Process and mechanism of depositing silver paste spot via laser-induced forward transfer

Laser-induced forward transfer (LIFT) can realize the laser transfer of silver paste, copper foil, aluminum, and other metal pastes as well as the interconnection of metal structures, which has wide application prospects in the microelectronics industry such as surface coating and circuit printing. In this work, compared with ultrafast laser, a cheaper YAG nanosecond laser was used to transfer silver paste and the effects of process parameters such as laser energy, paste viscosity, gap height and donor thickness, as well as the surface morphology of deposition points on morphology were studied. The feasibility of laser transfer printing from low-viscosity to high-viscosity paste has been proven, and it was also revealed that the deposition threshold of high-viscosity paste varies with the process parameters. In addition, two different voxel deposition mechanisms of the LIFT variable-viscosity silver paste were summarized and analyzed. First, the cavitation bubble pushes the complete jet to form a threshold deposition or no deposition without contacting the substrate. Second, under the instantaneous action of the laser, the cavitation effect is too large to form a jet completely; however, a hollow liquid column is formed and directly contacts the receiving substrate.