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Browse Prior Art Database

Design for Minimum Chip Joint Stress

IP.com Disclosure Number: IPCOM000037225D
Original Publication Date: 1989-Dec-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 2 page(s) / 39K

Publishing Venue

IBM

Related People

Niu, TM: AUTHOR [+2]

Abstract

Chip joints are subject to thermal stress during machine cycling because of the difference in thermal expansion coefficient between the chip and the substrate. The effect is reduced by incorporation of low expansion plates, but this technique is only partially successful because the expansion coefficient of the substrate cannot be reduced to that of the chip with any practical combination of materials.

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Design for Minimum Chip Joint Stress

Chip joints are subject to thermal stress during machine cycling because of the difference in thermal expansion coefficient between the chip and the substrate. The effect is reduced by incorporation of low expansion plates, but this technique is only partially successful because the expansion coefficient of the substrate cannot be reduced to that of the chip with any practical combination of materials.

The stress may be reduced to near zero by the inclusion of a resilient layer between the joint and the low expansion plates. It is then necessary to properly dimension the finger to which the joint is attached, and it must be of copper or some material with a comparable expansion coefficient. Stress is essentially eliminated by employing a finger of suitable length. The substrate expands and the plated through hole which anchors the finger expands with it. The finger, having a higher expansion coefficient, expands in the opposite direction. If the finger length is chosen correctly, the end of the finger at the joint will not move relative to the chip and no stress results.

This will be obtained if the ratio of finger length to chip size is equal to the ratio of Substrate Coefficient of Thermal Expansion (CTE) minus Chip CTE to Copper CTE minus Substrate CTE. The calculation assumes that the elastic modulus of the resilient layer is equal to zero; some adjustment will be necessary for real materials.

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