Reliability Assessment of Temperature Sensors Integrated on Elastic Substrate

Advanced manufacturing techniques combined with state-of-the-art materials and conventional surface mounted devices enable producing of stretchable electronic systems with conformable form factor. This paper investigates various temperature sensor implementations on elastic substrates and assess their reliability under mechanical loadings. The primary sensor implementations include printed sensor, glob-topped SMD sensor (thermistor) and glob-topped SMD sensor (thermistor) on a flexible interposer integrated on an elastic substrate. The study features iterative rounds, in which mechanical structures and material combinations change for the fixed design, and their effect with respect to baseline is quantified. The reliability assessment for test vehicles is performed with maximum and cyclic stretching tests including torsion. The conducted mechanical tests and failure analysis pinpoint critical interfaces with cracking under elongation. The quantitative results show unsatisfactory performance of printed sensors under linear elongation (<5%) and about 20% maximum stretchability for baseline SMD based structures. Iterative designs prove increased stretchability up to 30% in linear elongation for thermistor structures by means of a softer glob-top material. This also results in a change in major failure mechanism from rigid-soft interface to a printed conductor. Investigating the novel substrate and conductor materials records significant improvement in maximum stretching tests. The best material combinations withstand over 150% stretching. The study includes also test method development for stretchable electronics. A setup used for linear elongation test upgrades with a rotating base enabling torsional force. The advanced test setup mimics operational environment of elastic electronic systems more realistically and quantifies the difference between linear elongation and multi-dimensional forces.