Development of high accuracy classification system for small particles

点击翻译按钮获取中文摘要

High accuracy size classification is required to improve the performance of functional particles such as solder balls. In this work, a size classifier with twin rollers has been developed to realize high accuracy size classification of spherical particles. Since the two rollers are arranged at a certain inclination with a small opening angle, particles move along the gap between the rollers, and fall down at a point where the width of the gap exceeds the diameter of the particles. To clarify the dynamic behavior of the moving particles along the gap, microscopic observation was carried out, and images recorded with a high-speed digital camera (1/8000 fps) were analyzed. The particle velocity is proportional both to the angular velocity of the rollers and to the tangent of the inclination angle. The particles are not only rolling but also jumping slightly but continuously. The small jumps are caused by the friction between the particle and the rollers, and the forward movement of particles is due to the axial component of the gravitation. The particle velocity can be formulated with the circumferential velocity of the rollers, inclination angle, and friction coefficient. In addition, the particle movement depends on the operational conditions, i.e. directions and rotational velocities of the rollers, but not on the diameter of the particles if they are several hundred micrometers. For smaller particles, the adhesion between a particle and the roller affects the particle movement. The small surface roughness of the rollers lowers the adhesion and 98% of particles were able to be classified even though the particle diameter is 100 μm, and the accuracy of the size classification was within 1 μm.