Mechanically floating multi-chip substrate

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TECHNICAL FIELD

The present invention relates generally to integrated circuit packages and more particularly to modules having an array of integrated circuit chips electrically connected to a substrate contained within a housing.

BACKGROUND ART

Multi-chip modules play an increasingly important role in the electronics industry. Integrated circuit chips within a module may be functionally equivalent, such as an array of memory chips to provide a capability of forty megabytes. Alternatively, the chips may be functionally related, such as a chip set comprising a read only memory chip, a random access memory chip, a microprocessor and an interface chip.

As the number of chips confined within a single module increases, the importance of providing adequate cooling also increases. U.S. Pat. No. 5,006,924 to Frankeny et al., U.S. Pat. No. 5,001,548 to Iversen, U.S. Pat. No. 4,879,629 to Tustaniwskyj et al. and U.S. Pat. No. 4,750,086 to Mittal all describe use of a liquid coolant that is forced to flow through a multi-chip module to absorb thermal energy, whereafter the liquid coolant is removed from the module at an outlet port. Providing a liquid coolant loop through a module is an effective may of ensuring adequate cooling, but is an expensive cooling method. Requiring a mechanism for providing a forced flow of liquid coolant would be cost inefficient in such applications as computer workstations.

For small and medium scale applications in which forced liquid cooling is not a cost-efficient option, heat exchangers, or heat sinks, are used to dissipate thermal energy into the atmosphere surrounding a multi-chip module. Particularly for high power chips that generate a significant amount of thermal energy, this places an importance on the heat transfer interface of the chips to the heat exchanger. Ideally, contact is made between the integrated circuit chips and the structure that begins the thermal path to the surrounding atmosphere. A difficulty with this ideal is that during the fabrication of a manufacturing lot of multi-chip modules, there will be dimensional differences among the modules and even among the various chips within a single module. For example, chips are often encased within a chip carrier before being mounted to a component surface of a substrate that is attached to the heat exchanger. The carriers may have slight differences in height and/or the mounting of the carriers to the substrate may result in slight variations in height or angle with respect to the component surface of the substrate. Various fabrication and machine tolerances are additive, so that the carriers within a multi-chip module will not have coplanar upper surfaces. Bellows assemblies with forced liquid cooling for adaptation to individual chips or carriers of a module, such as described in the Mittal and Tustaniwskyj et al. patents, may be used where cost is not a major concern, but ensuring adequate contact between individual chips and a heat dissipating structure is more difficult in many applications.

Alternatively, thermally conductive pillows may be placed between the heat spreader and the chips, as described in U.S. Pat. No. 5,000,256 to Tousignant, U.S. Pat. No. 4,997,032 to Danielson et al. and U.S. Pat. No. 4,092,697 to Spaight. For example, Spaight describes an electrically nonconductive film contacting a single chip at a first side of the nonconductive film and containing a thermal liquid material at a second side.

It is an object of the present invention to provide a multi-chip module that achieves an adaptive heat transfer interface in a reliable, cost-effective manner. A further object is to provide a multi-chip module that achieves an adaptive heat transfer interface without forcing liquid cooling and that provides an electrical path to semiconductor chips of the module.

SUMMARY OF THE INVENTION

The above object has been met by a stand-alone module in which integrated circuit devices, such as semiconductor chips or chip carriers, are caused to be displaced in order to adapt to a heat exchanger. The adaptive displacement may be in response to a difference in thermal expansion of the heat exchanger relative to the integrated circuit devices, and is accomplished by mechanically floating a substrate on which the devices are mounted. Thus, rather than incorporating a structure to allow a heat exchanger to adapt to relative positions of integrated circuit devices, any adaptations at the device/exchanger interface are made by repositioning the substrate.

The integrated circuit devices are mounted on a component surface of the substrate. The substrate is contained within a chamber of the heat exchanger and is mounted to allow the substrate to "float" within the chamber. In one embodiment, the substrate is biased to press the integrated circuit devices against an upper wall of the chamber. For example, one or more springs may be disposed between the substrate and a lower wall of the chamber to press the substrate upwardly. Preferably, the spring is packed within a thermal grease to provide a thermal path from the substitute to the heat exchanger, thereby providing a thermal flow path for the dissipation of thermal energy. This thermal flow path is in addition to the path originating at the device/exchanger interfaces.

