# OCT Capillary Depth Measurement in Copper Micro Welding Using Green Lasers
###### Abstract  
The transition of the powertrain from combustion to electric systems increases the demand for reliable copper connections. For such applications, laser welding has become a key technology. Due to the complexity of laser welding, especially at micro welding with small weld seam dimensions and short process times, reliable in-line process monitoring has proven to be difficult. By using a green laser with a wavelength of \(\lambda=$515\,\mathrm{nm}$\), the welding process of copper benefits from an increased absorption, resulting in a shallow and stable deep penetration welding process. This opens up new possibilities for the process monitoring. In this contribution, the monitoring of the capillary depth in micro copper welding, with welding depth of up to \(1\,\mathrm{mm}\), was performed coaxially using an optical coherence tomography (OCT) system. By comparing the measured capillary depth and the actual welding depth, a good correlation between two measured values could be shown independently of the investigated process parameters and stability. Measuring the capillary depth allows a direct determination of the present aspect ratio in the welding process. For deep penetration welding, aspect ratios as low as 0.35 could be shown. By using an additional scanning system to superimpose the welding motion with a spacial oscillating of the OCT beam perpendicular to the welding motion, multiple information about the process could be determined. Using this method, several process information can be measured simultaneously and is shown for the weld seam width exemplarily.  
laser welding; green laser; copper welding; micro welding; optical coherence tomography; laser weld monitoring +
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process strategies, like spatial beam modulation, must be applied, providing additional challenges. In recent investigations, the authors of Reference [4] showed the difficulties for measuring the welding depth in micro welding with a spot diameter of 82 \(\upmu\)m, while, for many applications, even smaller focal diameters are being used.  
In latest developments, beam sources with shorter wavelength are reaching the required performance for laser micro welding of copper, opening up new possibilities. Using a green laser with a wavelength of 515 nm, the authors of Reference [5] were able to achieve comparable welding depth compared to state-of-the-art fiber micro welding processes. The use of shorter wavelengths in copper welding is beneficial due to an increased absorption of the laser beam in the material [6]. This allows larger focal diameters to be used, resulting in a shallow and wide capillary and a stable welding process. The resulting capillary shape increases the accessibility compared to micro laser welding with fiber lasers, which benefits the measurement of the capillary depth and the correlation to the welding depth.  
In the following, the capillary depth measurement and the correlation to the welding depth in copper micro welding, using a green laser is presented. By measuring the capillary depth, the aspect ratio of the welding process can be determined directly and will be presented. Lastly, the possibilities of a scanned OCT system to measure several process information are presented and exemplarily shown for the weld seam width.