| The drive toward higher density of input/output (I/O) in the electronic packaging industry has led to an explosive development of chip-on-board assembly technology, in which the solder bumps are required to connect the silicon die to the printed circuit board (PCB). The reliability of the solder joint, in particular at the interface of the solder/substrate, is of great concern in this new technology.; For more than 50 years, lead-bearing solders, primarily eutectic or near-eutectic Sn-Pb alloys, have been used almost exclusively throughout the electronics industry because of their perfect performance. However, such solders are now coming under scrutiny to comply with the legislation introduced in Europe and Japan, which will start on July 1, 2006.; As electronics manufacturers scramble to meet these tough restrictions, a lead-free, eutectic or near eutectic Sn-Ag-Cu solder alloy with a relatively low melting point (217°C), developed and patented by the U.S. Department of Energy's Ames Laboratory, is widely used in electronic packaging to replace lead-bearing solders. However, new research has shown that this alloy has a tendency to become brittle after repeated or prolonged heating cycles. Researchers from IBM and Intel have observed other defects when using this alloy. It was found that large Ag3Sn plates could subtend the entire cross section of the solder bumps and significantly influence the mechanical deformation behavior of the solder joints.; In this dissertation, research efforts have revealed a number of weaknesses for lead-free Sn3.5Ag solder, which is one of the lead-free solder candidates recommended by the NIST. These weaknesses are: (1) a large needle-like Ag3Sn phase growing from the interface between the solder and the substrate of copper, (2) brittle failure when subjected to tensile test, and (3) thermal-induced cracks occurring at the interface after high-temperature thermal aging.; To improve these weaknesses, a new type of composite solder, based on Sn3.5Ag alloy, reinforced by nanoparticles, has been developed in this dissertation. A number of tests, including slow cooling, rapid cooling, tensile test, and thermal aging performance, have been performed to evaluate the behavior of the reinforcement of nanoparticles in lead-free Sn3.5Ag solders. New findings in this dissertation may be summarized as two-folds:; Less than one percentage of copper nanoparticles, which produced in-situ Sn-Cu intermetallic compounds Cu6Sn5, can only refine the primary Sn-phase for Sn3.5Ag solder. They cannot effectively modify the needle-like Ag3Sn compounds. Rapid cooling, combined with the addition of copper nanoparticles, could produce a spherical morphology of Ag3 Sn compounds, and therefore strengthen the solders.; The addition of nickel nanoparticles, less than one percentage, could refine the primary Sn phase and simultaneously produce a spherical morphology of Ag3Sn phase. This composite solder has demonstrated a significant improvement in the mechanical properties without rapid cooling. The formation of Sn-Ag-Cu-Ni compounds at the interface of the solder/substrate, which have been found to suppress the formation of a large needle-like Ag3Sn phase, shows excellent resistance to thermal aging performance without causing brittle failure. |
