New numerical evaluation techniques for laser processed ceramic substrates

To date laser drilling has not been clearly defined by a standard set of me asurement parameters that fully define the features they produce. This arti cle presents criteria for solving some of these issues, which in turn assis t in the scientific characterization of the laser drilling process. The mea surement criteria presented here fall into four categories: laser parameter , material science, in-process, and geometry measurements. Generation of th ermal damage is the Achilles heel of laser processing of ceramics. The fund amental process of laser radiation interaction with ceramic substrates gene rates a severe thermal gradient between the recast layer and bulk substrate . This in turn leads to the stresses, which cause cracks to form. The numer ical quantification of these cracks, in terms of size and distribution, is essential for developing an understanding of how cracks form, and how they can be avoided. It was found that the width of these cracks fell into two d iscrete size ranges. scanning electron microscope micrographs showed that w hen the crack width was 7 mum or more, the cracks propagate through the rec ast layer and into the bulk substrate, significantly weakening the componen t. These cracks were termed "macrocracks." The cracks having a width smalle r than 7 mum were termed "microcracks," which were only found in the recast layer, and did not propagate into the bulk material. The numerical evaluat ion techniques presented here now provide a numerical route for the compari son of strength testing data, with respect to the change in size and distri bution of cracking seen in laser processed thermally sensitive materials. T his article also presents results from a study in techniques for the quanti fication of geometry features produced by the laser drilling process. This work employed high/low power optical and electron microscopy, linked with a n image processing system, to define these features. The study focused on d eveloping numerical methods for analyzing the surface hole dimensions (diam eter, area, shape), and axial hole geometry (cross-sectional area, and mate rial removal rate). (C) 2000 Laser Institute of America. [S1042-346X(00)003 06-5].