Characterization and Reliability of Touch Panels

The purpose of this master’s thesis is to develop a test method and a setup to characterize touch button interfaces for household appliances and to assess their reliability by means of accelerated life tests. The characterization and the reliability assessment are aimed at the validation of the different touch technologies in use on the touch panels. The theory behind touch systems is reported in the first part, with the difference between mutual and self capacitance sensing methodologies in Sec. 2.1. These sensing methodologies have been used as a reference in the context of touchscreens and touchpads in Sec. 2.2, of buttons in Sec. 2.3, and of sliders and wheels in Sec. 2.4. Particular attention is given to self capacitance buttons, their design, and configurations in Sec. 2.3.2. The first theoretical part concludes with a point of view on the touch drivers in Sec. 2.5, which are a fundamental component of the touch interface. Four case studies with different characteristics have been analyzed in Sec. 3. All of them are button panels and use self capacitance. Their characteristics are described, with reference to sensor technology (springs or printed electrodes), the touch driver, and the communication between the driver and the microcontroller (parallel, UART, or I2C). One case study is a floating ground system, and it needs further consideration. The testing setup for the characterization described in Sec. 5 is realized by converting a Cartesian Robot System that controls a touch bar, which simulates the human finger. An accurate control was needed for a meticulous characterization, that considers various parameters affecting the performance, reported in Sec. 4. To assess the reliability of the touch performance accelerated life tests were developed. The selection of the most touch-relevant tests is in Sec. 6.2, together with an overview of the most used tests in the industry. Before, a mathematical theory regarding reliability is reported in Sec. 6. The last part presents the results of the characterization in Sec. 7 and of the reliability tests in Sec. 8 for all the case studies. Finally, the conclusion regarding this effort is reported and possible future work is mentioned (Sec. 9).