WO1997016849A1

WO1997016849A1 - Ball grid array integrated circuit socket

Info

Publication number: WO1997016849A1
Application number: PCT/US1996/015045
Authority: WO
Inventors: Carol L. Cearley-Cabbiness; John S. Burg; David S. Hardcastle; Kurt H. Petersen; Gayle R. T. Schueller
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1995-11-02
Filing date: 1996-09-19
Publication date: 1997-05-09

Abstract

A socket for an integrated circuit device is provided which includes solder ball projections for surface-mount attachment to a circuit board. The socket includes a circuit board having conductive pads to which terminations of the integrated circuit device connect. The conductive pads are then connected to the solder balls either directly or through circuitry on the circuit board.

Description

Ball Grid Array Integrated Circuit Socket

Field of the Invention

The present invention relates generally to sockets for integrated circuit electronic devices, and more particularly, devices having a regular array of terminations, usually a rectangular or square pattern of solder balls.

Background of the Invention

Conventional sockets for either the temporary or permanent connection of an integrated circuit device to other circuitry, usually a circuit board, have included stamped terminals which contact the device at one end and include a "contact tail" at the other end. This tail extends through holes in the circuit board and are soldered thereto. There are several limitations associated with the traditional type of stamped contact interface. The construction of the formed contacts greatly increases the size of the socket relative to the integrated circuit device and so requires substantial footprint area on the printed circuit board. To combat this size penalty, the size of the contact tail has been reduced in cross-sectional area as the integrated circuit package pitch has decreased over the last twenty years. This reduction in cross-sectional area has made the contact tail more fragile, increasing the possibility of damage during handling or insertion into the printed circuit board. Also, solder can migrate up the tail of the contact, contaminating the opposing integrated circuit interface surface or interfering with the spring mechanism of the contact. Finally, the ability to reroute connections between the device and the board or change or enlarge the pattern of the integrated circuit device terminations is limited by the physical construction of the contact pin.

Summary of the Invention

The present socket requires less footprint area than one using standard stamped metal contacts, is lower in profile, is more resistant to damage in handling and assembly, limits the migration of solder to a confined area, and allows for broader routing differences between the socket contact pattern and integrated circuit device pattern.

In particular, the invention provides a socket for accepting an integrated circuit device having an array of terminals and connecting the integrated circuit device to a circuit board including a housing having a top surface, a bottom surface, a well defined by an opening in the top surface for accepting the integrated circuit device, a circuit board having a substrate, conductive pads attached to the substrate and an array of projections connected to the pads, wherein the circuit board is disposed to engage the integrated circuit device disposed within the well with a different one of the pads in electrical contact with a different one of the integrated circuit device terminals, and wherein the array of projections project from the housing and may be used to connect the socket to the circuit board.

The circuit board may be located either outside the housing attached to the bottom surface 14 or inside the housing well, and the circuit board, if located within the well, may be oriented so that its contact pads are adjacent the integrated circuit device or the housing. Preferably the circuit board is attached to the housing with an adhesive, more preferably epoxy, and the socket includes a lid to retain the integrated circuit device within the well. The projections connected to the circuit board pads and extending from the housing may be of solder or a hard metal such as nickel or copper and may assume a variety of shapes including spheres, buttons, columns, blocks, pyramids and.so forth.

Brief Description of the Drawings

The present invention will be described with respect to the accompanying drawings, wherein like numbers refer to like parts in the several views, and wherein:

Figure 1 is a perspective view of a socket according to the present invention;

Figure 2 is a cross-sectional view of a portion of the socket of Figure 1;

Figure 3 is a cross-sectional view of a portion of an alternate embodiment of a socket according to the invention;

Figure 4 is a cross-sectional view of a portion of a second alternate embodiment of a socket according to the present invention; and

Figure 5 is a cross-sectional view of yet another embodiment of a socket according to the present invention.

