Drop-on-Demand Laser Sintering with Silver Nanoparticles for Electronics Packaging

This paper proposes a dry laser-sintering method and discusses characteristics of a laser-sintered silver thin film on a polyimide or a copper substrate. This novel technology consists of the following processes: first, ink-jet printing of metal nanoparticles with dispersants and solvents for minute patterning; second, short preheating to remove organic substances in the ink; and finally, millisecond-order laser-beam irradiation under atmospheric conditions with the flow of argon gas for metallization. Regarding the wiring, visible lasers with high absorption on the ink develop rapid metallization and activate solvent evaporation, resulting in a rough surface with large pores. Interface adhesion is increased by the anchoring effect in the course of laser irradiation. In contrast, near-infrared lasers with low absorption heat the ink from the polyimide interface, yielding a dense, low-specific-resistance structure. Regarding pad formation on the copper leadframe without any surface pre-treatments, interdiffusion takes place at the Ag/Cu interface and increases adhesivity. The structural quality of the laser-sintered silver pad is almost the same as that of an electroplated one, so that no difference in good wire-bondability is obtained when the near-infrared continuous-wave laser is irradiated for a short time of a millisecond order per lead.