Cavitation in Unfired Ceramic Sheets by Pulsed Nd:YAG Laser
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CAVITATION IN UNFIRED CERAMIC SHEETS BY PULSED ND:YAG LASER Rong-Fuh Louh and Relva C. Buchanan Department of Materials Science and Engineering University of Illinois at Urbana-Champaign, Urbana,
IL 61801
ABSTRACT Single shot pulses from a Nd:YAG laser were used to generate different sized holes and patterns in tape-cast ceramic sheets for electronic packaging applications. The ceramic sheets were based on formulations in the BaTiO 3 and MgO-Al 2 O3 -SiO 2 systems. The single shot laser pulses were targeted using a computer controlled mirror deflection system and a programmed (cavitation) pattern which could readily be modified. Morphology and structure of the drilled holes before and after sintering were examined by SEN. The surface radial temperature profile resulting from the laser pulse was directly mapped using a lacquer thermocoat. Effects of the laser pulse energy and beam spot size on laser/ceramic interactions and cavitation process were also explored. INTRODUCTION Laser cavitation or hole drilling has been applied to a wide variety of materials. The current laser drilling techniques involve mainly the use of CW laser incidence or multiple pulses on the irradiated targets, typically consisting of dense sintered ceramics[I-7]. The laser pulse duration is normally greater than 100 nsec, but this long pulse generates complicated energy coupling interactions between the laser-induced plasma and the incident laser beam, with decreased heating of the workpiece . The Nd:YAG laser used in this study had a short pulse duration (10 nsec) and a high peak power output (>10 MW). This allowed single shot cavitation of the target materials,
typically tape-cast unfired ceramic sheets with a polymer binder content between 35-40%. Close control of the laser input energy was required to minimize localized cracking. The temperature profile
sintering, on the
directly mapped using a lacquer thermocoat, spectroradiometry technique[8]. Objective
of this
interaction mechanisms
study
was to
target
warping and microsurface was
in contrast to a
investigate
the
laser-solid
and the energy profiles necessary to per-
mit cavitation and possible line sintering in thin and thick films, with application potential tronics packaging.
deposited ceramic to microelec-
EXPERIMENTAL
Results reported in this paper were obtained using a pulsed Nd:YAG laser and the second harmonic wavelength of 532 nm. Peak power was on the order of 10 MW for 10 nsec pulse duration with up to 250 mJ pulse energy. The average unfocused beam diameter was less than 1 cm (1/e 2 power diameter), with pulse rate and pulse duration set to be constant during the experiment. Similar to a computer controlled laser marking machine [8], the laser processing system shown in Fig 1 was programmed to Mat. Res. Soc. Symp. Proc. Vol. 100. @1988 Materials Research Society
660
scan the beam across the target surface. The laser beam scanner, having an X-head and a Y-head, was connected to the controller (a digital-to-analog converter) which received the encoded sig
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