Improvement of micro-drilled hole quality for printed wiring boards

In the current manufacturing of printed wiring boards (PWBs), micro-drills are generally used to make smaller diameter through-holes in PWBs, which are desired for the miniaturization of equipment. However, a problem has emerged in that copper plating degraded by hole drilling can reduce the reliability of the electrical connection between layers. The surface roughness of drilled hole wall is one of the important factors affecting the plating quality. However, there are many factors, which can affect complexly the surface roughness, such as drilling conditions and material properties of PWBs. On the other hand, few reports previously describe the factors which influence on the micro-drilled hole wall quality of PWBs. The purpose of this paper is to indicate the proper method in order to improve the drilled hole wall quality by clarifying the factors which affect the surface roughness of micro-drilled hole wall of PWBs. First, the drill temperature was evaluated in drilling monitored by a thermography under various drilling conditions such as cutting speed and feed rate in order to show the influence of the temperature in drilling on the drilled hole wall quality. Second, the thrust force and the torque in drilling were measured with a dynamometer in order to evaluate the cutting phenomenon in detail and to clarify the relationship between drilling process and the variation of cutting forces. Especially, the influence of the workload, calculated using measured torque in drilling, on the drill temperature and the surface roughness of drilled hole wall was evaluated. As results, the following conclusions were reached: (1) the surface roughness of drilled hole wall increases with the drill temperature in drilling. (2) The drill temperature tends to increase with the workload on drill caused by friction between hole wall and the land or margin of drill. Therefore, the reduction of the workload by the friction seems to be effective in order to obtain the high quality micro-drilled hole wall.