Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
2001-11-21
2002-11-19
Tolin, Gerald (Department: 2835)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C257S719000, C403S270000
Reexamination Certificate
active
06483708
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a system for attaching a heatsink to an integrated circuit device, and more particularly, to a system for clamping a heatsink to an integrated circuit device that applies controllable force onto the integrated circuit device.
2. Description of the Related Art
The use of integrated circuits is becoming more prevalent every day. Integrated circuits (ICs) are used in a multitude of different devices from household appliances to computer applications. However, ICs are also rather fragile. They are generally thin pieces of silicon on which circuits are constructed. These IC “chips” are subject to corrosion, environmental damage, physical shocks, and other damage mechanisms. For this reason, IC chips are packaged using a variety of different materials and package styles to protect them from possible damage during transportation and use.
Conventional protective packaging is generally a plastic or ceramic material used as a base for the IC chip and serving as a means of expanding (“fanning out”) the electrical connections of the IC chip. The connections between the IC chip and the package are typically accomplished using wire bonds or, in the case of “flip chip,” solder balls. In a “flip chip” arrangement, the top of the IC chip is flipped over face down onto the base package. Solder balls placed between the face of the chip and the base package provide electrical connections between the chip and the base package. Additionally, a lid may be attached over the IC to provide protection for the chip. The choice of protective packaging will be determined by various factors including the parameters of the chip itself, the IC application, and the packaging material cost.
In addition to providing chip protection, a component is also needed to allow the chip and package to make electrical connections to other devices. These connection components function as electrical components, with circuits that connect the chip to the Printed Circuit Board (PCB) or other device to which the chip is attached.
There are several different types of connection components allowing a chip and package to make electrical connections to a PCB. The selection of the appropriate electrical connection component will depend to a large extent upon the particular design of the chip itself, the number of connections required, and the size of the package. For example, for connections with a chip encased in a protective package that is no larger than 32 mm square (1.59 in
2
) area, an array of solder balls (a ball grid array package or “BGA”) may be used to make the electrical connections. Similarly, for packages no larger than 42 mm square (2.73 in
2
) area, an array of solder columns (a column grid array package or “CGA”) may be used to make the electrical connections. Both the ball and column grid arrays can be directly soldered to a PCB.
Both the ball and column grid array methods have a size limitation stemming from the long-term reliability of the connection. This limitation is mainly a function of the coefficient of thermal expansion (CTE) mismatch that typically exists between the IC package construction material and the PCB construction material. As a package gets larger the outermost connections from the center of the package may be stressed beyond their yield point as the IC chip heats the package. Therefore, for larger chips with a high lead count, or for more durable chip package systems, it is preferable to use an interposer component. An interposer provides an array of compressible contact points, such points being comprised typically of a polymer with imbedded electrically conductive metal or comprised of a metal spring structure. The required electrical connection is accomplished through contact pressure rather than a rigid bond. By being a compliant connection the interposer thus effectively decouples heat expansion stresses that can occur between the package and the PCB. However, when using an interposer to provide an electrical connection between a packaged IC and a PCB, considerable pressure must be applied to ensure a good low resistive electrical contact connection is achieved.
Certain IC chips also require the capability to dissipate a large amount of heat energy. For example, some high-powered chips may give off over 100 watts of heat energy. For such high-powered chips, the cooling provided by ambient air is not sufficient to prevent the chip from overheating. An additional component for removing the heat from the chip is required. A heatsink is commonly attached to an IC chip package, with a thermal interposer material in-between, in order to provide superior heat dissipation. The thermal interposer provides good thermal conductivity between the device and heatsink. In a conventional system, the heatsink may be attached, with a thermal interposer material in-between, to the package lid protecting the chip, or if no lid is used, directly to the chip itself.
The entire assembly consisting of the heatsink, the thermal interposer, the chip, the base package, and the electrical connection components, is clamped together to ensure proper electrical connections and heat transfer capabilities. Significant clamping force (often exceeding several hundreds of pounds) is required to clamp the chip-package-interposer-PCB arrangement tightly enough to ensure a proper electrical connection through the interposer. The heatsink-to-chip connection does not require a similar clamping force to provide heat transfer capability. However, because the entire assembly is clamped together when a conventional heat sink structure is used, all components are subjected to the same clamping force. This clamping force could damage the chip itself, even though a lid may be used, since the lid could compress into the underlying chip.
In the conventional packaged chip and heatsink arrangement where the chip is covered by a lid, the heatsink and thermal interposer do not contact the chip directly. Heat must pass through the air layer or other conductive layer between the chip and the lid, as well as through the lid itself, and the thermal interposer, before being dissipated by the heatsink. Such an arrangement makes it difficult to effectively cool certain high-powered chips. A preferable arrangement is to provide contact directly between the heatsink and the chip itself. This arrangement provides for superior heat transfer properties. However, without a lid present to absorb some of the compressive forces, extreme care must be taken to ensure that the chip is not crushed in this situation due to the clamping force required for the rest of the assembly.
Thus a high-powered chip packaging assembly that includes both a heatsink and an electrical interposer has two different, competing clamping force needs. Significant clamping force is required for achieving proper connections in the chip-package-electrical interposer-PCB portion of the assembly. However, significantly less clamping force is desirable between the chip-thermal interposer-heatsink assembly, to avoid damaging the chip. Thus, there is a need for a system that decouples the clamping forces between these two sections of the overall assembly.
SUMMARY OF THE INVENTIONS
The present invention provides a system that decouples the clamping force in an electrical circuit assembly coupled to a heatsink. A heatsink clamping assembly applies controllable and predictable force on the electrical circuit assembly including a packaged microelectronic integrated circuit device (“chip”). The applied force is controlled to effectively ensure intimate contact between the chip and the heatsink to facilitate efficient chip cooling. The force applied to the chip is decoupled from the much higher force required to clamp the interposer interconnection between the electrical circuit assembly and the printed circuit board. There are certain instances where the base of the heat sink may be a hollow chamber or heat pipe structure, and as such could be damaged if the full clamping force were required to be imposed
Ali Hassan O.
Bechtel Richard L.
Fenwick & West LLP
Fujitsu Limited
Tolin Gerald
LandOfFree
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