Reliability Challenges in Fabrication of Flexible Hybrid Electronics for Human Performance Monitors: A System-Level Study

Abstract

Flexible hybrid electronics (FHE) interface rigid electronic components with flexible sensors, circuits, and substrates. This paper reports the reliability improvement of an FHE human performance monitor (HPM), designed to monitor electrocardiography (ECG) signals. The $50.8 {\,\,\mathrm {mm}} \times 50.8 {\,\,\mathrm {mm}}$ HPM is fabricated on Kapton HN polyimide (PI) substrate with flexible gold (Au) ECG electrodes on one side of the substrate and rigid electronic components for signal processing and communication mounted on the other side of the substrate. Our previously reported HPMs demonstrated reliability issues due to: 1) cracking of the copper (Cu) circuitry and 2) thinning and lack of adhesion at the printed Au and plated Cu interface that connected printed sensors to the Cu circuitry. Both failure mechanisms resulted in electrical opens in the circuit, which caused device failure. We explored effects of different design parameters, such as PI substrate thickness (50 versus $125~\mu \text{m}$ ), Cu circuit thickness (2 versus 6 $\mu \text{m}$ ), solder reflow temperature [205 °C for Tin-Lead (Sn-Pb) versus 175 °C for Tin-Bismuth (Sn–Bi) solder], solder pad design, and optimized inkjet printing (printing on bare Cu versus Au-plated Cu) on improving FHE reliability. Test vehicles (TVs) with different combinations of these factors were fabricated and bend-tested to determine the most robust configuration. TVs with 50- $\mu \text{m}$ -thick PI substrate, 6- $\mu \text{m}$ -thick Cu circuit, Sn–Bi solder, redesigned solder pads with rounded corners, and printed Au traces on Au plated Cu pads demonstrated the best reliability results.