| Global pressure based on environmental and health concerns regarding the use of Pb-bearing solder has forced the electronics industry to develop Pb-free alternative solders. Eutectic Sn-Ag solder has received much attention as a potential Pb-free candidate to replace Sn-Pb solder. Since introduction of surface mount technology, packaging density increased and the electronic devices became smaller. As a result, solders in electronic modules are forced to function as a mechanical connection as well as electrical contact. Solders are also exposed to very harsh service conditions such as automotive under-the-hood and aerospace applications.; Solder joints experience thermomechanical fatigue, i.e. interaction of fatigue and creep, during thermal cycling due to temperature fluctuation in service conditions. Microstructural study on thermomechanical fatigue of the actual eutectic Sn-Ag and Sn-4Ag-0.5Cu solder joints was performed to better understand deformation and damage accumulation occurring during service. Incorporation of reinforcements has been pursued to improve the mechanical and particularly thermomechanical behavior of solders, and their service temperature capability. In-situ Sn-Ag composite solders were developed by incorporating Cu 6Sn5, Ni3Sn4, and FeSn2 particulate reinforcements in the eutectic Sn-Ag solder in an effort to enhance thermomechanical fatigue resistance. In-situ composite solders were investigated on the growth of interfacial intermetallic layer between solder and Cu substrate growth and creep properties.; Solder joints exhibited significant deformation and damage on free surface and interior regions during thermomechanical fatigue. Cracks initiated on the free surface of the solder joints and propagated toward interior regions near the substrate of the solder joint. Crack grew along Sn grain boundaries by grain boundary sliding. There was significant residual stress within the solder joint causing more damage. Presence of small amount of Cu and Pb phase resulting from Sn-Pb finish on electronic component did not affect thermomechanical fatigue deformation of the eutectic Sn-Ag solder joints.; In the aging study up to 5000 hours, solder matrix consisting of Sn phase and Ag3Sn particles showed extensive coarsening. Intermetallic particles of Cu6Sn5 Ni3Sn4, and FeSn2 particles in the eutectic Sn-Ag composite solder joints were effective in retarding the interfacial intermetallic layer growth at low temperature less than 150°C. Ni3Sn4 and FeSn2 particles were stable during aging, whereas Cu6Sn5 particles exhibited coarsening.; In-situ composite solder joints exhibited different creep behavior depending on the type of incorporated particle, compared to the eutectic Sn-Ag solder joints. Cu6Sn5 and Ni 3Sn4 reinforced composite solder joints showed weakening at room temperature creep, whereas composite solder joints reinforced with FeSn2 particles exhibited strengthening at the same temperature. However, the creep strength of composite solder joints at high temperature of 85 and 125°C was similar to that of eutectic Sn-Ag solder joints, indicating that there is no substantial effect of particle at high temperatures. Cyclic creep by loading/unloading was found to significantly increase creep strength of solder joints. |
