Broadband Flash Lamps to Achieve Non-Equilibrium Soldering and Assembly Over Wide area Using Conventional Solder Alloys

The drive to enhance human interactivity and reduce the weight of electronic systems has led to the use of non-conventional substrates. As the substrates become thinner, more flexible, and economical, the thermal stability of the working substrate is significantly lowered. As such, the conventional modes of component attachment are no longer functional. To bridge this gap, anisotropic adhesives and tapes, as well as low-temperature solders and conductive epoxies, have been developed. However, in terms of electrical performance and mechanical robustness, conventional reflow soldering is still the benchmark. One of the ways to combine traditional soldering techniques with thermally sensitive substrates is laser soldering. However, technical challenges, combined with the high costs of lasers, continue to create barriers to broader adoption. This discussion focuses on photonic soldering, which uses high-intensity flash lamps to overcome the disadvantages of laser soldering; while still enabling soldering on a wide range of substrates. Similar to laser soldering, photonic soldering utilizes selective absorption of light to enable conventional solders to affix commercial packages on the underlying thermally unstable substrate. In this work, we present a comparative study to compare joints created through photonic soldering to joints created in conventional reflow ovens. The parameter space of the photonic soldering space is explored with respect to various soldering alloys and fluxes. Thermal profiles as a function of the photonic soldering parameters and their effect on attaching a variety of components are studied. Cross-sectional SEM and X-ray imaging of solder joints are used to compare the photonic soldering outcomes with the conventional reflow soldering results. Elemental analysis is performed to investigate the effect of photonic soldering parameters on intermetallic formation. Furthermore, photonic soldering as a way to attach conventional surface mount components to thermally unstable substrates is explored. An innovative application for this process lies in the field of wearables. Sensors and actuators attached to commonly used fabrics could enhance health and wellness monitoring as well as packaging and fashion. In this work, we showcase TPU as an inlay for wearable applications with a functional circuit built on top. Photonic soldering allows the utilization of these wearables to be stretched, folded, bent, and run through washing and drying cycles. Such performance criteria are not feasible with other attachment methods such as adhesive tapes.