Fundamental investigation and development of lead-free nanosolders for nanowire assembly and nano-joining

Tin/lead (Sn/Pb) solders have long been used as interconnect materials in electronic and microelectronic industry. Due to environmental concerns and health and safety issues of lead (Pb), "lead-free" (Pb-free) solders are being implemented and becoming more widely used. Unfortunately, many Pb-free solders are not eutectic formulations, which normally require higher temperature processing, making it difficult to create reliable joints. Thus, there is a tremendous need to develop Pb-free solders with a higher quality than (or comparable quality to) the traditional Sn/Pb solders for electronics and other industries. On the other side, to the requirements for high-performance interconnects in advanced packaging, the development of nanotechnology has to meet the dramatic scaling down of the electronics packaging size from micron-scale to nanoscale. In this sense, solder-based connections have great potential and provide opportunities to form robust joints between individual nanocomponents and integrate them into a nanosystem with desired functionality and geometry for device applications.;This thesis work is devoted to investigating the chemical and physical properties of nanoscale solders ("nanosolders") for microelectronic/nanoelectronic assembly and packaging applications with an emphasis on fundamental studies of nanosolder synthesis, reflow processing and oxidation prevention, 1-dimensional diffusion mechanism and wetting on reactive substrate.;Tin-based single and multi-segment nanowire nanosolders were synthesized by a templated electrodeposition method. Tin (Sn) and tin/silver (Sn/Ag) alloy spheroids were formed on non-reactive substrate at an oxygen-free environment. Nanosoldering is shown to be useful for the construction of functional nanostructures from nanomaterials by utilizing conventional reflow process with proper selection and utilization of industrial relevant fluxes. An one-dimensional two-segment copper/tin (Cu/Sn) nanowire system was established to evaluate the confined diffusion between metallic nanowires under electron-beam irradiation. Such 1-D cylindrical nanowire structure provides the critical information to process the joining techniques in micro/nano electronics packaging to devices in such small scale. The interaction between lead-free tin-based nanosolders onto metallic nanowires and reactive substrates such as copper were studied. The melting and wetting behavior of nanosolders on a reactive surface provide primary model for nanoscale joining on a 2-dimensional planar structure. A magnetically aligned metallic nanowire network jointed by Sn nanosolders was built through a liquid phase solder reflow process. This assembly study demonstrated the feasibility of Sn-based lead-free nanosolders for nanoscale interconnect applications in microelectronic packaging.;This thesis provides fundamental information on nanosolder melting on reactive and nanoreactive substrates, oxidation effect on nanosolder melting, 1-dimensional diffusion, nanosolder based nanowire assembly and nanojoint formation. To the best of our knowledge, this is the first doctoral dissertation devoting to lead-free nanosolders and their application for nanowire assembly and interconnection. These results provide necessary knowledge for future research and applications of nanoelectronics assembly and interconnect formation.