Abstract:
A ball grid array electronic package is attached to a substrate by means of solder balls and solder paste. Connection is made between a contact on the ball grid array and a solder ball by means of a first joining medium, such as a solder paste. Connection is made between a solder ball and a contact arranged on the substrate by means of a second joining medium. The contact arranged on the substrate is substantially quadrilateral in shape, and preferably substantially square in shape. Connection to the substrate, e.g., using round solder balls, is much more easily detected, e.g., by x-ray, than when using round pads, especially those having a smaller diameter than the balls.

Description:
TECHNICAL FIELD 
     The present invention relates to printed circuit boards and more particularly to printed circuit boards having a plurality of conductive pads for the attachment of an electronic package by means of solder balls. 
     BACKGROUND OF THE INVENTION 
     An electronic package typically includes a circuitized substrate with one or more active devices attached thereon. The use of attach materials, such as an adhesive, is common in electronic packaging applications for attaching the devices to the substrate, particularly in Ball Grid Array (BGA) packages. 
     BGA packages are a relatively recent development in the electronic packaging industry, replacing current products such as Quad Flat Packs (QFP). The main difference is the connection system to the printed circuit board (PCB), also called second level attachment, that is made through eutectic tin-lead solder alloy balls arranged in a matrix layout on the bottom side of the BGA package, instead of metal leads placed along the peripheral corner of the plastic component body. BGA and QFP packages are described in “Circuits Assembly (USA)—Vol. 6, No. 3, March, 1995 pages 38-40”. 
     U.S. Pat. No. 5,574,801 discloses a method of inspecting an array of solder balls used as connections in integrated circuit modules. The method checks the deviation of the balls from a best fitting plane through the balls in order to ensure that the array is flat. If the array is not flat, then one or more of the balls may fail to make an electrical connection with the PCB. 
     In order to check that all of the electrical connections have been made between the balls and the PCB, the completed assembly may be inspected with the use of X-Rays. The size and location of the connections, especially those internally positioned, do not allow inspection with the naked eye. However, it can be very difficult to determine, when inspecting from above with X-Rays, whether a solder connection has formed successfully or not. A round dark image appears of the ball and corresponding circular pad whether or not a solder connection is made, making detection difficult. 
     One solution to this problem is to use X-Ray Laminography. An X-Ray Laminography machine can produce synthetic X-Ray sections through the connection, allowing the 3-dimensional shape to be assessed and a determination made as to whether a good solder connection has been made. However, such X-Ray Laminography equipment is relatively expensive. 
     It is believed, therefore, that an enhanced pad design for an electronic package in which a simple X-Ray process would readily identify those connections which have not been made, without recourse to an X-Ray Laminography machine, would represent a significant advancement in the art. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is a primary object of the invention to enhance the art of electronic substrates and particularly those of the PCB variety. 
     It is another object to provide an enhanced conductor pad design for such a substrate which is especially adapted for receiving solder balls. 
     It is a still further object of the invention to provide such a substrate structure which can be produced economically and does not require complicated testing equipment or methods. 
     In one aspect of the invention, there is provided a substrate for the attachment of ball grid array electronic packages thereto by means of solder balls and a joining medium wherein connection is made between a contact on the ball grid array electronic package and a solder ball by means of a first joining medium and between a solder ball and a contact arranged on the substrate by means of a second joining medium and wherein the contact arranged on the substrate is substantially quadrilateral in shape. 
     According to another aspect of the invention, there is provided a method of attaching a ball grid array electronic package to a substrate comprising the steps of: applying a first joining medium to a plurality of contacts located on the ball grid array; locating a solder ball on each of the plurality of contacts of the ball grid array; heating the ball grid array to a temperature sufficient to join each of the solder balls to the corresponding contact on the ball grid array by means of the first joining medium; applying a second joining medium to a plurality of contacts located on the substrate; positioning the substrate and the ball grid array such that each of the solder balls is located adjacent its corresponding contact located on the substrate, the corresponding contact being substantially quadrilateral in shape; heating the ball grid array to a temperature sufficient to join each of the solder balls to the corresponding contact on the substrate by means of the second joining medium; and inspecting, by means of X-Ray, the connection formed between the ball grid array and the substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic, partial cross-sectional view taken through a known BGA module with solder balls attached thereto and corresponding contacts on a printed circuit board; 
     FIG. 2 is a schematic plan view of a known BGA module and printed circuit board; 
     FIG. 3 is a schematic partial cross-section of the known BGA module and printed circuit board of FIG. 2, after the connections have been reflowed successfully; 
     FIG. 4 shows an X-Ray image of the known BGA module and printed circuit board of FIG. 3; 
     FIG. 5 is a schematic partial cross-sectional view of the known BGA module and printed circuit board of FIG. 2, in which only one connection between the BGA module and the solder ball has been formed during reflow (solder melting); 
     FIG. 6 is a schematic plan view of a BGA module and printed circuit board having quadrilateral contact pads according to the present invention; 
     FIG. 7 shows an X-Ray image of the BGA module and printed circuit board of FIG. 6 after the connections have been reflowed successfully; and 
     FIG. 8 shows an X-Ray image of the BGA module and printed circuit board of FIG. 6 in which only the connection between BGA module and the solder ball has been melted during reflow. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a cross-sectional view of a known BGA module having a substrate  100  with solder balls  106  attached to a planar array of contacts  102  located on the substrate, the solder balls adapted for being coupled to corresponding contacts  112  on a printed circuit board  110  is shown. A solder paste  104  is also shown between balls  106  and contacts  102 . The contacts  102  are typically round to match the shape of the respective balls  106  and to allow a close spacing sufficient to reliably prevent solder bridging. Contacts  102  are made of a conductive material and may be deposited by known screening or photolithographic processes. The solder balls  106  are joined to the contacts  102  by means of solder paste  104 , which is typically a tin-lead solder. The solder paste  104  is applied to the contacts  102 , followed by a layer of sticky flux (not shown). The solder balls  106  are located on the contacts  102  and held in place by the sticky flux. 
