Characterization of Laser Cleaning of Copper for Soldering Processes

The laser cleaning of copper surfaces with a Nd:YAG Q-switched laser pulse for the improvement of solder quality on printed circuit boards (PCBs) has been monitored and characterized by sensing the acoustic emission during the process and one-dimensional mathematical model analysis. It was found that selective removal of copper oxides from the surface was achieved by the laser operation, which was described theoretically by the model and was confirmed experimentally by the acoustic monitoring. The acoustic monitoring provided not only threshold laser fluence and optimal process window for the cleaning of copper but also the clear possibility for real-time surface monitoring of the process. Different features at the two laser wavelengths used (1064 nm and 532 nm) were observed in the surface morphology, i.e., the laser pulse with 532 nm wavelength produced a lightly sputtered wide area around the laser crater. From the theoretical and experimental investigation of laser cleaning mechanisms at different wavelengths, it was found that the mechanical effect induced by the intense shock waves was much more dominant at 532 nm wavelength than at 1064 nm.