Laser drilling of micro-vias in PCB substrates

Traditional mechanical drilling of PCB vias is practical only for through h oles with diameters greater than 200 mum. With the emerging global demand f or higher packaging densities, extensive R&D to produce micro-vias as small as 50 mum is being carried out worldwide. The laser drilling of micro-vias is accepted as a feasible method and has e ven seen limited use in production. It offers greater resolution over the m echanical technique with its ability to produce via sizes well below 50 mum . The smallest via size that can be drilled with lasers is very dependent o n the laser wavelength, beam energy density, composition and thickness of t he substrate material. As most PCB materials have finite absorption charact eristics with respect to the laser wavelength, only certain laser classes a re compatible with the commonly-used substrate materials. This paper will describe the process of fabricating micro-vias reliably in BT, FR4, polyimide and alumina substrates using the RF and TEA CO2, 3(rd)-h armonic Nd:YAG, and KrF excimer lasers, respectively. Both blind and throug h-vias were drilled and evaluated for taper, wall-angle and smoothness thro ugh optical microscope, SEM and cross-sectional analyses. The beam/material interaction mechanisms for the various lasers and PCB substrates were exam ined. The effects of the substrate composition such as fiber density distri bution and copper cladding were also studied, as well as, the platability o f the via holes 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.