Drilling is an industrial process in which holes are produced by removal of material. This process is relatively well established for macroscale machining. However, microscale mechanical drilling is a more challenging process, especially in parts made of difficult-to-cut materials such as nickel-based superalloys. Although laser drilling and electrical discharge machining (EDM) have been reported as alternatives, mechanical drilling continues to be widely used for industrial macroscale drilling. However, mechanical microdrilling suffers from premature drill breakage due to the fragile nature of the microdrill. Furthermore, mechanical-drilled holes are inherently associated with geometry and metallurgy defects such as burr and subsurface damage, respectively. In laser percussion drilling, the challenge is how to improve the quality of the hole by minimising taper, recast layer, and heat affected zone formation. In addition, drilling hollow parts such as airfoil blades without introducing damage to the back wall is a major challenge in laser drilling. In drilling, the accuracy of the process and the quality of the surface finish are of great importance for both the manufacturer and the customer. Hybrid machining has been identified as a promising process which combines the benefits of different machining processes especially when applied to machining of superalloys. This Thesis presents a novel method to microdrill an Inconel 718 alloy, at both normal and inclined angles to the surface, using laser followed by mechanical drilling (sequential drilling). The method was aimed at extending the twist drill life and improving the quality of the hole when compared with existing techniques. The effect of laser predrilled-hole geometry on the quality of the produced hole were studied and evaluated. Continuous wave (CW) fibre and pulsed Nd:YAG lasers were used to produce holes with different geometry (blind, positive and negative tapered holes) as a pilot hole for mechanical drilling. CW fibre and Nd:YAG laser microdrilling of Inconel 718 alloy were implemented and evaluated before conducting the sequential drilling process. Taguchi methods were employed to design the experiments and analyse the results to establish the optimum set of parameters that yields an acceptable level of the response target. The standard commercial statistical software package MINITAB was used to evaluate the results. Initial experiments on the use of CW fibre laser drilling showed a great improvement in the quality of the hole and drilling speed. Those encouraging results inspired more experimental work and further evaluation of microdrilling of an Inconel 718 alloy. This unprecedented work was aimed at establishing the optimum conditions of laser and process parameters for hole taper, recast layer, and machining time. The results proved that the CW fibre laser drilling mechanism could be considered as a keyhole laser welding before material breakthrough. Furthermore, the process gas must be used to push away the molten material through the hole exit. The results also showed that a near zero tapered hole with very small recast layer and free of micro-cracks could be achieved with air process gas. This would have huge economical and environmental impacts since air is cheap and also an abundant resource. In the case of Nd:YAG laser microdrilling, the results proved that using assisted gas in laser drilling would not always increase the drilling speed or improve the quality of the hole. It was also found that the quality of the holes produced by air process gas is sufficient to meet the requirements for mechanical finishing. The sequential laser mechanical technique reduced the width of cut compared to mechanical drilling and relieved the load on the drill point resulting in a decrease in the thermal and mechanical stresses on the cutting tool. When compared with pure mechanical microdrilling, mechanical finishing of near zero laser drilled hole resulted in 100-330% increase in the tool life, up to 75% reduction in burr height, and significant improvement in surface integrity. In addition, the sequential laser and mechanical drilling of laser blind holes would be an effective technique for decreasing burr size and avoiding the back-wall problem in laser drilling of hollow parts especially when the exit surface of the components to be drilled has a closed cavity or is hard to access. It was also found that a smaller predrilled hole provided stability to the twist drill at the entry stage. However, burr size at the exit side decreased when the size of the predrilled hole was increased. Therefore, the mechanical finishing of negative tapered hole technique was developed to maintain the stability of the drill, extend the drill life, improve the burr size and surface integrity. The burr size for the mechanical finishing of negatively tapered laser predrilled holes was measured to be 6 times smaller than that of pure mechanical drilling. Finally, the results proved that the new technique alleviated the indentation and secondary cutting edge action. This would enable manufacturers to grind drills to thicker web thickness, which in turn, will increase the drill strength.