Mechanical floating can also be achieved by use of a conformal mechanism at the surface of the substrate opposite to the integrated circuit devices. A thin membrane that is parallel to the substrate is used to entrap a fixed volume of liquid. Preferably, the liquid is under pressure by the entrapment between the heat exchanger and the membrane, so that the liquid presses the membrane outwardly. The pressure on the membrane ensures a compression contact of the membrane against the substrate, so as to press the integrated circuit devices against the heat exchanger. The membrane and the liquid preferably are thermally conductive. For example, the membrane may be a stainless steel member and the liquid may be distilled water having a concentration of an additive to retard oxidation of the membrane and the heat exchanger.

In another embodiment, an arrangement of a conformable membrane and entrapped liquid is formed at both the upper and lower walls of the chamber that houses the substrate. Thus, the substrate and its integrated circuit devices are mechanically floated between the two conformable membrane-and-liquid arrangements.

An advantage of the above-described embodiments is that the floating substrate allows displacement of the integrated circuit devices in a manner to first achieve and then maintain desired device/exchanger interfaces. The position of the membrane can be adjusted to compensate for variation in the heights and/or the angles of the integrated circuit devices relative to the component surface of the substrate. Moreover, during operation of the module, compensation is possible for differences in thermal coefficients of expansion for the integrated circuit chips, the substrate and the heat exchanger. Depending upon the thermal coefficients, the ability of the substrate to float within the chamber may provide a strain release or may provide a means for causing the integrated circuit devices to follow the outward expansion of the heat exchanger.

The module preferably includes a flexible cable that channels signals and utilities to and from the substrate without interfering with the ability of the substrate to float within the chamber.

In another embodiment, the module includes a substrate onto which the semiconductor chips are mechanically and electrically mounted. While not critical, the substrate is a silicon substrate and the chips are surface mounted using a solder bump technique. A silicon substrate provides a high degree of flatness, thereby reducing variations in thickness of the substrate as a source of non-coplanar chips. Moreover, silicon is better matched to the chips in terms of the thermal coefficients of expansion than are standard printed circuit board materials.

The heat exchanger is fixed to the substrate at the component surface of the substrate. A fluid-tight chamber is defined between the membrane and the heat exchanger. The fixed volume of liquid is contained within the fluid-tight chamber. Preferably, the liquid is under pressure by the entrapment between the heat exchanger and the membrane, so that the liquid presses the membrane outwardly. The membrane extends generally parallel to the chips. The liquid pressing against the membrane ensures a compression contact of the membrane against each semiconductor chip, regardless of variations in heights and angles.

The membrane is made of an electrically conductive material that forms an electrical path from the grounded heat exchanger to the back sides of the semiconductor chips. This grounding provides an advantage over typical prior art structures, since the grounding of chips containing CMOS devices is often desirable.

An advantage of the present invention is that it provides a conformal heat flow path from the chips to the heat exchanger. Heat is channeled from the chips to the thermally conductive membrane and liquid and then to the heat exchanger where the energy can be dissipated into the surrounding atmosphere. The conformal thermal interface not only allows adaption to differences in chip heights and chip angles resulting from manufacturing tolerances, but also provides a strain release for chip expansion during operation. The chip expansion varies with the thermal coefficients of expansion of the chips and the material used to form any chip carriers. Preferably, the stainless steel membrane has a thickness in the range of 0.0005 inch and 0.001 inch. A membrane that is too thin will be unreliably fragile, whereas a membrane that is too thick will not have the necessary conformity.

As compared to traditional packaging which merely employs a heat sink, the present invention achieves a greater cooling capability. This is particularly true where a second heat exchanger is attached to the substrate at the side opposite to the component surface. Optionally, integrated circuit chips may be mounted on both major surfaces of the substrate. The double-sided substrate can then be entrapped between two conformal interfaces, each comprising a thermally conductive membrane and a static body of liquid entrapped between the membrane and a heat exchanger.

Utilizing the present invention, higher power integrated circuit chips can be placed closer together at a lower and more uniform temperature. Closer component spacing allows higher performance products, since electrical paths can be shortened. Moreover, lower component junction temperatures yield higher performance as well as increased component reliability. It is predicted that an improvement of thirty-four percent in the gate delay for CMOS circuits can be achieved.

The present invention integrates structural support, protection from the external environment, radio frequency shielding and a conformal heat transfer interface. Thus, replacement merely requires removing a module from a motherboard and plugging in a replacement module. Plumbing connections to an external source of liquid coolant are not necessary. Nor is it necessary to provide secondary housing to contain RF radiation, since grounding the heat exchangers sufficiently protects against RF radiation leakage.

Also disclosed is a module having double-sided cooling. First and second heat sinks are mounted at the opposed major surfaces