Laser Cleaning of Flux Residues on Copper Surfaces in Electronics Production

With the manifold of different laser sources, their application in power electronics production to remove solder flux residues on direct copper bonded substrates is a promising approach to ensure adequate surface cleanliness for further processing. For many subsequent processes in the production chain of power electronic assemblies, like potting or ultrasonic wire bonding, flux residues are a major obstacle, which can lead to manufacturing and reliability problems. State of the art cleaning processes make use of water- or solvent-based liquids in a washing process, which is followed by a rinsing and drying step. As the utilized liquids have to be recycled or disposed after use and drying is often performed at elevated temperatures or in controlled atmosphere, the procedure is energy and resource consuming. An alternative way to remove flux residues and prepare the surface for further processing is a laser induced removal process. In this paper the applicability of a nanosecond-pulsed ytterbium-doped fiber laser and a continuous-wave carbon-dioxide laser to clean flux residues on copper surfaces is presented. The cleaning result was assessed via energy-dispersive X-ray spectroscopy, fluorescence measurement technology and laser confocal analysis. Results show, that the condition of the flux residues after soldering has a strong influence on the applicability of the laser sources.