The X-ray phase contrast imaging is a powerful method to understand the fundamental behavior of the melt and keyhole during the laser beam welding process. In this paper, the keyhole-induced vapor capillary formation in the melt pool is investigated by using an adjustable laser beam source. For this purpose, the aluminum A1050 specimen with a thickness of 0.5 mm is molten only with the heat conduction welding regime by using the ring-mode laser beam. Once the specimen is molten through, the core multi-mode laser beam is then applied to vaporize the melt and a transition to keyhole welding regime occurs. Therefore, the core multi-mode laser beam with an intensity value of 33.3 MW/cm² is investigated. The correlation between the keyhole-induced vapor capillary and the melt behavior is further investigated in this paper which was recorded with a high sampling rate of 19 kHz. In addition, a theoretical calculation about the keyhole depth is discussed in this paper.

The laser-spot welding process of aluminum alloy 1050A with a limited thickness is observed with the x-ray phase contrast method to investigate the melt dynamic especially when the melt penetrates the material. The laser-spot welding is investigated with two different wavelengths of the laser beam source: 515 and 1030 nm to investigate the influence of the absorptivity. The melt progressively penetrates the material during the spot-welding process until reaching the bottom side of the material and when the melt penetrates the lower side of the material, the so-called “lens-like” melt appears at the lower side due to the surface tension. At a comparable beam intensity value, the oscillation of the “lens-like” melt at the lower side of the material is driven by the expansion of vapor capillary. This expansion occurs inside of the material and directly above the “lens-like” melt. The shape of the expanded vapor determines the volume as well as the geometry of the resulting melt volume. Furthermore, the transition from the heat conduction welding mode to the keyhole welding mode is investigated by defocusing the laser beam for the beam source with a 515 nm wavelength. At a given variation, a clear difference between either mode is observed with the x-ray phase contrast method.

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.

The laser impulse metal bonding (LIMBO) process opens a new possibility to join a thick interconnector on a thin metal layer which lays on a sensitive substrate such as epoxy resin (FR4) material. Since the FR4 tolerates only limited amount of the thermal load during the joining process, the LIMBO process applies a novel approach to separate the melting and joining phase. Hence, the thermal load on the underlying substrate during the joining phase is minimized and therefore allows a welding approach even for electric as well as electronic components. However, the substrate is thermally affected due to the heat conduction which is induced during the joining phase. In this paper, the thermal cycle induced to the sensitive substrate during the LIMBO process and the overlap welding process for enlarging the weld joint area will be investigated. To evaluate the maximal amount of thermal load on the sensitive substrate below a thin copper layer by given setting, the thermal destruction threshold of substrate is exceeded on purpose. For the understanding of the heat distribution during the joining stage, the experimental results are validated with simulation results.