Study on Folding-Reliability of Wearable Biometric Band

The use of flexible electronics for wearable applications provides a number of advantages including compactness, non-planar architectures, and light-weighting. There is increased interest in dealing with the process and reliability issues related to the use of flexible electronics in wearable applications under the stresses of daily motion. Stresses of daily motion have the potential to cause the premature failure of the electronics systems and sensors. Wearable electronics may be subjected to twisting, flexing and folding under normal operational use-life. In this study the effect of the stresses of daily motion have been studied through the development of test protocol for dynamic folding imposed on a flexible biometric band with printed sensor and surface mount components. This research study involved the calibration, characterization, and reliability study of wearable biometric bands subjected to dynamic folding. The wearable band used for the study combines a biometric sensor unit, a micro-controller unit with a wireless connection, and a printed thermistor unit. The sensors have been calibrated by actual temperature and biometric signals. Furthermore, the folding test was conducted utilizing multiple boards. Due to multiple components and printed lines of the PCB, optical images were taken in order to confirm which parts failed and the reasons for the failures. An FEM analysis was developed in order to understand the locations of stress concentration the PCB and which parts are stressed during the folding process. Fatigue failure analysis on folding reliability of the wearable biometric band was conducted using experimental analysis, microscopy analysis, and simulating analysis. A correlation of the stress change in the biometric band with the progression of dynamic folding was determined using the finite element model.