Description of the Preferred Embodiment

Sockets for integrated circuit devices are usually provided for two purposes. The first is for placement in a piece of equipment such a computer so that the user may add or replace integrated circuit devices in a convenient manner. The second is to allow rapid testing of integrated circuit devices prior to final placement in consumer equipment. Such testing may include, for example, elevated temperatures. The primary difference between the types of sockets are the materials from which they are made, whether the socket is primarily designed for ease of changing devices or security of the device once installed, and the robustness of the socket components.

Figures 1 and 2 illustrate a socket, generally indicated as 10, intended primarily as an end-use socket. It provides an interconnect between the printed circuit board in electronic equipment and an integrated circuit device. End-use sockets give equipment manufacturers and their customers the enhanced ability to repair or upgrade components. In accordance with the continuing trend towards more compact electronic equipment, the socket 10 must also be compact, utilizing little more than the area of the integrated circuit device.

The socket 10 includes a housing 12 having a bottom surface 14, a top surface 16 and a well 18 defined by a opening in the top surface 16. The well 18 accepts an integrated circuit device 20 having an array of terminations on its bottom surface. A lid 22 engages the housing 12 to retain the integrated circuit device 20 within the well 18. Springs 24 urge the integrated circuit device 20 into firm engagement with the bottom surface of the well 18 within the housing 12 and forces the terminations of the integrated circuit device 20 into holes 26 formed in the housing 12. Attached to the bottom surface of the housing 12 is a circuit board 28 which may be either rigid or flexible. The circuit board 28 is attached to the housing 12 by means of an adhesive 30, preferably epoxy. The circuit board 28 includes conductive pads 32 which engage the terminations of the integrated circuit device 20. Figure 2 shows an additional contact piece 34 disposed within the hole in the housing 12 to ensure contact . Such a piece may be necessary to extend the conductive pad 32 if the bottom wall of the housing 12 is very thick.

Formed on the conductive pad 32 on the side of the circuit board 28 opposite the socket housing 12 is a conductive projection 36. The projection 36 may be hard metal, such as nickel or copper, solder, or hard metal having a solder-coating. Although a spherical shape for the projection 36 is shown, other shapes are possible and may be preferred in some applications. The projections 36 may assume the shape of buttons, columns, blocks, crowns, pyramids, donuts and so forth. The circuit board 28 may include a layer of solder resist 38 or cover coat applied to the surface of the circuit board 28 having the projections 36. This layer would be particularly desirable if solder is used for the projections 36 to aid in the placement of the solder and to prevent the solder from bridging together during the process of soldering the socket 10 to the equipment circuit board. Figure 3 illustrates an embodiment of a socket 40 wherein the circuit board 42 is located within the well 44 of the housing 46 rather than on the bottom of the housing 46. In this embodiment, the circuit board 42 is provided with a via 48 and a conductive pad 50 which closes off the via 48. The via 48 allows the terminations of the integrated circuit device 20 to make contact with the conductive pad 50, and the pad 50 allows attachment of a solder projection 52 while preventing solder from flowing into the via 48. Figure 4 illustrates an embodiment of a socket 60 wherein the orientation of the circuit board 62 is reversed so that the conductive pad 64 is disposed upwardly, away from the housing bottom 66. This construction ensures accessibility of conductive pad 64 to the terminals of the integrated circuit device 20, but requires alignment features in the socket 60 since holes are not present to capture the integrated circuit terminations.

Figure 5 illustrates a more robust socket 70 which is particularly adapted to test and burn-in applications and which utilizes the unique properties of flexible circuitry. The socket 70 houses an integrated circuit device 72 within a well 74 and includes solder projections 76, or projections 76 of other conductive materials, on the bottom surface 78. Within the well 74 is an elastomeric layer 80 which forces a flexible circuit 82 into contact with the terminations 84 of the integrated circuit device 72. The base of the socket is built in sections to allow the flexible circuit 82 to be routed to solder balls disposed beyond the limits of the integrated circuit device 72. Thus the pattern of the terminations 84 of the integrated circuit device 72 can be rerouted to a different or larger pattern at the circuit board to which the socket 70 is attached. It will be recognized that in this configuration, unlike those described above, the solder balls 76 are not connected directly to the conductive pads which contact the terminations of the integrated circuit device. Rather, the conductive pads and the solder balls 76 are connected to each other by a conductive trace on the flexible circuit . Such an arrangement rather than the one-to-one connection shown is possible with any of the embodiments described above, and would be used if it were desired to, for example, enlarge or change the pattern of the solder balls extending from the socket with respect to the pattern of the terminations on the integrated circuit device.

