Opposed Ball Grid Array Mounting

This invention relates generally to techniques for providing electrical components on printed circuit boards or cards.

In a wide variety of electronic devices, space is often at a premium in order achieve small device size. This is particularly true in a variety of computer applications and most particularly true in connection with portable computers.

The demands for available board space has increased with the introduction of the Pentium.RTM. II brand processor from Intel Corporation. This processor uses Gunning transceiver logic ("GTL"), which requires parallel resistive terminations for all GTL signals. The number of GTL signals between the processor and bridge can be in the range of 150, each of which must be connected to a parallel pull up resistive termination. Decoupling capacitors are typically utilized to provide high frequency capacitive decoupling in computer systems. From 30 to 40 decoupling capacitors may be needed in a portable computer using GTL. Thus, the demands for limited PCB or card space are strained even further by the additional device requirements brought on in recent vintage processors.

Ball grid array ("BGA") packages have become increasingly popular for a large variety of applications Fundamental limitations on wire bonding technology and pad pitch have made some existing industry standard packages, such as plastic quad flat pack ("PQFP") packages, technology roadblocks when the input/output pin count must exceed 250 to 300. In addition, BGA packages can have internal ground and power planes for high frequency operation. These internal planes provide impedance control, limiting the effective inductance seen by the silicon device. This may increase the useable operating frequencies of the silicon device due to the reduction of package related noise and cross-talk.

While there are a wide variety of options available in the general field of BGA's, there are two well known BGA packages. One BGA package uses an over molded pad array carrier ("OMPAC") while the other uses a substrate material that is ceramic ("CBGA"). Generally, BGA packages include a die carrier and a plurality of solder balls or bumps beneath the die carrier which connect to contacts formed on a board or card. These contacts may communicate via metal lines with vias which extend through the board. The portion of the board on the underside of a BGA package is generally too populated with via holes to provide useful space for additional devices.

In accordance with one aspect of the present invention, a board for connecting electronic devices includes a plurality of feed-through vias extending through the board. A first ball grid array package is connected to the vias on a first side of the board. A second ball grid array package is connected to the first ball grid array package through the vias, the second ball grid array package positioned on a second side of the board.

In accordance with another aspect of the present invention, a computer system includes a board with a processor mounted thereon. A plurality of electronic components are mounted on the board in electrical communication with the processor. A first ball grid array package is mounted on the board in communication with the processor. A second ball grid array package is mounted on the board underneath the first ball grid array package.

In accordance with yet another aspect of the present invention, a method for connecting ball grid array packages to a board includes the step of securing a first ball array package to a first surface of the board. A second ball grid array package is secured to the board on the opposite side of the board directly beneath the first ball grid array package.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and description below. Other features, objects and advantages of the invention will be apparent from the description and drawings, and from the claims.

FIG. 1 is a partial enlarged side elevational view of one embodiment of the present invention;

FIG. 2 is an enlarged, cross-sectional view taken generally along the line 2--2 of FIG. 1;

FIG. 3 is a schematic view of a computer system;

FIG. 4 is a cross-sectional view taken generally along the line 4--4 of FIG. 2; and

FIG. 5 is a depiction of a computer system.

Referring to the drawing wherein like reference characters are utilized for like parts throughout the several views, a printed circuit board (PCB) or card, hereafter collectively referred to as a board 10, has mounted on opposing sides, a first BGA package 12 and a second BGA package 14. Each of the BGA packages 12 and 14 includes a plurality of solder balls or bumps 16. Each package also includes a PCB 18 and a die 52 containing structure 20, which could be, for example, epoxy over molded in the case of OMPAC packages or substrate material-ceramic for CBGA packages. If desired, the same pattern and arrangement of solder balls 16 can be used for both packages 12 and 14.

As shown in FIG. 2, the solder balls 16 are arranged generally in an array around a series of vias 22 which extend through the board 10. Connections can be made to metal lined vias 22 by metal traces 26. Thus, communication between the top surface 40 of the board 10 and the lower surface 50 of the board 10 can occur through the metal lined vias 22. Advantageously, connections with power or ground planes on the underside of the board 10 can then be made using these vias 22. In addition, some packaged electronic devices 24 may be situated where permitted by the arrangement of vias 22 underneath the BGA's 12 or 14. However, the high population of vias 22 otherwise makes connection of electronic devices under BGAs impractical.

Referring now to FIG. 3, a microprocessor 28, which may be a Pentium.RTM. II microprocessor available from Intel, communicates with a typical bridge 34. The bridge 34 may connect by a bus 37 to one or more input/output devices 38. A computer system 58 which could include, a disk drive 68, a CD ROM drive 70, a monitor 60, or a keyboard 63, for example, connected by cables 64, as shown in FIG. 5.

In the case of the Pentium.RTM. II processor, GTL signal paths 30 connect the processor 28 and the bridge 34. While only a few paths 30 are shown in FIG. 3 for illustration purposes, a number of such paths, perhaps exceeding 100 signal paths, may be utilized. Each of the paths includes a parallel resistive pull up termination 32 in conventional GTL designs. In addition, a plurality of high frequency decoupling capacitors 36 are utilized in connection with the bridge 34 in conventional GTL designs.

Thus, the already crowded board 10 must now accommodate a large number of resistive terminations 32 all connected to the power plane and a large number of decoupling capacitors 36 connected to the power plane and to the ground plane.

By using the opposed configuration of BGA packages 12 and 14, the real estate on the underside 50 of the board 10 may be effectively utilized despite the presence of multiple vias 22. As a result, more effective utilization of the entire available board 10 area may be achieved.

Moreover, in one specific application, the resistive terminations 32 and decoupling capacitors 36 may all be provided in the lower BGA 14. The devices 32 and 36 do not need to extensively connect to other components inside the package 12. The necessary connections can readily be made through the metal lined vias 22, as illustrated in FIG. 4, by way of metal traces such as the traces at 26 in FIG. 2.

Since the same feed-through vias 22 are used for both packages 12 and 14, the opposed BGA layout is easy. Moreover, the otherwise unusable space on the underside board 50 underneath the upper BGA package 12, is effectively utilized. While the arrangement shown in FIG. 2 has been described in connection with implementing a GTL design involving multiple resistive terminations and decoupling capacitors, other applications may exist which can utilize the space saving arrangement described herein.

While the present invention has been described with respect to a single preferred embodiment, those skilled in the art will appreciate numerous modifications and variations and it is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the present invention.