Photonic Flash Soldering of Thin Chips and SMD Components on Foils for Flexible Electronics

Ultrathin bare die chips and small-size surface mount device components were successfully soldered using a novel roll-to-roll compatible soldering technology. A high-power xenon light flash was used to successfully solder the components to copper tracks on polyimide (PI) and polyethylene terephthalate (PET) flex foils by using a lead-free solder paste. Results are compared with oven-reflowed solder joints on PI substrates. The delicate PET foil substrates were not damaged owing to the selectivity of light absorption, leading to a limited temperature increase in the PET foil while the chip and copper tracks were heated to a temperature high enough to initiate soldering. The microstructure of the soldered joints was investigated and found to be dependent on the photonic flash intensity. Reliability of the photonically soldered joints during damp heat testing and dynamic flexing testing was comparable with the reflowed benchmark and showed increased reliability compared with anisotropic conductive adhesives bonded on PET foils.