Evaluation of silver and copper sintering of first level interconnects for high power LEDs

Silver sintering, well known in power electronic applications, is migrating into optoelectronic assembly, to replace other interconnect materials like the eutectic Au80Sn20 or SnAgCu solder. Sintering offers an interconnect that can be formed at low temperature while at the same time can operate at high temperature. The goal of the research of this paper is to develop a sintered interconnect which can replace traditional AuSn or SnAgCu (SAC) solder, offering low thermal resistance, sufficient shear strength and thermo-mechanical fatigue resistance. Silver offers excellent thermal properties and can be an effective replacement in case of low mechanical stress applications. But copper is in case of applications with high thermo-mechanical stress the material of choice due to its higher yield strength and in general due to lower material cost. Process conditions in case of silver sintering under pressure and pressureless silver sintering have been established. A stable interconnect matching the reference SAC305 solder in terms of mechanical and thermal performance has been realized returning shear strength values averaging 59MPa for silver sintering under pressure and 42MPa for pressureless sintering as against the reference SAC305 solder averaging 56MPa. The thermal performance, measured by transient thermal analysis (TTA) reveal the lower thermal resistance of 0,8K/W and 0,5K/W for silver sintering under pressure and pressureless silver sintering respectively, as against the reference SAC305 solder as expected based on the thermal conductivity of the material. Particles size, binding material, bonding force and bonding atmosphere are shown to have a major impact on the quality of the silver sintered interconnect. Pastes consisting sub-micron and nanoscale silver particles provide less porous interconnect compared to paste containing solely micron sized particles in case of pressureless sintering. A major challenge with regards to copper nano-powder based sintering is to ensure sufficient penetration of the reducing gas during the sintering process. This is observed also in terms of the mechanical (4MPa) as well as the thermal performance. The paste used in the experiments has not sufficient reducing agents. Based on the results strategies to improve the activation need to be developed for the binder chemistry.