Drilling of printed circuit boards: factors limiting the use of smaller drill sizes

The process of making connections between the different layers in a printed circuit board requires holes to be drilled which are subsequently plated. A trend towards miniaturization of board detail requires smaller holes to be drilled. However, the smaller-size drills are inherently more prone to breakage and must be used within a restricted range of drilling and process parameters. In a typical cycle, a drill encounters alternate layers of copper separated by a glass-resin composite. Drill forces, comprising an end-load and a torque, depend upon the process conditions and the composition of the material being cut. This paper examines the forces during a typical drilling cycle. Force trends associated with wear, glass content, chip-load and aspect ratio are presented together with an indication of the contribution from the copper layers. Recommendations for the selection of process parameters to avoid drill breakage due to excessive forces are given.