The increasing electrification of automotive powertrains demands reliable, high‑precision joining technologies, particularly for inverter and power module production. As power electronic converters become more compact and achieve higher power densities, joining processes must minimize heat input due to the proximity of temperature sensitive components. This is especially critical when welding copper busbars to thin copper foils, where pronounced thickness ratios challenge process stability and reproducible weld formation. This work investigates the capability of different laser sources, specifically near‑infrared and visible wavelengths, for joining copper with high thickness ratios. Owing to the thin lower joining partner, the process imposes strict requirements on energy input and weld depth stability. This work examines how wavelength‑dependent absorption, applied energy density, and varying focus diameters influence process consistency and seam geometry. Special emphasis is placed on the correlation between laser wavelength and the reproducibility of weld penetration depth. To complement the quantitative analysis, high‑speed imaging of the melt pool provides insight into melt pool dynamics and their impact on process robustness. The recordings enable the identification of characteristic instabilities and support interpretation of wavelength‑induced differences in energy coupling. The investigation further includes an evaluation of mechanical and electrical properties of the welded joints, providing a comprehensive assessment of connection quality. Overall, this study advances understanding of wavelength‑dependent joining processes for copper in applications with significant thickness ratios and offers guidance for designing robust, laser‑based contacting strategies in automotive power electronics.