Characteristics of chip formation in the micro-drilling of multi-material sheets

Abstract

In order to meet the requirements of electronic product miniaturisation, the use of thinner and smaller printed circuit boards (PCBs) will be required. To achieve this, many more micro-holes must be drilled in a smaller area than before. PCBs are anisotropic multi-material sheets consisting of a dielectric layer (resin/glass fibre cloth) and a high purity metal conductor (copper foil). It is difficult to achieve high machining precision, surface quality, drilling efficiency and long drill life when drilling PCB micro-holes. In this study, micro-drills with a diameter of 0.1 mm were used to drill the PCBs at rotational speeds of up to 300 krpm. The drilling process was digitally photographed. The chip morphology and the hole wall were observed. The quality of the hole wall, such as hole wall roughness and nail head and exit burr formation were observed and measured. The influence of drilling condition on the drilling process and hole quality were studied. It was found that the morphology of the chips and the hole wall surface depended on the material properties of the printed circuit board. Chips were formed normally as conical helical chips from the aluminium entry board, conical helical chips from the copper foil and discontinuous chips from the glass fibre and softened resin. The hole wall through the copper foil seemed much smoother than that of resin/glass fibre cloth layer. However, nail heads and burrs were formed at the copper foil layers, which decreased the hole quality. Chip morphology and hole quality were affected by feed rate, spindle speed and tool wear. The basic removal mechanism of the PCB micro-holes was analysed, and this study provided a firm foundation for further work in this area.