Development of the combined spatial and temporal pulse modulation for the laser impulse metal bonding process

The laser impulse metal bonding (LIMBO) process is a novel laser spot welding process to form a weld joint between a massive interconnector and thin metallization on a thermally sensitive substrate. As for the LIMBO process, the gap between joining partners in overlap configuration allows energetic separation of the melting and joining processes, whereas the underlying joining partner is only thermally stressed during the welding process. The LIMBO process shows, therefore, potential to substitute the ultrasonic wire bonding and the soldering process for joining interconnectors on metallized substrates such as direct bonded copper or PCB. Due to a separation between joining partners, the LIMBO process is divided into three stages that are achieved with temporal pulse modulation. The initial stage prepares a melt pool on an upper joining partner which penetrates through the material. After achieving a local melt pool through the upper joining partner, the temporal intensity modulation of the laser beam is initialized for the second stage. The enhanced laser beam intensity on the melt pool surface vaporizes the melt surface rapidly and induces a recoil metal plume. Hence, the recoil metal plume pressure accelerates the melt toward the underlying joining partner. After the contact, further energy input is required for the last stage to melt the underlying joining partner and subsequently to form a weld. Therefore, the laser beam intensity is once again temporally modulated to lower the intensity value. In this paper, the LIMBO process is extended with a further stage which combines spatial and temporal pulse modulation. From the outgoing position from the last stage of the LIMBO process, the laser beam is spatially modulated to perform a circular movement around the weld joint. Therefore, the spatial pulse modulation allows us to overcome the limitation of the spot welding process. In combination with temporal pulse modulation during the circular movement, the weld joint is expanded continuously. During the extended stage, the accessed heat is also considered, so the thermal threshold of the sensitive substrate is not exceeded. In addition, the overlap pulsed welding process is applied with the extended LIMBO process to achieve a linear weld joint area.