| Solder has given the designer of modern electronic systems remarkable flexibility for interconnecting electronic components, and has received explosive attention as the result of rapid development of surface mount technology. Lead-bearing solder has a history going back several thousand years, but concern over the toxicity of lead has led to the introduction of Pb-free solders in microelectronic devices. The most widely adopted of these are high-Sn alloys.; In this study, creep mechanisms of pure Sn solder joints were first investigated at different stresses and temperatures to determine the stress exponents and activation energies. The effects of substrate metallization on creep properties of solder joints were also systematically investigated. The increase in creep resistance when Cu is replaced by Ni is attributed to a decrease in grain boundary mobility due to the introduction of a dense array of Ni3Sn 4 intermetallic compounds (IMCs) along the grain boundaries. Finally, the growth kinetics of abnormal IMCs on the nickel side were studied for the Ni|Sn|Cu joints with respect to reflow time.; Based on an understanding of the deformation mechanism, we studied how low stress exponents in creep properties can contribute to fast stress relaxation and high fracture resistance. In this study, Transformation Induced Plasticity (TRIP) was explored as a potentially effective way to control the stress exponent.; To do this, we first explored the possibility of TRIP plasticity in Sn alloys with low indium contents. The morphology and variant relationship of the martensite formed after quenching in liquid nitrogen was examined. Second, we determined whether TRIP exists in low-indium Sn alloys by simple shear tests with respect to temperature. The strain to fracture had a clear maximum as a function of temperature, and the platelike structures commonly seen during martensitic transformations were observed only in samples showing abnormally high strain to fracture. We discuss how TRIP effects contribute to the stress exponent in creep and how the stress exponent is related to fast stress relaxation and high fracture resistance. Finally, we investigated ways in which the substrate metallization (Cu and Ni) and aging process affected creep properties of the low-indium solder joints. |
