Implementing Fringing Field Sensors in PCB Technology

Abstract

Interdigitated electrode structures are well suited for realizing capacitors where a significant percentage of the total capacitance in due to the out of plane fringing fields. An object that interferes with the fringing fields will then change the measurable capacitance between the electrodes. Therefore this configuration can be used as a sensor for the object that interferes with the fringing fields and is called a capacitive fringing field sensor. These types of sensors have been used in many applications, such as water detection, moisture content measurement and as proximity switches. Printed circuit board (PCB) technology is particularly useful for realizing this type of sensor architecture. The interdigitated electrode structures can be patterned in the Cu cladding on one or both sides of the substrate. The solder mask coating can then be used to insulate the electrodes to prevent shorting in the presence of water or other conductive substances. The size and spacing of the electrode structures can be optimized to adjust the sensitivity of the sensor to the measurand of interest. The implementation of this type of sensor in PCB technology has several advantages when compared to traditional MEMS technologies. External electronics can readily be attached to the PCB substrate. Very large sensor panels can be manufactured easily and economically. Other types of MEMS sensors have been implemented in PCB technology in recent years. This sensor is compatible with those types of sensors and could be used to augment sensor suites implemented in PCB MEMS technology. Demonstration prototype fringing field sensors were implemented in PCB technology for measuring the mass of small quantities of water, for measuring soil moisture content and for use as capacitive touch switches.