Thermal enhancement of flip-chip packages with radial-finger-contact spring

Power dissipation, size and circuit density of integrated circuit chips are increasing and flip-chips are getting popular. Flip-chips may be joined to the substrates using small solder balls (commonly known as controlled collapse chip connections or C4s) on the circuit side of the chips, and are capable of providing a very large number of input and output connections. This paper describes a radial-finger-contact (RFC) spring for thermal enhancement of packages with C4 type flip-chips. This spring, in the shape of a disk, is made from a thin sheet of high conductivity copper alloy. A number of radial slots are cut in an alternating pattern to make it flexible. It is plated with nickel to prevent corrosion and formed into a dome shape such that when placed inside 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 the chip to the cap. To establish the feasibility and thermal reliability of this concept, a number of flip-chip packages were built with and without the RFC spring and tested for thermal performance. In one set of packages with the RFC springs, a silicone gel was used at the chip interface to further improve the thermal performance. The packages were exposed to a number of accelerated stresses and the package thermal performance (internal or the chip-to-case thermal resistance) was periodically measured. The results show that significant improvement in thermal performance is possible with the use of the RFC spring and this remains stable after exposure to accelerated stresses. This method of thermal enhancement is easily extendable to multi-chip packages with flip-chips.