     For a ceramic BGA, the solder balls  106  have a higher melting point than the solder paste  104 . The solder balls  106  are then reflow joined to the contacts  102  by applying a temperature high enough to melt the solder paste  104 , but not high enough to melt the solder balls  106 . A planar array of contacts  112  is also arranged on substrate  110 . Each of the contacts  112  on substrate  110  corresponds to a contact  102  on substrate  100 . Substrate  110  is typically a printed circuit board. A solder paste  114  is deposited on contacts  112 . 
     For a plastic BGA, the above process is followed, except that the solder ball  106  melts and combines with the solder paste  104  to form a single complete ball. 
     FIG. 2 shows a plan view of the BGA module and printed circuit board of FIG.  1 . The contacts  102  can be seen as a circle. The ball  106  can be seen as a slightly larger circle. The contacts  112  (shown hidden) can be seen as a circle of size comparable to that of contacts  102 . 
     The substrate  100  and the substrate  110  are brought together so that corresponding contacts on the two substrates are aligned. A temperature high enough to melt the solder paste  114 , but not high enough to melt the solder balls  106 , is then applied. FIG. 3 shows a cross-section of the BGA module and printed circuit board of FIG. 2 after the connections have been reflowed successfully. Connections exist between each of the solder balls  106  and the contacts  102  by means of the solder paste  104  and between each of the solder balls  106  and the contacts  112  by means of the solder paste  114 . 
     The above BGA module and printed circuit board, with connections successfully made, are now subjected to an X-Ray test to ensure that all the connections are satisfactory. FIG. 4 shows an image of an X-Ray which is taken from above the module, looking through the contact  102 , solder paste  104 , solder ball  106 , solder paste  114  and contact  112 . The resulting image shows at the center a darker area (where contact  112  lies over a central portion of the larger ball  106 ) which corresponds to that of the solder ball  106 . A concentric, slightly lighter ring around the darker area corresponds to the contact  112  and the solder paste  114 . 
     FIG. 5 shows a cross-section of a BGA module and printed circuit board in which only the connection between BGA module and a singular solder ball has been melted during reflow. Connections exist between each of the solder balls  106  and the contacts  102  by means of solder paste  104 . Connection exists between the solder ball  106  and the contact  112 , located second from the left hand edge by means of solder paste  114 . However, no connection was made between the solder paste  114  and the solder balls  106  in the other three locations. 
     The BGA module and printed circuit board with its connections are now subjected to the aforementioned X-Ray test to ensure that all the connections are correctly made. The main purpose of this X-Ray test is two-fold: (1) to ensure that there are no short circuit connections; and (2) that none of the solder balls have been displaced from their intended position. When BGA modules and associated printed circuit boards are examined under X-Rays, any open circuit connections to the PCB are difficult to see as a uniform ball  106  is created over the round pad  112  of the PCB, such that all of the connections look round on the X-Ray, whether the connection has been formed or not formed. 
     In the present invention, the footprint of the contacts for Ball Grid Arrays on the printed circuit board is redesigned from being round such as is shown by  112  in FIGS. 1-5 to being square such as is shown by  712  in FIGS. 6-8. By using a square footprint, faulty or open circuit connections are easier to spot under X-Ray examination. If a square pad  712  is used on printed circuit board  110 , then a correctly formed connection will present a square appearance if it has been properly soldered. If it has not wetted to the pad, then the solder ball  106  will remain spherical and thus appear round on the X-Ray. This obviously substantially improves the reliability of inspecting BGAs under X-Ray. 
     FIG. 6 is a schematic plan view of a BGA module and printed circuit board having square contact pads according to the present invention. The round contact  102  on the substrate  100  can be seen, together with the round shape of the solder ball  106 . The printed circuit board differs from the board described in FIGS. 1-4 in that the contact  712  is a substantially square shape, rather than being round. In a variation of the embodiment shown, the pad may be rectangular in shape or it may have a slight radius applied to the corners of the square pad. However, the pad should remain substantially quadrilateral in shape, compared to the initially round solder balls. 
     As further seen in FIG. 6, the corners of the quadrilaterals project slightly beyond the profile (shape) of the respective round solder balls  106 , the quadrilateral shapes thus having a slightly greater diameter in the corners thereof than that of the corresponding ball. As understood from the following, it is these corresponding comparative shapes that allows a quadrilateral shape to appear during X-Ray inspection to thus indicate a sound connection between ball and substrate pad, in comparison to simply a round shape for the ball to show no such connection has occurred. 
     FIG. 7 shows an X-Ray image which is taken from above the module, looking through the contact  102 , solder paste  104 , solder ball  106 , solder paste  114  and contact  712 . The image shows the substantially square shape of the contact pad  712  clearly visible, indicating that the contact pad  712  has been wetted and a good connection made. 
     FIG. 8 shows an X-Ray image of the BGA module and printed circuit board of FIG. 6 in which no connection between the right side part of the BGA module and respective solder balls (only the two on the right side) has occurred. The image is taken from above the module and printed circuit board looking through the top contact  102 , first solder paste  104 , solder ball  106 , second solder paste  114  and bottom contact  712 . The image shows the round shape of the solder ball  106  clearly visible, but not the substantially square shape of the underlying contact  712 , indicating that the contact  712  has not been wetted and a good connection has not been made. 
     While there have been shown and described what are at present the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.