Trend Analysis of Dissipated Electrostatic Discharge Energy in Touchscreen Displays

Touchscreen displays can be susceptible to sparkless electrostatic discharge events. The energy observed by sensitive touchscreen circuitry can vary significantly with design parameters like the glass thickness, the capacitance between the sensor pad and the ground structure, and the resistance of the traces and sensor terminations connected to the pad. The energy dissipated in resistive structures within the display can lead to damage. Methods are presented to estimate the maximum energy dissipated in the touchscreen circuitry during a spark-less discharge to the display. The trends in the energy with variations in design parameters are analyzed using traditional curve-fitting techniques. The analysis was performed using measured data obtained for 20 touchscreen configurations when the ESD gun was charged to 9 kV and 15 kV. The analysis helps the designer to understand the trends and to predict how future design decisions may impact ESD susceptibility. Results suggest that immunity can be maximized by increasing the glass thickness, reducing the load resistance, and reducing the distance between the sensor pad and the PCB return plane.

I. Introduction

ESD events in touchscreen displays can result in device failures. Typically, IEC 61000-4-2 [1] test is used to evaluate the immunity compliance of the product. Various modeling and measurement methods have been proposed to study the characterization of ESD events [2]–[6]. The influence caused by different parts of the human body is shown in [2], indicating the effect of the source. A discharge event on the glass may cause current coupling to the touch screen matrix and then to the traces of the IC [3]. From the device perspective, the capacitance between a mobile device and a metallic coupling plane strongly affects the discharge current [4]. The charge, the discharge position, the cleanliness of the screen and the discharge polarity show a strong influence on the discharge to display screens, as well [5]. Most of these parameters, however, only indirectly impact the ESD event. For an in-depth analysis of an ESD event, energy, fields, voltage peak, current peak and the rise time of the ESD pulse should be analyzed. The total dissipated ESD energy is particularly important in some products [7]. In the study presented here, methods are developed to predict the energy delivered to the load resistor of a touchscreen sensor during a spark-less discharge to the display. The highest priority was placed on predicting the energy since it is expected that a high energy discharge is likely to damage the device.