LASER DRILLING


WATER-SPRAY ASSISTED LASER DRILLING (SALaD): INSIGHTS


PRIYANKA GHOSH ET AL.*, THE MANUFACTURING TECHNOLOGY CENTRE a


b


Figure 2: Transverse cross section of a) SALaD and b) SOTA laser drilling; 55 NoP, 16 J, 65 Hz, 1.5 ms


Figure 1: Comparison of hole characteristics of SALaD vs SOTA processes INTRODUCTION


A critical manufacturing requirement for aluminium nitride-based devices is the drilling of through-holes with diameters ranging from ~100 μm to several millimetres. There is a clear gap in existing laser drilling technologies for achieving both high-speed and high-quality drilling of brittle materials like aluminium nitride (AlN). To address this technology and knowledge gap, the proposed study will develop and investigate a coaxial water- spray assisted millisecond fibre laser drilling approach.


EXPERIMENTS


The water-spray assisted laser drilling (SALaD) experiments were conducted on commercially available AlN ceramic substrates with a thickness of 0.6 mm. A high-power quasi-continuous wave (QCW) ytterbium-doped fibre laser system (YLS-2000/20000- QCW) was used. The focal spot diameter for this configuration was 125 µm (approx.). A commercial coaxial laser drilling head (Precitec) was modified to incorporate a water-spray delivery system. The original nozzle tip was re-engineered to allow injection of pressurised water. Compressed air, ranging from 1-5 bar, and oxygen assist gas, supplied at a controlled pressure (1-7 bar), mixed with the water at the nozzle tip, was used, resulting in atomisation and formation of a fine water spray.


Key process parameters including spray conditions, number of pulses, pulse energy, and pulse duration were systematically varied to investigate their influence on SALaD of AlN, including hole morphology, material removal, and process consistency. Comparative analysis with state-of-the-art (dry) laser drilling was also performed to understand and benchmark the improvements achieved using the proposed technique. A large volume of data was collected, and multiple analyses were conducted, which can be assessed in the full research publication [1].


26 | LASER USER 118 DECEMBER 2025 RESULTS AND DISCUSSION


A comprehensive optimisation trial was conducted for the dry, state-of-the-art laser (SOTA) drilling process, and the final optimised parameters (20 NoP, 8 J, 120 Hz, 1.5 ms) were used to enable a representative comparison. Figures 1 and 2 provide a comparative assessment of hole characteristics produced using the SALaD technique versus the SOTA drilling approach. The optimised SALaD configuration employed 55 pulses at 16 J, 65 Hz, and a pulse duration of 1.5 ms, while the SOTA drilling condition used 20 pulses at 8 J, 120 Hz and 1.5 ms.


As seen from Figures 1a and 1b, the SALaD process produces holes of larger diameter and minimal taper. The entrance diameter for percussion-drilled holes is approximately 0.65- 0.80 mm, whereas the SOTA laser drilling produces holes in the range of 0.20-0.25 mm. The plasma-assisted material removal also contributed to the improved hole characteristics observed with SALaD.


In addition to size, SALaD exhibits noticeably less spatter, cleaner edges, and reduced thermal artefacts. The scanning electron microscope (SEM) images in Figures 1b and 1d reveal that the dry-drilled holes exhibit a relatively thick spatter layer and visible cracks within the spatter. In contrast, the water- assisted holes are significantly cleaner, with minimal spatter (Figures 1a and 1c).


Cross-sectional SEM images (Figure 2) further emphasise the benefit of SALaD compared to the SOTA dry laser drilling in terms of the thermal damage. In Figure 2b, a visible recast layer approximately 10-20 µm thick lines sidewalls, accompanied by a layer of heat affected zone (HAZ) extending into the substrate. In several locations, microcracks were observed over the recast. Conversely, the recast layer was negligible in the SALaD sample


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