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Flexible Heat Conducting Sheet Material for Semiconductor Packages

IP.com Disclosure Number: IPCOM000045633D
Original Publication Date: 1983-Apr-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 4 page(s) / 32K

Publishing Venue

IBM

Related People

Lacombe, RH: AUTHOR [+2]

Abstract

With ever increasing circuit densities on silicon chips, the need to be able to dissipate heat out of the chip is becoming ever more critical. The drawing above illustrates one technique for conducting heat out of chip mounted on an MLC (multilayer ceramic) substrate.

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Flexible Heat Conducting Sheet Material for Semiconductor Packages

With ever increasing circuit densities on silicon chips, the need to be able to dissipate heat out of the chip is becoming ever more critical. The drawing above illustrates one technique for conducting heat out of chip mounted on an MLC (multilayer ceramic) substrate.

Two paths exist for heat conduction out of the chip: one being through the solder joints to the underlying MLC substrate, and another path exists from the base of the chip to the aluminum cap or cover which covers the module. This path is viable, assuming that a suitable thermally conductive material can be placed between the chip and the cap. A material is needed which can be put in place before the cap is sealed to the MLC substrate. This material must form a good heat conductivity path between the chip and the cap. It must further be chemically inert so as not to cause corrosion or to contaminate the module in a deleterious manner. Further, the material must be suitably soft so as not to crush the chip when the cap is set in place, while not being so soft as to flow over the border of the chip and contaminate the rest of the module.

The material must also exhibit suitable long term physical and chemical stability so as not to degrade appreciably over the service lifetime of the semiconductor module in which it is placed. Finally, the material must be cleanable by suitable means so that chip replacement and rework on the module is possible.

These objectives are met by a novel particle binder system which is designed to meet all of the above criteria. It consists of a matrix of ceramic particles in a polymer binder. The particles have a high thermal conductivity, very low electrical conductivity, and a high degree of chemical inertness. The binder provides an overall cohesiveness to the matrix and imparts suitable mechanical properties to the composite, such as coating and flow behavior. The binder is also a chemically inert material of low electrical conductivity.

One further consideration must be added to the list of requirements which was outlined above. Suitable provisions must be made for proper treatment of the surfaces between which the thermal compound is to conduct heat. Surface preparation is necessary to avoid air gaps or other defects which can give rise to a high thermal resistance at the interfaces between the chip and the thermal compound and between the thermal compound and the aluminum cap.

Boron nitride particles (in a range from 1 to 15 microns in size) mixed into a binder of polyisobutylene (PIB) provide a suitable thermal medium.

PIB is a viscoelastic resin with the following properties: (see original)

As seen from Table I, PIB at room temperature is a viscous liquid or a rubbery solid depending on the molecular weight.

Boron nitride is a white ceramic powder. There are two crystalline forms: hexagonal and cubic. In this approach the

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hexagonal form is contemplated w...