A new procedure for laser spot welding of thin copper stripes was developed based on measuring the calibrated reflectivity coaxially to the laser beam and using this signal for in situ control of the laser power. Experiments with a power ramp of r=3.1 MW/s were performed on 100-μm-thick commercially available copper stripes of 99.9% purity. The experimental process was divided into four phases: analysis, heating/melting, melt-spreading/welding, and cooling. Overall, results confirm that the new technique allows achieving smaller weld spot diameter without affecting reliability.