Optimization and evaluation of a three-step pretreatment process for PCB microdrills

The traditional Murakami two-step and three-step acid pretreatment processes were introduced to reduce the negative impact of Co on the cemented carbide tools during deposition of the diamond coatings. However, treatment methods requiring a longer etching time and a higher etching solution concentration lead to the formation of pores and cavities on the surface of the substrate, which would damage the substrate and thus significantly decrease the fracture load of the tools, especially microdrills with a small edge diameter. In this study, the pretreatment process of microdrills is optimized based on the traditional three-step process. The surface morphologies and Co content of the microdrills after pretreatment are investigated via scanning electron microscopy and energy-dispersive spectroscopy. The effect of the pretreatment on the fracture toughness of the microdrills and the adhesion of the diamond coatings is evaluated. The results show that the surface microstructure and fracture load of the microdrills are inevitably damaged during the acid and Murakami etching processes. Furthermore, the adhesion of the diamond coating depends on the final step of the acid etching process. The optimized three-step process consists of first etching with an acid solution for 10 s, then etching with a Murakami solution for 4 min, and finally etching with the acid solution for 30 s. This process improves the pretreatment efficiency and coating adhesion without significantly reducing the fracture load of the microdrills.