Light Wave Interference During Laser Drilling of PWB's.

A CO sub 2 laser has been used to drill semiburied vias in a 150microns (5.9 mils) thick dielectric (PDD 81-44) layer coating a copper clad substrate. A typical hole is 100 to 150microns (4 to 6 mils) in diameter. The holes in this study were each made with a single laser pulse of 10.6microns wavelength, duration 100 or 200microns, 4 to 20 mj energy. Two superimposed sets of periodic ripples have been observed on the hole walls. The shorter wavelength varies from 4.0microns at the top of the hole to 5.3microns at the bottom of the hole. The longer wavelength to be 13.2microns and is attenuated as the wave propagates towards the copper substrate. The experimental data are compared with values calculated using a model that considers the interference of a standing wave inside the hole with radiation propagating through the dielectric surrounding the hole.