Investigation of the laser-based joining technique for PCBs with locally thickened layers using a cold gas spraying process
The laser beam welding process is not commonly used for joining interconnectors on metallized substrate such as Printed Circuit Board (PCB) due to the presence of the vapor capillary and high energy input which exceed the thermal destruction threshold of the substrate. To perform a welding process between interconnectors and PCBs, a metallization with sufficient thermal mass is required. The cold gas spraying process is used to spray a copper layer on a thin metallization to increase its thickness and the thermal mass. In this paper, a copper interconnector is laser-welded on a metallization of PCB which is cold gas sprayed with copper powders. The characterization and transfer of the welding technology to the joining of spray layer is investigated. The main challenges of the welding process on spray layer on substrate are the uncontrolled surface roughness and the inhomogeneous heat distribution of the spray layer compared to the bulk material. The influence of the sur face roughness on the void formation is investigated by considering roughness values of the sprayed layer. A correlation between the void formation and the surface roughness is shown. Also, the void formation at the weld joint increases with the higher laser beam power. The increased laser beam power leads to a deeper vapor capillary and it is assumed that the inhomogeneous heat distribution of the lower joining partner induces varying solidifications speed. By reducing the surface roughness value and lower laser beam power, we could significantly reduce the void formation at the weld root.