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 λ=515, 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 , 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.

Algorithms for Weld Depth Measurement in Laser Welding of Copper with Scanning Optical Coherence Tomography

“…As all data processing methods show a trend towards lower RMSE value for an increase in the OCT measurement position behind the TCP, at a laser power of 4000 W and a welding speed of 15 m/min, we expect a more accurate weld depth measurement for these parameters for a further increase in OCT measurement position behind the TCP. This is in accordance with findings by Beck et al [13] who could show that an alteration in laser power and welding speed leads to a change in the OCT measurement beam position behind the TCP.…”

“…One solution to observe the weld penetration depth inline is optical coherence tomography (OCT). OCT is an interferometric measurement technology that enables the measurement of weld penetration depth coaxially to the processing laser in a fixed optic [5][6][7][8][9][10][11][12] or scanning optic [13][14][15].…”

“…Such alignment is not universal and needs to be adjusted for each welding task with respect to the keyhole shape [3,17]. For instance, the higher the welding speed and the higher the laser power, the higher the distance between OCT measurement beam from the tool center point (TCP) of the processing laser, in the case of the laser welding of copper [13].…”

Towards process control by detecting acoustic emissions during ultrashort pulsed laser ablation of multilayer materials

“…For example, a commonly used and sophisticated system for monitoring welding processes is optical coherence tomography (OCT). [1][2][3] Yet, the use of OCT requires a sufficiently large key hole, which precludes its application from drilling micro-holes, e.g. for processing modern printed circuit boards [4][5][6][7] , using USP lasers.…”