Recent advances in microelectronics and packaging industry are characterized by a progressive miniaturization in response to a general trend toward higher integration and package density. Corresponding to this are the challenges to traditional manufacturing processes. Some of these challenges can be satisfied by laser micromachining, because of its inherent advantages. In laser micromachining, there is no tool wear, the heat affected zone can be localized into a very small area, and the laser micromachining systems can be operated at a very wide range of speeds. Some applications of laser micromachining include pulsed Nd:YAG laser spot welding for the photonic devices and laser microdrilling in the computer printed circuit board market. Although laser micromachining has become widely used in microelectronics and packaging industry, it still produces results having a variability in properties and quality due to very complex phenomena involved in the process, including, but not limited to, heat transfer, fluid flow, plasma effects, and metallurgical problems. Therefore, in order to utilize the advantages of laser micromachining and to achieve anticipated results, it is necessary to develop a thorough understanding of the involved physical processes, especially those relating to microelectronics and packaging applications. The objective of this Dissertation was to study laser micromachining processes, especially laser drilling and welding of metals or their alloys, for the microscale applications. The investigations performed in this Dissertation were based on analytical, computational, and experimental solutions (ACES) methodology. More specifically, the studies were focused on development of a consistent set of equations representing interaction of the laser beam with materials of interest in this Dissertation, solution of these equations by finite difference method (FDM) and finite element method (FEM), experimental demonstration of laser micromachining, and correlation of the results. The contributions of this Dissertation include: 1)development of a finite difference method (FDM) program with color graphic interface, which has the capability of adjusting the laser power distributions, coefficient of energy absorption, and nonlinear material properties of the workpiece as functions of temperature, and can be extended to calculate the fluid dynamic phenomena and the profiles of laser micromachined workpieces, 2)detailed investigations of the effect of laser operating parameters on the results of the profiles and dimensions of the laser microdrilled or microwelded workpiece, which provide the guideline and advance currently existing laser micromachining processes, 3)use, for the first time, of a novel optoelectronic holography (OEH) system, which provides non-contact full-field deformation measurements with sub-micrometer accuracy, for quantitative characterization of thermal deformations of the laser micromachined parts, 4)experimental evaluations of strength of laser microwelds as the function of laser power levels and number of microwelds, which showed the lower values than the strength of the base material due to the increase of hardness at the heat affected zone (HAZ) of the microwelds, 5)measurements of temperature profiles during laser microwelding, which showed good correlations with computational results, 6)detailed considerations of absorption of laser beam energy, effect of thermal and aerodynamic conditions due to shielding gas, and the formation of plasma and its effect on laser micromachining processes. The investigations presented in this Dissertation show viability of the laser micromachining processes, account for the considerations required for a better understanding of laser micromachining processes, and provide guideline which can help explaining and advancing the currently existing laser micromachining processes. Results of this Dissertation will facilitate improvements and optimizations of the state-of-the-art laser micromachining techniques and enable the emerging technologies related to the multi-disciplinary field of microelectronics and packaging for the future.

This research studies microdrilling of biocompatible materials including commercially pure titanium, 316L stainless steel, polyether ether ketone (PEEK) and aluminum 6061-T6. A microdrilling technique that uses progressive pecking and micromist coolant is developed that allows drilling of 127 micrometers diameter microholes with an aspect ratio of 10:1. The drilling parameters, dominant wear pattern, hole positioning accuracy and effect of AlTiN tool coating are experimentally determined. The experimental data trend agrees with classical Taylor's machining equation. Despite of fragile and long microdrills, the progressive pecking cycle and micromist allowed deep hole drilling on all the tested materials. Drill wear is more pronounced at outer cutting edge due to higher cutting speeds. However, when drilling 316L stainless steel attrition wear at chisel edge is dominant. Hole quality degradation due to formation of built up edge at the drill tip is observed. Coated drill improves tool life by 122% and enhances hole quality when drilling 316L stainless steel. The hole positioning accuracy is improved by 115% and total hole diameter variation decreased from 0.11% to 0.003% per mm of drilling distance.

"Laser micromachining is essential in today’s advanced manufacturing, of e.g., printed circuit boards and electronic components, especially laser microdrilling. Continued demands for miniaturization, in particular of high-performance MEMS components, have generated a need for smaller holes and microvias as well as smaller and more controllable spot-welds than ever before. All these neeeds require smaller taper of the microholes and more stable and controlled laser micromachining process than currently available. Therefore considerable attention must be focused on the laser process parameters that control critical specifications such as accuracy of the hole size as well as its shape and taper angle, all of which highly influence quality of the laser micromachining processes. Determination of process parameters in laser micromachining, however, is expensive because it is done mostly by trial and error. This Dissertation attempts to reduce the experimental time and cost associated with establishing the process parameters in laser micromachining by employing analytical, computational, and experimental solutions (ACES) methodology."