Reliability Study and Finite Element Modeling of a Wearable Sensor Patch (WSP) to Monitor ECG Signals

Flexible hybrid electronic devices, which interface flexible sensors, circuits and substrates with rigid electronic components present a unique reliability challenge due to their high conformity which induce high stresses and strains. This paper focuses on work done to improve reliability of a wearable sensor patch (WSP) designed to monitor ECG signals and skin temperature. The original device was fabricated on a 2" X 2" Kapton® polyimide (PI) substrate. Sensors were printed on one side of the flexible PI substrate (sensor side) and rigid electronic components were mounted on the other side for signal processing and communication purposes (component side). The sensors were connected to the rigid electronic components using Through Hole Vias (THVs) and flexible Cu circuit electroplated on the PI substrate. The device was susceptible to cracking of the Cu circuitry near the front end signal conditioning chip. It was determined that substrate thickness, Cu circuit thickness, type of solder used and solder pad design were the variables that might affect reliability of the WSP. Multiple test vehicles fabricated using either 2 or 6 μm thick Cu circuitry, 50 or 125 μm thick PI substrate and Sn63Pb or Sn42Bi solder were bend tested to determine the most robust assembly. Test vehicles fabricated using improved solder pad design were also bend tested to study its effect on device performance. Finite element modeling was done and an effort was made to correlate experimental and simulation results. It was observed that devices with 6 μm thick Cu circuit, 50 μm thick PI substrate and Sn42Bi solder had no defects as a result of assembly processes and performed best under bend testing. Improved solder pad design helped improve reliability of the device. Simulation results showed excellent correlation with experimental results.