Customized flipped quasi-Bessel beams for infrared ultrafast laser drilling through scattering FR4 composite

• Ultrafast laser drilling of FR4 fiberglass epoxy resin by custom Bessel-like beams. • Beam intensity distribution simulation to find the beams design parameters. • Dedicated passive beam shaping by axicon-lens doublets. • Light-scattering reduction and drilling enhancement by flipped Bessel axial profile. The light-scattering behavior of heterogenous transparent materials impedes the proper propagation of laser beams, affecting the formation of a precise below-surface focal spot and, therefore, their processing with ultrafast laser beams. In this work, the interest of using and customizing non-diffractive Bessel beams as a solution to mitigate in-volume propagation scattering losses during bulk laser processing of a composite material is investigated. For this purpose, we assess the compensation ability on the ultrafast laser drilling performance of holes through a translucent fiberglass epoxy resin (FR4) substrate, a material of interest for printed electronic circuits. Using simple optical elements like axicons and lenses, customized regular and flipped quasi-Bessel beams are generated, which respectively show decreasing or growing on-axis intensity profiles. Their performance to drill through-holes in a 200-μm thick FR4 substrate is systematically investigated using different pulse numbers and energy. The interest of sample-long flipped-profile Bessel beams is shown, obtaining repeatable through-holes between 10 and 95 μm wide, a lower drilling energy threshold, and a reduced heat-affected zone around the holes. These results demonstrate the benefit of adapted beam shaping as a solution to mitigate scattering during laser processing of heterogeneous materials, unveiling a new range of accessible materials and potential applications by ultrafast laser processing.