Land grid array socket actuation hardware for MCM applications

Details Land grid array socket actuation hardware for MCM applications Land grid array socket actuation hardware for MCM applications

2001-09-07

2002-11-05

Luebke, Renee (Department: 2833)

Electrical connectors

With coupling movement-actuating means or retaining means in...

For dual inline package

C439S076200, C439S342000

Reexamination Certificate

active

06475011

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electrical connectors and more specifically to a device for connecting a multi-chip module to a printed wiring board.
2. Description of the Prior Art
Land grid array socket assemblies are common in the electronics industry for mounting single chip modules to printed wiring boards. The interconnection of a land grid array (LGA) module to a printed wiring board (PWB) requires the accommodation of a high area density of electronic contacts and must result in a highly reliable electronic connection over a range of operating environments. One method of interconnecting an LGA module to a PWB is by using a conductive interposer. The interposer has an array of electrical contacts on one surface which mirrors those of the LGA module and, on the opposing surface, an array of electrical contacts which mirrors those of the PWB. The mounting of the LGA module is then accomplished by aligning the electrical contacts of the LGA module, interposer and PWB and mechanically compressing the interposer.
A land grid array socket assembly using an interposer has several advantages over other more traditional methods of component mounting. The modules may be changed or easily upgraded in the field. Also, system assembly and rework costs may be reduced during production. The interposer reduces the effects of thermal expansion mismatch between the chip modules and the PWB by acting as a compliant member between the chip module substrate and the PWB surface. This compliant property of the interposer ensures electrical connectivity of the assembly over a range of thermal and dynamic operating environments.
The demand for higher performance in electronic equipment has led to the development of LGA socket and interposer assemblies for multi-chip module applications. However, the mounting of a multi-chip module presents challenges due to the greater number of electrical contacts and larger substrate size inherent with this type of electronic component.
A key challenge in using LGA sockets with interposers for multi-chip modules is the creation of a consistent mechanical clamping force to compress the interposer between the multi-chip module and the PWB. A consistent clamping force is required to ensure positive electrical connections between the components and to maintain the alignment of the assembly over various operating environments. A multi-chip module requires a high density of electrical contacts over the surface of the module substrate. This high density of contacts necessitates an initial accurate alignment of the assembly and a controlled and predictable compression force to maintain the multi-chip module, interposer and PWB in electrical contact.
Various hardware configurations have been employed to achieve the compression of the LGA socket, interposer and multi-chip module assembly. Typical existing systems use a spring member to compress the components together. The components are assembled upon the PWB and a spring member is deflected by spring actuation hardware thus clamping the components in place. One problem inherent in this approach is the range of spring deflections achieved, and hence the range of clamping forces generated, due to the mechanical tolerances presented by the assembly. The mechanical tolerances of the actuation hardware, multi-chip module, interposer and printed wiring board all directly effect the spring deflections generated in the complete assembly.
One example of an existing system for securing a multi-chip module in an LGA socket connection upon a printed wiring board is shown in FIG.
1
. In such a system, a multi-chip module body
110
includes a substrate portion
112
, upon which a plurality of integrated circuit chips are mounted, and a housing, which typically includes a heat sink. The substrate portion
112
is mounted upon a printed wiring board (PWB)
116
using an interposer
114
. An interposer
114
is a thin sheet with a plurality of electrical contacts, arranged to mirror the electrical contacts of the substrate
112
and the printed wiring board
116
, passing therethrough that facilitates electrically coupling the substrate
112
to the printed wiring board
116
. The multi-chip module
110
is clamped into position by load posts
120
, spring elements
122
, and actuation nuts
124
. The spring elements act upon a load plate
118
positioned on the underside of the PWB
116
. As the actuation nuts
124
are tightened, the spring elements
122
are compressed between the load plate
118
and the actuation nuts
124
. The actuation nuts
124
create a tensile load on the load posts
120
and the load plate
118
is compressed up against the PWB
116
. The tensile load in the load posts results in a downward force on the multi-chip module body
110
which compresses the substrate
112
, interposer
114
and PWB
116
together.
The spring actuation hardware typically includes a threaded actuation member which is used to compress the spring member. To compress the spring, the clearances in the assembly are first removed by advancing the actuation member. The actuation member is then further advanced a given number of turns to create a known deflection of the spring member. One source of uncertainty in this approach is that the determination of when the tolerances have been removed from the assembly is a subjective judgment. A second source of uncertainty is associated with monitoring the turn count of the actuation member. The end result is an imprecise displacement of the spring element and a resulting uncertainty in the compressive force applied to the multi-chip module, interposer and PWB assembly.
As further demonstrated in
FIG. 1
, existing systems apply the compressive force about the periphery of the assembly only. This non-uniform application of force results in an uneven deflection of the multi-chip module substrate
112
, PWB
116
and interposer
114
. This deflection of the components allows a corresponding variance in the compressive force seen by the individual electrical contacts across the surface of each component. The result is that the electrical contacts at the center of each mating component face are not as tightly compressed as the electrical contacts about the edges of the assembly, demonstrated by arrows
126
. This variance in contact pressure reduces the integrity of the electrical connection when exposed to a range of operating environments.
Therefore, there is a need for a device that predictably applies even force to a multi-chip module and a printed wiring board.
SUMMARY OF TILE INVENTION
The disadvantages of the prior art are overcome by the present invention which, in one aspect, is an apparatus for applying force to a multi-chip module having a substrate, a printed wiring board having a first side and an opposite second side and an interposer. The interposer facilitates electrical contact between the substrate and the printed wiring board through the interposer. The multi-chip module and the interposer are disposed on the first side of the printed wiring board.
The apparatus includes a plurality of elongated spaced-apart load posts, a load transfer plate, a spring member, a backside stiffener plate, and a spring actuator. The load posts are affixed to the multi-chip module and pass through a plurality of post holes defined by the printed wiring board from the first side to the second side of the board. Each load post has a proximal end affixed to the multi-chip module and an opposite distal end that defines an engagement surface. The load transfer plate is disposed opposite the multi-chip module and spaced apart from the second side of the printed wiring board. The load transfer plate defines a plurality of openings through which the distal ends of each of the plurality of load posts pass. Each of the plurality of openings is shaped so as to engage the engagement surface of the distal end of a corresponding load post.
The spring member is disposed adjacent the load transfer plate between the load transfer plate and the printed wiring

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