Effect of silver and copper on tin-silver-copper interconnects

Environmental concerns over the toxicity of lead-bearing solders have stimulated research activities in seeking lead-free (LF) alloys. Sn-x%Ag-y%Cu alloys have become the most promising LF candidates.; Our understanding about Sn-x%Ag-y%Cu interconnects is limited compared to lead-based interconnects. For example, even the discovery of Sn-x%Ag-y%Cu comes with a group of alloys, discernible by x and y.; The objective of this research was to understand the effects of Ag and Cu content on Sn-x%Ag-y%Cu interconnects, especially to understand how Ag and Cu apply their effects. The research was conducted from aspects of alloy thermal characteristic behavior, alloy phase formation, solder-substrate interactions, interconnect microstructural development and their impact on the interconnect mechanical performance and reliability.; During the research, surface microetching microscopy (SMM), a brand-new three-dimensional microanalysis technique, was developed to closely examine the interfacial microstructures.; The research indicated:; All Sn-x%Ag-Cu alloys possess a similar melting point about 217.4°C. The latent heat increases as Ag content increases. The alloy undercooling behaves similar to a quasi-parabolic function of Ag content. A correlation between alloy undercooling and Ag3Sn plate occurrence exists. Ag 3Sn plates can grow only if the undercooling is large. Ag3Sn plates can grow with Ag content as low as 1.2 wt.%. This content is much lower than the reported threshold value (3.13 wt.%).; Sn-x%Ag-y%Cu interconnects can have different interfacial microstructures, depending on Ag and Cu contents. High Ag and/or high Cu content make the interconnects brittle through interfacial microstructural development.; With assistance of SMM, the mechanisms via which Ag and Cu apply their influences were revealed: There is a coupled-growth between Ag3Sn plates and Cu6Sn5 IMC. Ag applies its influence via creating Ag3Sn plates which are brittle due to their incompatible geometry to that of surrounding solder and promoting Cu6Sn 5 IMC growth; Cu implements its influence through establishing Cu 6Sn5 IMCs which are intrinsically brittle and promoting Ag 3Sn plate growth. Such mutually-promoted growth of intermetalic Ag 3Sn and Cu6Sn5 phases and their brittleness can degrade the interconnect macro performance and reliability.