A Spring-Loaded Chip Interface for Flexible Stripline Cryogenic Readout

Abstract

Scaling astronomical detectors and quantum devices to larger and larger sizes will require more readout lines. A problem with extra readout lines is the complexity and cost of many coaxial cables in parallel. Furthermore, the thermal loading on the coldest stages via thermal conductivity should be kept at a minimum. Delft circuits provides an eight-channel flexible cabling solution based on stripline technology with Ag or NbTi conductors and polyimide dielectric (flexline) as a solution to these problems. However, a compact and light-tight flex-to-chip interface is lacking, and this would be key to unlocking its full scaling and lightweight potential. Here, we present the design and measurements of a flexline to PCB interface for cryogenic detectors. The flexline is connected securely via a spring-loaded pressing tool, ensuring reliable contact even after thermal contraction. To reduce stray radiation, the chip and connector assembly sits in a light-tight setup. The connector spans a width of 20 mm along one edge of the chip, illustrating the compact design. We show that the flexline connector has an insertion loss of -2 dB at 7 GHz using a probe measurement at room temperature. We also demonstrate the connector with a test chip with kinetic inductance detectors in a dilution refrigerator cooled to 40 mK.