Mechanical model of back-drilling high-speed printed circuit boards with eccentricity effects

Abstract

The rapid development of fifth-generation (5 G) communication technology is placing greater demands on the performance of high-speed printed circuit boards (PCBs). Back-drilling plays a crucial role in achieving high-frequency and high-speed signal transmission and is a key technology for improving the signal integrity of high-speed PCBs. The thrust force is considered the main factor affecting hole quality when drilling PCBs. Accurately predicting the thrust force is essential for optimizing the back-drilling process. However, back-drilling thrust force is strongly coupled with eccentricity, presenting a significant challenge in predicting its accuracy. In this paper, an ideal state mechanical model (without eccentricity effects) and an actual state mechanical model (with eccentricity effects) are proposed for the first time to predict the thrust force when back-drilling high-speed printed circuit boards. First, a length model for the effective cutting edge in back-drilling is established for the first time based on back-drilling characteristics. The contact conditions and back-drilling mechanism between the materials (glass fiber-reinforced plastic, resin and copper) and the microdrill (tungsten carbide) are further analyzed in terms of the effective cutting edge and contact mechanics. Then, by considering the eccentricity effects, mechanical models under ideal and actual states are proposed for predicting the thrust forces when back-drilling high-speed PCBs. In addition, by setting different feed rates and spindle speeds and using 4 types of microdrills with different structural parameters, a back-drilling experiment is performed on high-speed PCBs to determine eccentric distances, actual thrust forces, and stub lengths. Compared with the actual thrust force, the maximum error of the ideal model is 8.03%, while the actual model has a better applicability with a maximum error of 5.51% and a minimum error of 1.33%. The experimental results indicate that the thrust force first decreases and then increases as the eccentric distance increases when the chisel edge is involved in the cut during back-drilling. Moreover, the stub length increases with increasing eccentricity and decreasing point angle, while the back-drilling diameter has a negligible effect.