| At present, the ball grid array (BGA) packages are widely applied to the field of microelectronic packaging, so the surface mounting technique (SMT) gradually enters into the epoch of assemblies for BGA. The failure of solder joints is the main reason for destruction of BGA devices. Along with enhancing of people's consciousness for environmental protection, using lead-free solder will be the development tendency of the microelectronic industry. Recently, the study for reliability of SMT lead-free solder joints at work has attracted great attention because of these reasons. The study for reliability of lead-free solder joints has been becoming an utmost concern by researchers, lead-free solder SnAgCu is most likely regarded as a substitute for traditional SnPb solder as the material of SMT solder joints. However, SnAgCu solder with high content of Sn is bound to cause rapidly growth of IMC between solder and Cu pads, larger thickness of IMC will lead to weakening interface of solder joint and cracking in the IMC layer. Thus, the study for reliabilities of lead-free solder joints of BGA has theoretical significance.In this paper, Anand constitutive equation was employed to depict visco-plastic behavior of Sn3.8Ag0.7Cu and Sn37Pb under the temperature cycles. The method of FEM was used, and the octant finite element three dimensional model was constructed. The regulation of distribution for solder joints was analyzed to locate the key joint of solder joints. The key solder joint, where the failure of fatigue was prone to occur, was located in the solder joints array, and the failure mechanism of key joint of Sn3.8Ag0.7Cu was analyzed. Thermal fatigue life of the key solder joints of two types of materials was predicted with Coffin-Manson model based on plastic strain and Morrow model based on plastic energy, respectively. It was revealed that the thermal fatigue life of the key solder joint for Sn3.8Ag0.7Cu was about 2 to 3 times more than that of Sn37Pb, and that Sn3.8Ag0.7Cu showed better property of thermal fatigue resistance.The experiments on soldering reaction and aging were conducted respectively at the interface between Sn3.8Ag0.7Cu solder and Cu pads. It was concluded that the composition of IMC layer was Cu6Sn5 after soldering and a thin layer of Cu3Sn was found near Cu pad after aging at 125℃for 100h. The results of measuring thickness of IMC indicated that the growth of IMC layer displayed a nearly linear relationship to Square root of aging time, which virtually conformed to the Fick's diffusion law.The model of the key solder joint was simulated after rational simplification for IMC layers. The results of numerical simulation indicated that the equivalent stress of inner layer of IMC increased with its thickness. On the contrary, the stress of solder had no conspicuous change, but the quantity of visco-plastic strain and accumulated plastic work increased dynamically with increasing of IMC layer. The fatigue life of solder joint decreased substantially with the increasing of thickness of IMC layer.
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