The Simons Observatory: mechanical and electrical robustness of the universal focal-plane modules

The Simons Observatory (SO) is a ground-based cosmic microwave background survey experiment that consists of three 0.5 m small-aperture telescopes and one 6 m large-aperture telescope, sited at an elevation of 5,200 m in the Atacama Desert in Chile. The SO focal planes will be tiled with 49 universal focal-plane modules (UFMs), in which transition-edge sensor detectors are coupled to microwave SQUID multiplexing readout components. These detector modules contain a stack of silicon wafers and chips, which are encased in an aluminum shield and electrically connected with over 10,000 wire bonds. To ensure the UFMs will maintain their electrical and mechanical integrity throughout their expected lifetime, we have developed a program of robustness testing. This program involves repeated cryogenic cycling to mimic a lifetime of operation in the field. We also describe electrical validation tools that enable the debugging of electrical shorts that can appear during assembly and device screening. As a result of these tests and developments, we expect that the UFMs will maintain operability through in-lab module screening and at least five seasons of observation in Chile.