Preparation and Cutting Performance of Diamond-Coated Microdrills for Microhole Machining in High-Frequency Microwave Printed Circuit Boards

Driven by the rapid deployment of 5 G and emerging 6 G communication technologies, high-frequency printed circuit boards (HFPCBs) have become indispensable substrates in communication base stations and artificial intelligence servers, imposing more stringent requirements on signal transmission efficiency and thermal stability of the substrate materials. Owing to their excellent dielectric properties and physical stability, ceramic-reinforced polytetrafluoroethylene (PTFE) copper-clad laminates are widely utilized to fabricate high-frequency microwave printed circuit boards (HFMPCBs). However, since these composites incorporate over 60% hard ceramic fillers, they form a typical multiphase heterogeneous laminated structure that induces severe tool wear and degrades micro-drilling quality, rendering them a new generation of difficult-to-machine materials. To address these challenges, diamond coatings are widely recognized as an effective solution for micro-cutting applications due to their ultra-high hardness, outstanding wear resistance, and low friction coefficient. In this study, diamond coatings with varying grain sizes were deposited onto 0.50 mm-diameter cemented carbide microdrills via hot filament chemical vapor deposition (HFCVD). The coating microstructures and morphologies were characterized systematically, and their mechanical properties and micro-drilling performance in HFMPCBs were comprehensively evaluated. The results demonstrate that varying the coating grain size effectively mitigates tool wear and enhances hole positional accuracy. Specifically, the hole wall surface roughness and burr height are reduced by up to 66.8% and 88.6%, respectively, while a stable drilling capacity of up to 20,000 holes is achieved. Nanocrystalline diamond (NCD) coatings provide a smoother surface and a lower friction coefficient, making them highly suitable for high-precision microhole finishing. Conversely, microcrystalline diamond (MCD) coatings exhibit superior hardness and wear resistance, rendering them ideal for applications requiring extended tool life.