Hold-down collar for attachment of IC substrates and elastomeric material to PCBs

Abstract: A land grid array package (21) or a ball grid array package is electrically connected by means of an elastomeric type layer (26) containing alternating elements of conductive and non-conductive silicone, to a printed wire board (31) or motherboard. A hold-down collar (36) engages the peripheral edges (24) of the package (21) and snaps into holes (33) in the board (31) in such manner as to create pressure on the elastomeric layer (26), causing the layer (26) to compress and create through-going electrical paths between exposed lands (22) or conductive elements on the bottom of the package (21) and exposed conductive elements (32) on the wiring board (31). In one embodiment, solder balls adhere to the exposed conductive elements on the bottom of the package. In this embodiment the collar is initially in two pieces which are fitted together to engage both the top and bottom edges of the substrate.

Description: The invention relates to an IC package assembly comprising the features set out in the preamble of claim 1. Packages using inexpensive chips normally are soldered to a printed wire board. When changing a chip is required, major reworking of the board is necessary and usually this cannot be done in the field. Hence it is desirable, particularly for expensive chips, to provide means for removing one package and replacing it with another. The systems heretofore used for such replacement employ sockets which impart considerable force to the substrate to the hold it in place and hence the devices are large, complex and expensive. Furthermore, such sockets interfere with the use of heat sinks on top of the package. An IC package with the features of the preamble of claim 1 is known from DE-A-3701310. A similar IC package is known from US-A-3885173. These IC packages comprise a printed wire board, a hold-down collar and an intermediate ring of elastomeric material, wherein the intermediate ring is made up of alternating conductive and non conductive rings for producing a conductive connection between the printed wire IC package and contact zones on the printed wire board. This type of arrangement has a low level of accuracy and an automated assembly of these elements is difficult to achieve. More specifically, in large scale production, particularly where robots are used, it would be difficult or impossible to properly align these elements. It is thus an object of the present invention to improve this prior art arrangement in that an auto-mated attachment of IC substrates to printed wire board is achieved. This object is met by the invention as set out in new independent claim 1. Through the combination of providing holes in the elastomeric layer, an exact alignment of the layer and its conductive lands is achieved with respect to the printed wire board and the IC package through the fact that the legs may pass through the apertures, thus providing an exact positioning for the layer between the IC package and the printed wire board. This structure is ideally suited to robotic assembly. When necessary, the collar may be disengaged in order to disconnect the package from the board so that a replacement may be installed. One type of package used by this invention is a land grid array (LGA) wherein metallic lands are exposed below the surface of the package. Another type of array is a ball grid array (BGA) wherein minute balls of solder are attached to the lands on the underside of the package. The present invention accommodates both types of arrays. Where a ball grid array is used, the collar grips both the upper and lower surface of the package. Hence the collar must be split and later fitted together in order to engage the package. For land grid array packages, a split collar is not necessary. BRIEF DESCRIPTION OF THE DRAWINGS: The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description serve to explain the principles of the invention. FIG. 1 is a greatly enlarged vertical sectional view through a portion of the package elastomeric layer, printed wire board and collar of the present invention. FIG. 2 is a further prospective view of a collar used with the structure of FIG. 1. FIG. 3 shows a modified structure using balls of solder attached to lands on the underside of a BGA package prior to assembly. FIG. 4 is a sectional view showing the structure of FIG. 3 assembled. FIG. 5 is a view similar to FIG. 2 of a modified collar used with the structure of FIGS. 3 and 4. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT: Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to those embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. An IC package 21 may be of various constructions. Essentially it consists of a substrate 21 having an array of lands 22 on the underside thereof, such lands being exposed approximately circular conductive elements of about 0,66 mm (26 mil) dia. about 0,035 mm (1.4 mil) thick. Various forms of encapsulation 23 are used to protect the package. The marginal edges 24 of the package are exposed. Beneath the package 21 is a thin soft elastomeric layer 26. Such a layer may be of a material produced by Elastomeric Technologies, Inc. of Hatboro, Pennsylvania, known as STAX or MOE. The STAX version is made of alternating vertical elements of conductive and nonconductive silicone, there typically being 78 to 98 elements per cm (200 to 250 elements per inch). Such material has been used to connect printed wiring boards to flex circuits, flat paneled displays, LCD displays and other PWBs, as well as in IC sockets. MOE (metal-on-elastomer) has metallic contact such as gold or nickel which are layered on skins of nonconductive plastic. These layers have also been used for redundant contact in board-to-board chip carrier, direct-to-board, and other applications requiring low resistance and close spacing. Holes 27 are formed adjacent to each corner of layer 26. Beneath layer 26 is a printed wire board 31 such as a PCB, motherboard or the like. Board 31 has an array of lands 32 of conductive material exposed on its upper surface. Lands 32 are vertically aligned with lands 22. In accordance with the invention, four slots 33 are formed in the board in vertical alignment with the comers of package 21 and the holes 27 of layer 26. Collar 36 is made of a strong, flexible plastic material in a shape slightly longer than package 21. It has top horizontal flanges 37 to engage the margins of edges 24 of package 21. Vertical sides 38 extend below flanges 37 and at the corners of the collar 36 there are depending legs 39 corresponding in spacing to holes 27 and slots 33 having prongs 41 at their lower ends. The collar 36 is pushed downwardly, the legs 39 extending through the slots 33 until the prongs 41 snap under the lower surface of board 31, thereby detachably locking collar 36 in place. For such purpose slots 33 are at least as large as prongs 41 and are so located that, as prongs 41 enter slots 33, legs 39 are biased outward. When prongs 41 clear slots 33, legs 39 return to vertical position and prongs 41 underlie board 31, holding collar 36 attached to board 31. The dimensions of collar 36 are such that the space between the underside of flange 37 and the upper surface of prongs 31 is slightly less than the distance between the top surface of package 21 and the bottom surface of board 31. This results in compression of layer 26, causing the conductive materials therein to engage both lands 22 and 32, thereby establishing redundant electrical contact therebetween. If necessary, the collar 36 may be removed, permitting replacement of package 21 with a different package without requiring major operation on the board 31. Directing attention now to FIGS. 3-5, the package 21 is of the ball grid array type wherein minute balls of solder 46 (about 26 mil diameter) have been caused to adhere to the lands 22 a on the underside of package 21 a . In other respects the package 21a, layer 26 a , and board 31 a resemble those of the modification of FIG. 1. Collar 36 a , however, is somewhat modified from the collar 36 of the preceding modification. A horizontal lower flange 48 is formed below flange 37 a . The spacing between flanges 37 a and 48 is such that the edge 24 a of package 21 a is engaged. The thickness of flange 48 is slightly less than the distance between the underside of package 21a and the upper surface of layer 26 a . In order to fit the collar 36a over the package 21a, it is best split. One form of split is best illustrated in FIG. 5 wherein there is a vertical parting line 49 through two sides of the collar 36 a . To facilitate assembly thereof, tongues 51 in one half of the collar interlock into grooves 52 in the other half, as by a dovetail structure. After the collar 36 a has been placed engaging the edges 24 a , the two halves of the collar are suitably joined together either with or without use of an adhesive. In this embodiment, the lands 22 a are in electrical engagement with the solder balls 46 which are in turn in engagement with the conductive stripes in the layer 26 a . In the modification of FIGS. 1-2, a conductive member on the package comprises land 22; in the modifications of FIGS. 3-4 it comprises land 22 a and ball 46. In other respects, the modification of FIGS. 3-5 resembles that of FIGS. 1 and 2 in the same reference numerals followed by the subscript a are used to indicate corresponding parts. The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.

