Hybrid erosive jet micro-milling of sintered ceramic wafers with and without copper-filled through-holes

Abrasive slurry jet micro-machining (ASJM) in combination with abrasive air jet micro-machining (AJM) was used to mill micro-pockets in three different ceramic wafers: sintered alumina, aluminum nitride, and zirconium tin titanate. A composite substrate of aluminum nitride with an array of copper-filled through-holes was also machined using a hybrid process of AJM and ASJM that capitalized on the significant differences in the erosive characteristics of each method. The objective was to investigate an alternative to laser micro-milling. The sintered ceramics were found to erode in a brittle manner by the dislodgment of grains upon abrasive particle impact. The eroded profiles produced by ASJM and AJM were modeled analytically.ASJM could make 100 μm deep flat pockets, 500 μm wide with 60° sidewall angles in the sintered ceramics using overlapping parallel channels. The shapes of the pockets could be predicted accurately as long as the depth of each machined pass was less than 50 μm. Pockets of the same size and roughness (Ra = 0.4 μm) could also be machined and modeled accurately using masked AJM. The surface roughness of the sintered ceramics was found to be insensitive to particle size, being controlled by the size of the sintered grains.Similar pockets could be machined in the aluminum nitride containing 180 μm diameter copper through-holes using ASJM provided that the maximum depth was about 25 μm. Beyond that, the secondary slurry flow away from the jet footprint created unwanted etching of the copper-filled through-holes leading to a lack of flatness. Deeper pockets in these substrates were machined using a hybrid AJM–ASJM methodology, in which AJM was used to selectively erode the brittle sintered ceramic without etching the ductile copper, followed by the leveling of the protruding copper pillars to the depth of the ceramic using ASJM. Computational fluid dynamics models were used to explain the results in terms of the large differences in the local particle impact angles and velocities in ASJM and AJM.