Abstract
Shape memory alloy (SMA) actuators are attractive for their exceptional actuator and sensory capabilities, especially in industries like aerospace and automotive where achieving lightweight construction is crucial. SMAs, with their high-power density, play a vital role in meeting these demands. The shape memory effect (SME) is utilized to induce actuator stroke, typically activated through resistance heating or a surrounding medium. While semi-finished products such as wires, tubes, sheets, or foils are commonly used for actuator applications, their transformation into functional actuators requires expertise in SMAs. Mechanical anchoring, often achieved through processes like crimping, is essential for transmitting generated forces. Additionally, electrical connections are frequently needed for activation, leading to significant custom development work and expertise in the complex field of SMAs, incurring both time and cost. Utilizing laser welding to bond NiTi onto printed circuit boards presents a novel method to optimize the attachment of NiTi wires, ensuring both mechanical and electrical connectivity. Leveraging widely used PCBs provides a well-researched platform for standardized actuator components, streamlining engineering processes for new actuator systems. Laser welding of nickel-titanium (NiTi) alloys onto PCBs is a reliable method due to its small heat-affected zone, enabling modification of shape memory properties. Challenges in welding NiTi and Cu, such as brittle precipitates and the inclusion of Sn in the Ni-Ti-Cu-Sn system, require careful consideration. Moreover, the thickness of tinned copper layers on printed circuit boards is typically 70 μm or smaller, posing an additional challenge, especially when combined with the commonly used FR4 base material. Reinheimer et al. initially used a theoretical approach to underscore the fundamental feasibility of a welded joint on a printed circuit board. Subsequently, they successfully validated their approach through blind joints, demonstrating the absence of delamination defects. The key challenge lies in controlling heat input during the welding process, as excessive heat can lead to delamination processes within the FR4. To address this concern, a nanosecond laser is employed, in contrast to a previous publication that used a modulable continuous-wave fiber laser. This choice of laser technology aims to enhance precision in heat control, mitigating the risk of delamination processes in the FR4 substrate. Traditional soldering methods, often used for bonding components to PCBs, face challenges with NiTi due to poor wettability, necessitating aggressive fluxes or ultrasonic soldering. In contrast, laser welding with nanosecond lasers, as explored in this paper, offers a stable joint with controlled heat input.
