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

Flexible hybrid electronics (FHE) interface rigid electronic components with flexible sensors, circuits, and substrates. This paper reports the reliability improvement of a FHE Human Performance Monitor (HPM), designed to monitor electrocardiography (ECG) signals. The 50.8 mm × 50.8 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 effect of different design parameters, such as PI substrate thickness (50 μm vs 125 μm), Cu circuit thickness (2 μm vs 6 μm), solder reflow temperature (205 °C for Tin-Lead (Sn-Pb) vs 175 ° for Tin-Bismuth (Sn-Bi) solder), solder pad design, and optimized inkjet printing (printing on bare Cu vs 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 μm thick PI substrate, 6 μm 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.