Laser drilling of micro-vias in PCB substrates

Traditional mechanical drilling of PCB vias is practical only for through holes with diameters >200 μm. With the emerging global demand for higher packaging densities, extensive R&D to produce micro-vias as small as 50 μm is being carried out worldwide. Laser drilling of micro-vias is accepted as a feasible method and has even seen limited use in production. It offers greater resolution over the mechanical technique with its ability to produce via sizes well below 50 μm. The smallest via size that can be drilled with lasers is very dependent on the laser wavelength, beam energy density, and substrate material composition and thickness. As most PCB materials have finite absorption characteristics with respect to the laser wavelength, only certain laser classes are compatible with commonly-used substrate materials. This paper describes the process of fabricating microvias reliably in BT, FR4, polyimide and alumina substrates using the RF and TEA CO₂, 3rd-harmonic Nd:YAG, and KrF excimer lasers, respectively. Both blind and through-vias were drilled and evaluated for taper, wall-angle and smoothness by optical microscopy, SEM and cross-sectional analyses. The beam/material interaction mechanisms for the various lasers and PCB substrates were examined. The effects of substrate composition such as fiber density distribution and copper cladding were also studied, as well as the via hole platability in terms of via dimensions and features. Process parameters were then further optimized to determine the required laser pulse energy, peak power, number of pulses and energy density