Power dissipation, size and circuit density of integrated circuitchips are increasing and flip-chips are getting popular. Flip-chips maybe joined to the substrates using small solder balls (commonly known ascontrolled collapse chip connections or C4s) on the circuit side of thechips, and are capable of providing a very large number of input andoutput connections. This paper describes a radial-finger-contact (RFC)spring for thermal enhancement of packages with C4 type flip-chips. Thisspring, in the shape of a disk, is made from a thin sheet of highconductivity copper alloy. A number of radial slots are cut in analternating pattern to make it flexible. It is plated with nickel toprevent corrosion and formed into a dome shape such that when placedinside a flip-chip package it makes contact with the chip and the cap,thus providing a direct heat transfer path from the back side of thechip to the cap. To establish the feasibility and thermal reliability ofthis concept, a number of flip-chip packages were built with and withoutthe RFC spring and tested for thermal performance. In one set ofpackages with the RFC springs, a silicone gel was used at the chipinterface to further improve the thermal performance. The packages wereexposed to a number of accelerated stresses and the package thermalperformance (internal or the chip-to-case thermal resistance) wasperiodically measured. The results show that significant improvement inthermal performance is possible with the use of the RFC spring and thisremains stable after exposure to accelerated stresses. This method ofthermal enhancement is easily extendable to multi-chip packages withflip-chips
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