We report on a comprehensive study of laser percussion microvia drilling of FR-4 printed circuit board material using ultrashort pulse lasers with emission in the green spectral region. Laser pulse durations in the pico- and femtosecond regime, laser pulse repetition rates up to 400 kHz and laser fluences up to 11.5 J/cm2 are applied to optimize the quality of microvias, as being evaluated by the generated taper, the extension of glass fiber protrusions and damage of inner lying copper layers using materialography. The results are discussed in terms of the ablation threshold for FR-4 and copper, heat accumulation and pulse shielding effects as a result of pulse to pulse interactions. As a specific result, using a laser pulse duration of 2 ps appears beneficial, resulting in small glass fiber protrusions and high precision in the stopping process at inner copper layer. If laser pulse repetition rates larger than 100 kHz are applied, we find that the processing quality can be increased by heat accumulation effects.
We report on the characterization of a hybrid laser scanning system using acousto-optical deflectors in combination with galvanometer scanners for ultra-short pulse laser material processing. The hybrid scanning system is characterized by the roundness of static pulsed ablations of metal thin film on a transparent substrate within the acousto-optical scanning field at different galvanometer scanner deflection angles and laser focal positions. An ablation roundness of more than 90% is reached in a defocusing range of 200 $$\upmu \text{m}$$ μ m within a galvanometer scanfield of 900 $$\text{mm}^2$$ mm 2 , corresponding to approximately 74% of the usable scan area of the f-theta lens. A high maximum positioning speed of 843 m/s is pointed out within an acousto-optical scanfield of 0.4 $$\text{mm}^{2}$$ mm 2 by applying positioning frequencies of up to 1 MHz across a distance of 843 $$\upmu \text{m}$$ μ m . Consequently, the hybrid scanning system combines the advantages of optical and mirror-based scanners, enabling a highly dynamic and extremely precise laser beam positioning in a large processing area.
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