High repetition frequency micro hole drilling of metal foils using ultrashort pulse laser radiation

Abstract

This paper discusses latest results obtained in high-pulse repetition frequency micro hole percussion drilling of metal foils ranging between 25 μm and 100 μm thickness. In the investigations a high-repetition frequency, high average power ultrashort pulse laser source was applied, providing a near-infrared laser beam of 1.03 μm wavelength, 87.5 W maximum average laser power (at the processing plane), 51.48 MHz maximum pulse repetition frequency, 650 fs pulse duration, and 30 μm focal spot size. In the experiments, the process parameters pulse energy, pulse repetition frequency, and the pulse number were varied thus in order to evaluate their influence on both micro hole geometry (hole diameter, roundness), and drilling quality (thermal modification, melting residues). The minimum pulse number required to drill through the metal foils was detected by an in-house developed pulse counter. A significant lower pulse number to drill through the material was observed when irradiating ultrashort pulses of megahertz pulse repetition rates. On the one hand, this effect might be induced by thermal accumulation thus increasing the substrate temperature which, in turn, enhances the laser beam absorption or rather lowers the energy consumption needed for evaporation. On the other hand, pulse interactions with the still existing plasma at these very short pulse intervals cannot be neglected, so a higher amount of material might be ejected assisted by the higher plasma pressure. As another detrimental effect for megahertz pulses, a significantly lower processing quality was obtained that was due to intensive melting affected by heat accumulation, as the melt re-solidified within and/or surrounding the drilling holes. Surprisingly, by irradiating pulses at 20 MHz and above, the melt was completely ejected out from the drilling holes as the drilling efficiency was even the highest. Finally, based on the findings of this study, optimum parameter settings are presented with regard to highest machining quality and drilling throughput.