Thus there has been described a socket which is compact in size compared to those using stamped contacts, has high electrical performance due to short electrical path lengths, and is easily assembled to circuit boards because of its compatibility with existing surface mount assembly equipment .

Although the invention has been described with respect to only a limited number of embodiments, many modifications are possible. For example, other features could be added such as stand-off protrusions that ensure the collapsed height of the solder ball and provide support for the socket assembly, thus reducing stress on the solder joint, locating protrusions that ensure proper alignment of the socket to the printed circuit board and orientation protrusions that ensure proper orientation and alignment of the socket to the printed circuit board. Finally, although the invention has been described primarily with respect to an integrated circuit device having its terminations disposed on a flat bottom in a square of rectangular array, the socket can be used with other integrated circuit packages such as quad flat packs, wherein the terminations exit the sides of the package and bend to extend downwardly below the bottom surface of the device.

Claims

The invention claimed is:

1. A socket for accepting an integrated circuit device having an array of terminals and connecting the integrated circuit device to a surface of a second device comprising: a housing having a top surface, a bottom surface, and a well defined by an opening in the top surface for accepting the integrated circuit device; a circuit board having a substrate, conductive pads attached to said substrate and an array of projections connected to said pads; wherein said circuit board is disposed to engage the integrated circuit device disposed within said well with a different one of said pads in electrical contact with a different one of the integrated circuit device terminals; and wherein said array of projections project from said housing and may be used to connect said socket to the second device.

2. A socket according to claim 1 wherein said circuit board is attached to said housing bottom surface with said pads disposed adjacent said bottom surface and wherein said housing includes holes corresponding to said circuit board pads and extending from said bottom surface to said well to provide access of the integrated circuit device terminals to said circuit board pads .

3. A socket according to claim 2 further including an adhesive attaching said circuit board to said socket housing.

4. A socket according to claim 3 wherein said adhesive is an epoxy.

5. A socket according to claim 2 wherein said projections are of solder.

6. A socket according to claim 5 wherein said projections are spherical.

7. A socket according to claim 2 further including a lid for retaining said integrated circuit device within said well .

8. A socket according to claim 1 wherein said housing includes first holes extending from said bottom surface to said well and said circuit board is disposed within said well with said conductive pads aligned with said first holes.

9. A socket according to claim 8 wherein said conductive pads are disposed on said circuit board opposite said housing and said circuit board includes second holes extending from said conductive pads to communicate with said housing first holes.

10. A socket according to claim 9 further including an adhesive attaching said circuit board to said socket housing.

11. A socket according to claim 10 wherein said adhesive is an epoxy.

12. A socket according to claim 9 wherein said projections are of solder.

13. A socket according to claim 12 wherein said projections are spherical.

14. A socket according to claim 9 further including a lid for retaining said integrated circuit device within said well.

15. A socket according to claim 8 wherein said conductive pads are disposed on said circuit board adjacent said housing and said circuit board includes second holes extending from said conductive pads to provide access of the terminals of the integrated circuit device to said conductive pads.

16. A socket according to claim 15 further including an adhesive attaching said circuit board to said socket housing.

17. A socket according to claim 16 wherein said adhesive is an epoxy.

18. A socket according to claim 15 wherein said projections are of solder.

19. A socket according to claim 18 wherein said projections are spherical.

20. A socket according to claim 15 further including a lid for retaining said integrated circuit device within said well.