Effects of lanthanum doping on the microstructure and mechanical behavior of a tin-silver alloy.

摘要

SnAg and SnAgCu lead-free solders are used extensively as replacements for SnPb solders in microelectronics packaging. Extensive studies have been conducted to understand the electrical and thermomechanical behaviors of these Sn-based lead-free solders, and they have been found to have issues such as formation of intermetallic compounds (IMC) and poor wetting abilities. Work is therefore needed to modify the current lead-free compositions to further improve their performance. In this work, the effects of doping with rare earth elements (REs) on SnAg alloys are investigated. REs are well recognized as surface-active agents and previous researchers have found that RE doping can dramatically increase the wetting property of solder. It can reduce IMCs and their growth on solder/pad interfaces and also refine the microstructure of the alloy which results in improved mechanical properties of the solder.; This work systematically studied Lanthanum (La) doping on the microstructure and mechanical behavior of 96.5Sn3.5Ag wt% alloy. Quantitative relations were established not only for initial as-cast conditions, but also for thermal aged conditions.; A quantitative microstructure study has been performed on multiple scales. It was found that doping greatly reduces the grain size as well as the size of the intermetallic particles Ag3Sn. However, the inter-particle spacing remains relatively unaffected by the La doping amount and because of stoichiometry, decreasing Ag3Sn particles size increases the total number of Ag3Sn particles. A higher La doping level, therefore, leads to higher volume fraction of the eutectic region and lower volume fraction of the Sri dendrite phase in the solder alloy.; Creep tests at various temperatures and strain rates were conducted. A strain rate jump test was performed on the specimen of all conditions to cover the strain rate from 2x10-6/s to 2x10 -2/s range and these tests were performed in a constant temperature environment from -55°C to 125°C. The results show that La doping increases creep resistance of the SnAg alloy by approximate 15%.; The creep test result can be fit into a modified microstructure dependent Anand model, which is based on a uniform plastic-creep constitutive model with the modification of a particle-size dependent term. A one step data fitting method was developed to facilitate constant extraction from the complex nonlinear constitutive model.; A new constitutive law was also proposed to account for the hierarchal microstructure over multiple length scales. Specifically, at the smaller scale of the sub-micrometer level, the SnAg eutectic region was treated as a two-phase composite with the Ag3Sn being the particle and Sri as the matrix. At the larger length scale of the micrometer level, the solder alloy was treated as a two-phase composite with the Sri dendrite as the particle and the SnAg eutectic region as the matrix.; At the small scale, a micromechanics composite material model was used to calculate the creep properties of the eutectic region and the results are used as the phase properties for large scale. Several methods were used in this scale including micromechanics composite material model and a FEM based digital image (DIB) model and these models found that although the material has local anisotropic pattern, the overall behavior is close to isotropic. A good match was found between the models and tensile creep test result.; Finally, a fatigue test was performed on bulk samples. It was found that La doping increases the fatigue life of SnAn alloy by a factor of about 5. A 3D FEM model was established and validated with the fatigue test specimen. A fatigue model was established by combining the fatigue life from test, with the local deformation obtained from 3D FEM model.