NANOSECOND LASER-INDUCED THERMAL DIFFUSION DURING LASER ABLATION OF EPOXY FROM A MULTILAYERED SURFACE

Abstract

Interference ring marks were observed on a copper substrate consisting of plated layers of Au, Pd and Ni after laser ablation of epoxy material from the top surface. These ring marks are the result of laser-induced surface oxidation and need to be avoided or minimized in order to improve the solder wettability on the treated surface. Studies of laser-induced thermal diffusion through the plated layers were carried out using a 532 nm Q-switch Nd:YAG laser. The major process variables investigated were the beam profile, pulse repetition rate and beam scan pattern. Secondary ion mass spectroscopy (SIMS) was used to obtain a depth profile of the laser-treated layers. It revealed that interfusion of Pd and Ni by diffusion occurred during laser irradiation. Calculations of thermal diffusion length and temperature rise of the metal layers without considering the accumulated thermal effect of overlapping pulses are presented in this article. The study results are useful for a better understanding of the process of laser removal of epoxy flashes on leadframe heatsink surfaces.