Land grid array subassembly for multichip modules

Details Land grid array subassembly for multichip modules Land grid array subassembly for multichip modules

2001-08-09

2002-09-10

Chervinsky, Boris (Department: 2835)

Electricity: electrical systems and devices

Housing or mounting assemblies with diverse electrical...

For electronic systems and devices

C361S707000, C361S709000, C361S712000, C361S715000, C361S718000, C165S080200, C165S080300, C165S185000, C174S016300, C257S718000, C257S719000

Reexamination Certificate

active

06449155

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to the field of multichip modules of integrated circuits, and more particularly relates to a field replaceable unit of a multichip module having land grid array interconnections.
BACKGROUND OF THE INVENTION
In the world of integrated circuits, there are a multitude of electrical connections between the integrated circuits of an electronic device such as a computer processor and other integrated circuits within the processor and eventually to the “outside world.” As integrated circuits become more dense so must the electrical connections. Integrated circuits are mounted on printed circuit boards and may be connected to other circuits by traces embedded in a printed circuit board. These traces are interconnected with vias/microvias which allows connection of one trace on one circuit-board layer to a trace on a different layer. Varying and controlling the physics of each connection by controlling the dielectric used, the dielectric thickness, and the signal-path area of and within each layer can result in a specific, controlled characteristic impedance. These vias/microvias, however, degrade the continuity of a signal path by introducing variations in the electrical qualities of the signal transmnission.
There are a myriad of options to provide the electrical connections to/from integrated circuits with these considerations incorporated into the design. Some of the options include various small outline packages, plastic leaded chip carrier, dual inline packages, pin grid arrays, ball grid arrays, etc. The next generation of integrated circuits, such as a system-on-chip and other high density devices, however, require high density electrical interconnections. While these fine, high density circuit traces increase the density of a interconnect they also increase the inductance, resistance, and current-carrying ability of the interconnect. Previous packaging options, like pin grid arrays and quad flat packs all left something to be desired in accommodating the dense new technology of printed-wiring boards with trace widths as small as or even smaller than 0.003 inch. Even the ball grid array is not an option with high-end printed circuit boards requiring 1000 and more pin counts because the large footprints can not stand the forces on the solder joints that are caused by thermal mismatch, i.e., the materials of the semiconductor device have different coefficients of thermal expansion than those of the target printed circuit board. Thus, high-speed, high-density circuit boards are difficult to design, have exacting requirements, and are expensive to manufacture.
Area array socket connectors are an evolving technology in which an interconnection between mating surfaces is provided through a conductive interposer. One significant application of this technology is the socketing of land grid array (LGA) modules directly to a printed wiring board in which the electrical connection is achieved by aligning the contact array of the two mating surfaces and the interposer, and mechanically compressing the interposer. The advantages of LGA socket connectors include: the capability to upgrade electronics in the field; flexibility in starting up and diagnosing an electronic system; reduction of cost required to rework the previously assembly board; reduction of mismatch between the coefficients of thermal expansion between the module and the board; improvement of electrical performance; and the compactness and low profile of the electrical connector designs. Although not technically accurate, the easiest way to envision an LGA device is to picture a semiconductor with nothing but tiny round gold plated pads on the bottom whereas if the device were a ball grid array, a ball would be soldered to each pad. The biggest reason for terminating a device as an LGA is to achieve higher pin counts, i.e., number of outputs, with smaller packages. The “z-axis” connection configuration of a LGA, moreover, can overcome the thermal mismatch problems described above with respect to the ball grid arrays. In short, the LGA offers a viable interconnection for high speed, high density integrated circuits.
At a pitch of, e.g., one millimeter, a LGA has 1,225 interconnections in a 35×35 grid that is less than 1.5 square inches and 2,025 interconnections in a 45×45 grid that is less than 1.75 square inches. LGAs are easier and cheaper to manufacture because terminations such as pins or balls are no longer required. Land grid arrays have low inductance because the distance from the bottom of the device being socketed to the target board can be less than two millimeters with some LGA socket designs thus minimizing the electrical path of each connection. Co-planarity problems are eliminated because LGA sockets can be manufactured for spring movement of 0.015″ (0.4 mm), which “takes up the slack” when there is a problem with co-planarity on the bottom of the device.
Almost all LGA interconnections require use of mechanical elements that apply a controlled load to LGA connector elements. Examples of these mechanical elements include an interposer or socket component; something that possesses the specific LGA pattern of exposed contacts on top and bottom faces of the connector and mates to corresponding module and board surfaces to be interconnected. To ensure reliable LGA interconnection performance, both contact members in the interposer and mating surfaces of boards and module LGA contact pads must possess a noble surface finish that is both resistant to corrosion and provides low contact resistance within the contact load range necessary for mating of the connector. To provide these attributes on printed circuit boards, the surface of LGA contact pads may be plated with a nickel or gold or palladium or a combination thereof (Ni/Au/Pd). In many applications, including some backplane applications, these surfaces may be plated by selective deposition of electrolytic Ni/Au platings as opposed to use of electroless or immersion platings.
There are various type of LGA interposers, among them being the CINCH interposer which is based on fuzz button technology to provide a high-speed, Z-axis, compression mount interconnect which has a very low profile interconnect. The compressed signal path length may be as short as 0.8 millimeter to enable transmission of frequencies greater than 1 giga-Hertz. Palladium or gold plated molybdenum wire is randomly wound to form an individual contact which can be spaced one millimeter on center or greater, and which may be mounted in a plastic molded frame. Another LGA interposer is the TYCO metal particle interconnect technology, in which silver-filled elastomeric contacts are molded onto and through a KAPTON carrier which is staked onto a plastic frame. Yet a third type of LGA interposer is the cLGA product from Intercon in which C-shaped clips are individually inserted into cavities molded in a plastic frame.
LGA socket assemblies are prevalent today in the electronics industry, but are typically used to attach single-chip-modules to printed wiring boards. The demand for higher performance is driving a requirement to develop LGA socket applications for multichip modules to printed wiring boards.
SUMMARY OF THE INVENTION
Thus what has been disclosed herein is a land grid array, multichip module assembly comprising: a thermal transfer unit; an electronic multichip module; a module cap having shimmed load posts and adjustable brackets; a land grid array electrical connector to provide electrical interconnections to the electronic multichip module, wherein the land grid array is placed upon the adjustable brackets with the shimmed load posts providing a particular load to the electrical interconnections; and a protective cover.
The thermal thermal transfer unit may comprise a heat sink and a thermal interface between the heat sink and the module cap. The shimmed load posts are preferably positioned at several locations near a center of each dimension of the periphery of the module cap. The shimmed load posts may also be pos

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