| 其他摘要 | Low cycle fatigue behavior of the Sn-3.8Ag-0.7Cu lead-free solder was investigated as a function of plastic strain amplitude, strain rate, temperature and microstructure. Following the mechanical fatigue studies, effects of current stressing on microstructure and mechanical properties were explored in the Sn-3.8Ag-0.7Cu/Cu solder joints.
At the strain rates of 8×10-3/s~10-4/s, the Sn-3.8Ag-0.7Cu solder with an equiaxed structure exhibited distinct cycle-dependent softening over the total strain amplitude range of 0.2～1.2％, which started from the beginning of fatigue cycling. Numerous microcracks along grain boundaries were observed in the microstructure and number of the intergranular microcracks increased with continued cycling. By combining percolation theory with microcracking analysis, the cycle-dependent softening behavior was shown to result from accumulation of microcrack density with fatigue cycles.
The cycle-dependent softening of the equiaxed Sn-3.8Ag-0.7Cu solder was accelerated at higher temperatures. In addition to the intergranular microcracks, microcracking along the phase interface and the recrystallization also contributed to cycle-dependent softening of the solder, especially at temperatures higher than 90℃. While the fatigue life of the solder was insensitive to strain rates or frequencies, increasing temperature resulted in shorter fatigue lives.
Different from the equiaxed microstructure, the dendritic microstructure of the Sn-3.8Ag-0.7Cu solder exhibited a stable cyclic response, namely, reaching a stable maximum stress at a constant strain amplitude. The lower the strain amplitude was, the longer the stable stress stage. After the stable stage, cyclic softening occurred due to formation of the fatigue cracks, followed by the rapid propagation of the main fatigue crack.
At the same strain amplitude, the fatigue life of the dendritic Sn-3.8Ag-0.7Cu solder was longer than that of the Sn-3.8Ag-0.7Cu solder with an equiaxed structure. However, the fatigue resistance of the Sn-3.8Ag-0.7Cu alloy in both microstructures was higher than that of the Sn-Pb eutectic solder. The differences in the fatigue resistance resulted from the different cyclic deformation and fatigue damage mechanisms in the various solders. The deformation and damage mechanisms of the dendritic Sn-3.8Ag-0.7Cu solder consisted of shear bands along the maximum shear stress direction and microcracking along such shear bands, while the grain boundary sliding and intergranular microcracking constituted the dominant deformation and damage mechanism for the equiaxed Sn-3.8Ag-0.7Cu solder and Sn-Pb eutectic solder. Because of the intergranular fatigue damage, microcracks tend to develop at the fine grains of Sn and Pb in the Sn-Pb eutectic solder along grain and phase boundaries, resulting in low fatigue resistance of the Sn-Pb eutectic solder.
Under current stressing, the IMC at the anode interface between solder and Cu substrate was much thicker than that at the cathode interface in the Sn-Ag-Cu/Cu solder joint, creating a polarity effect. At the same time, segregation of Cu was found on the surface of the joint to form large Cu6Sn5 phase on the surface as copper migrated from the interior of the solder and from the dissolution of Cu6Sn5 IMC at the cathode interface of the solder joint to the surface.
The tensile strength of the solder joint after current stressing decreased markedly. Under the high current density, fatigue life was shortened greatly and the fraction of brittle fracture was increased with increasing time of current stressing. Following current stressing, fracture occurred at the cathode interface of the solder joint as voids accumulated at the cathode interface, in agreement with the decrease of tensile strength and the fatigue life of the solder joint after current stressing. |
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