Claims: An IC package, assembly comprising: an IC package (21) having an array of conductive members (22) in a first array on a lower surface and having peripheral top edges (24), a layer (26) of soft elastomeric material formed with a plurality of conductive and non-conductive vertical elements beneath said package (21), a printed wire board (31) below said layer (26) having a plurality of conductive lands (32) in a second array on an upper surface of said board, said board (31) underlying said layer (26) and being formed with hold-down collar engaging means (33), a hold-down collar (36, 36a) having in-turned flanges (37) engaging said top edges (24) of said package (21), legs (39) extending downward relative to said flanges (37) and having board-engaging means (41) co-operable with said collar-engaging means (33) to apply downward force to said flanges (37) and secure said collar (36, 36a) to said board (31), said collar (36, 36a) being vertically dimensioned so that when said collar (36, 36a) is secured to said board (31) said layer (26) is compressed and conductive elements (22) are exposed to establish electrical circuits between pluralities of said members (22) and corresponding pluralities of said lands (32), characterized in that said layer (26) is formed with holes (27), said legs (39) passing through said holes (27). The IC package of claim 1, in which said package comprises a substrate (21) and encapsulating material (23) on an upper surface of said substrate (21). The IC package of any of the preceding claims, in which said board engaging means comprise prongs (41) on said legs (39) and said collar engaging means comprise apertures (33) in said board (31) to receive said prongs (41). The IC package of any of the preceding claims, in which each said conductive member (22) comprises a second land (22a) on the underside of said package (21a) and a ball of solder (46) adhered to said second land (22a). The IC package of any of the preceding claims, in which said collar (36, 36a) comprises a vertical skirt (38, 38a) depending from said flanges (37, 37a), said legs (39, 39a) depending from said skirt (38, 38a). The IC package of claim 5 , in which said collar (36a) further combines second in-turned flanges (48) spaced below said first-mentioned flanges (37a) a distance substantially equal to the thickness of said package (21a) at said top edges (24a). The IC package of claim 6 , in which said second in-turned flanges (48) have a thickness less than the distance between the underside of package (21a) and the upper surface of layer (26a). The IC package of claim 6 or 7, in which said collar (36a) is initially formed in at least two pieces and reassembled together surrounding said package (21a), said first-mentioned flanges (37a) engaging said package (21a) above said top edges (24a) and said second flanges (48) engaging the bottom of said package (21a). The IC package of any of claims 5 to 8, in which said package (21, 21a), said layer (26, 26a) and said collar (36, 36a) are substantially rectangular.

Inventors: SELNA ERICH H, SELNA, ERICH H.
Applicants: SUN MICROSYSTEMS INC, SUN MICROSYSTEMS, INC.
Application year: 1997
Main classification: H05K3
IPC codes: H05K7, H05K3