Session: 02-03: Shape Memory Alloys

Paper Number: 90970

90970 - Micro Laser Welding of NiTi Shape Memory Wires and Printed Circuit Boards 

Shape memory alloy (SMA) actuators are attractive due to their outstanding actuator and sensory functionalities. Lightweight construction is a core requirement in demanding industrial sectors (aerospace, automotive), and SMA with their high-power density gain importance in this field. Here, the shape memory effect (SME) is utilized to generate actuator stroke, which is usually triggered electrically (resistance heating) or via a surrounding medium. Integration of SMA into actuator systems is still challenging, since mechanically and electrically anchoring are needed to provide stroke and trigger the 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 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. Furthermore, soldered connections can withstand mechanical loads to a limited extent. Micro laser welding can overcome these challenges and create a stable joint if the heat input can be sufficiently controlled during the process. Insufficient heat input leads to a weak connection that lacks mechanical strength (tensile strength, cyclic strength). Excessive heat input causes delamination processes due to excessive temperatures at the boundary layer between the 70 µm thick copper layer and the base substrate (FR4). This work investigates the influence of laser power on the welded joint. For this purpose, tensile tests, lifetime tests, and microsections of the joint area are used for characterization.

Presenting Author: Marvin Schuleit RUB