  of a BGA substrate, in accordance with a preferred embodiment of the present invention. As shown in  FIG. 1 , four quadrants labeled I-□ are established using an X-axis and a Y-axis that intersect at a middle of the substrate and that are orthogonal to each other. The surface layer  300  includes a layout area  310  and a margin area  320 . The inner layer  400  includes a metal foil  402  and an isolating film  404  covered on the metal foil  402 . A plurality of solder ball pads  301  is arrayed on the layout area  310  of the surface layer  300 . A plurality of interconnection vias  302  and an inner interconnection via  303  (i.e., not located at a periphery of the layout area  310 ) extend through the surface layer  300  and the inner layer  400 . The plurality of interconnection vias  302  is isolated from the metal foil  402  by the isolating film  404 , and each is electrically coupled to a corresponding one of the plurality of the solder ball pads  301 . The inner interconnection via  303  is located in quadrant II, and electrically coupled to a corresponding one of the plurality of the solder ball pads  301 . The plurality of interconnection vias  302  includes a first interconnection via  302   a  and a second interconnection via  302   b  located in quadrant II. The first interconnection via  302   a  and the second interconnection via  302   b  are laid on the margin area  320 , isolated from the metal foil  402  by the isolating film  404 , and are each electrically coupled to a corresponding one of the plurality of the solder ball pads  301 . No interconnection vias and isolating film  404  are arrayed between the inner interconnection via  303  and an edge of the margin area  320 . Thus, the inner interconnection via  303  is electrically coupled to the metal foil  402  directly without transmission line required.  
      It is believed that the present invention and its advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.