Analysis of 355 nm Nd:YAG laser interaction with patterned flexible circuit substrates

Abstract

This paper describes a new approach in modeling the interaction and ablation mechanisms of 355 nm Nd:YAG laser and polyimide material. Generating micro-structures by laser micromachining of inhomogeneous patterned flexible substrates is limited because only a rough estimation on etch rate is possible with a specific set of process parameters. In order to exploit the potential of laser micro-machining, i.e. to generate structures in the resolution range of the focal spot diameter (~10-20 &mu m), the effect of laser beam needs to be calculated pulse-by-pulse. The novelty of the model is that it incorporates the main beam properties of a solid-state laser, the structure of the substrate and the accumulated heat caused by successive pulses, while it eliminates the details of molecular processes which are still considered but only by means of experimental constants. The model also introduces a new conception of ablation threshold as it is defined as a temperature value instead of pulse energy. This allows the calculation of the amount of ablated material by determining the temperature distribution of the processed material after each laser pulse. This way the model can also take the patterned inhomogeneous structure of the substrate into account, thus allowing material processing by lasers at high resolution. Experimental and simulation results are compared and evaluated to obtain the model constants (like ablation threshold temperature, transformation efficiency of pulse energy into heat etc.) for 355 nm Nd:YAG lasers and polyimide materials and also for the verification of the model.