Ultrasonic dewatering in minute holes

Printed-circuit boards are widely used for miniaturizing electronic equipment. According to the demand, diameter of holes in the boards is decreasing. There is a problem in their fabrication processes, in which they should be cleaned in water. After the cleaning process, water remains in the small holes. The remained water contains chemical material and/or dusts and might cause damage of the boards in the future. Conventionally, air pressure has been used to remove the water in the boards. But the method is not valid for holes that have bottoms inside neither for much smaller holes. Heating could be an alternative method, but this method might leave chemical material in the water and cause deflection of the board. In this research, a new dewatering method has been proposed. The method utilizes standing wave of ultrasonic wave in air. An experimental apparatus was organized to observe the ultrasonic dewatering and consisted of a Langevin type ultrasonic transducer and a reflector. Proposed dewatering was observed successfully and recorded by a high speed camera. The sound pressure in the standing wave was measured by a microphone.