MICRO LASER WELDING OF NITI SHAPE MEMORY WIRES AND PRINTED CIRCUIT BOARDS

Integration of shape memory alloys (SMA) into actuator systems is still challenging, since mechanically and electrically anchoring are needed to provide stroke and trigger the shape memory effect (SME) in a controlled manner. A direct connection of SMA wires with printed circuit boards (PCBs) would offer many advantages. No additional material (e.g. crimps, clamp connections) and no additional steps for the electrical connection would be required. This would facilitate the engineering for new actuator systems because key integration challenges would be addressed through the use of the nickel-titanium (NiTi) PCB assembly. Soldering is traditionally used for bonding components with PCBs, but due to the poor wettability of the natural passivation layer, NiTi can only be soldered by using aggressive fluxes. In addition, soldered connections withstand mechanical loads to a limited extent. A stronger connection can be achieved using micro laser welding to create a joint, which provides mechanical and electrical connection for actuators. Controlling the heat input yield by laser radiation is especially important for this purpose. On the one hand, too intense heat input causes delamination processes due to excessive temperatures at the interface layer between copper layer and the PCB substrate. On the other hand, insufficient heat input leads to a weak connection that lacks mechanical strength, most importantly tensile and cyclic strength. In this work, the influence of laser power on the welded joint is investigated. In this context, tensile tests, lifetime tests, and microsections of the joint area are used for characterisation.