The Cryocooler Implications of Flexible, Multi-Channel I/O Cables for Low Temperature Superconducting Microelectronics

Abstract

The cryocooler performance implications are examined for a multi-channel, flexible, input/output (I/O) cable in a cryosystem for low temperature superconducting microelectronics operating at 10 K and 4.5 K. We show that the I/O cable contribution to the total heat dissipation in either cryosystem is large in comparison to the dissipation from the electronics. High frequency electronic components require very pure copper, short signal paths and the use of a ground plane running the length of the I/O cable. The contradictory demands of electrical and thermal properties for the cable signal lines and ground plane therefore require a quantitative understanding of the heat conduction problem. We have used the integral and thermal network (finite difference) methods to model the effects of the copper thermal conductivity and the signal to ground thermal linkage.

The integral method is a powerful tool for making calculations of heat transfer from the cable to the system, easily accommodating various conductor thicknesses, cable lengths, and spacing between heat sink stages. Additional detail is provided by the thermal network method which quantifies heat conduction more precisely than the integral method and additionally provides temperature profiles along the cable length. These profiles indicate the tendency, if any, of signal lines to attain temperatures different than those of the ground plane due to the fact that the signal lines are heat sinked through the cable dielectric. For a given electrical performance, there are fundamental limitations to the thermal performance of an I/O cable.