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DECEMBER 2021 I PCB007 MAGAZINE 31 thermal energy more effectively than other areas and result in a localized temperature increase. e temperature profile of the mate- rial being processed can be controlled by vary- ing the timing of the pulses (pulse length and delay between subsequent pulses). e tem- perature reached can be above the rated tem- perature of constituent parts of the device stack without damaging them, in part because the heating is noticeably short, and the device stack will revert to ambient conditions soon aer the light illumination has stopped. e soldering process (Figure 3) shows the ideal trade-off between light power density and flash durations. e soldering is accom- plished from one to four seconds depending on power density. Power settings from P1 to P9 will induce reflow from 4.5 to 8 seconds, with the shortest at P9 of 0.5 seconds. Spatial Selectivity e photonic soldering process is unique and provides a soldering process for a given mate- rial system (substrate, conducting track, sol- der, and component) that cannot be duplicated in a normal reflow oven. e average power is a function of the energy of a single light pulse Thermal Profile e photonic soldering process builds on the process of using high-intensity flashes of broad- spectrum light to heat up a multi-layered stack in a non-equilibrium process. is process was developed as part of enhancing the manufac- turability of flexible hybrid electronics (FHE) by sintering metal particle-based inks into con- ductive traces. e photonic soldering tools rely on extremely high average power delivery for a xenon gas-filled flash lamp. As such, flash lamps must be water-cooled to prevent run- away heating and detrimental damage to the system under high-duty use. Additionally, the flash lamp system needs to have digital controls to adjust for soldering of different-sized com- ponents under various thermal conditions. SAC-305 (Indium 8.9HF, type 4) solder paste was manually stencil-printed on the cop- per contact pads. Wet thickness of the applied solder paste was roughly 75 µm. Sulfur-toler- ant chip resistors in 0603 packages from Rohm Semiconductors (part number SFR03) were used as the main component (Figure 3). Areas which absorb parts of the spectrum more efficiently convert the light energy to Figure 3: Process development: Power density vs. time. (Source: NovaCentrix)