Pulsed laser processing of dielectric materials

Abstract

The thesis investigates the wavelength dependent laser ablation in dielectric materials used for the fabrication of high density Printed Circuit Boards (PCBs) in the electronics industry. Here the market for consumer and industrial products of ever-rising complexity has led to a demand for increased miniaturisation and low costs of multi- level printed circuit boards (PCBs) interconnected by microvias, which electrically connect the various circuit layers. Laser machining offers a potential solution to this need. The main objective of the research is to investigate the wavelength-dependence of the laser machining/drilling efficiency of two important sets of PCB materials, categorised as Organics and Ceramics using a carbon dioxide laser which can be tuned across its emission spectrum in the 9pm -II pin spectral region.. The organics include commercially available electronic materials with trade names such as Kapton, Arlon, FR4 and RCC and the ceramics materials studied are alumina and low temperature co- fired ceramic (LTCC). The aim is to determine the optimum laser wavelength for maximum processing efficiency i. e. to find the wavelength where the laser parameters are best matched to the optical, thermal and mechanical properties of each of the materials. A C02 laser machining system was constructed which incorporated a novel laser source developed in the research programmes. The laser source was a MOPA system with a line-tuneable cw oscillator and a five pass power planar waveguide rf discharge-excited power operating in the so-called enhanced power regime to produce maximum peak power. An Acousto-optic modulator between the master oscillator and the amplifier allowed convenient control of pulse amplitude and duration. The system enabled the wavelength dependent studies on the wavelength and pulse energy dependence of the laser ablation properties (e. g. ablation threshold fluence and ablation rates) - to derive the so-called 'ablation spectrum' of the selected materials A comparison is made of the wavelength dependence of ablation with the room temperature absorption spectrum measured for each material using ellipsometry. It was observed that the 'ablation spectrum' information does not always appear to match the simple expectations derived from the room temperature 'absorption spectrum' of the material. This disparity in results is likely due to the change of absorption properties of material because of rise in temperature, chemical decomposition or melting of material during ablation. However, the room temperature absorption spectrum (while not adequate alone), did provide a useful guide to the selection of a sub-set of the 40+ lines that would otherwise have to be studied. The results may be of direct application in the electronics industry to increase the efficiency of laser machining