The eutectic Sn-Ag-Cu ( SAC) alloy has become the most prevalent lead-free solder after a decade of intense research owing to it’s excellent mechanical and physical properties. At the same time, the density for electronic package assembly, such as BGA (Ball Grid Array) structure in Flip Chip, is getting higher and higher to achieve multifunctions and miniaturization of electronic products. In the BGA solder structure, the grain number in one single joint is becoming smaller, even down to several grains. Therefore, mechanical behavior of the solder directly determines the reliability of joint. Under the advanced package techniques it is meaningful to improve the reliabilities of package assemblies. This study has investigated the following aspects of SAC solder reliability: 1) rate dependent behavior of the joint; 2) current induced weakening in the joint; (3) microstructural coarsening induced by electric current; (4) the anisotropic behavior of the joint.
Because the room temperature is higher than 0.6Tm (melt point) of the SAC solder alloy. It’s mechanical property was highly rate dependent. The rate dependence was examined by conducting single lap joint tests on Sn3.8Ag0.7Cu (SAC)/Cu solder joints. It was found that strain rate had a strong influence on the shear strength and fracture mode. With increasing strain rates, the failure mode transited from cohesive manner to solder/IMC interface. The reason for the fracture mode transition was explained by a strain-rate hardening effect. In finite element analysis, the maximum equivalent plastic zone was shifted to the interface, which rationalized the experimentally observed failure path.
Coarsening of the microstructure in SAC solder joints under current stressing was observed experimentally and modeled by a dislocation-creep model which incorporates the coarsening of second phase particles in lead-free solder alloys. Both the effects of electric current and strain-enhanced coarsening were considered in the model. The straining effect took into account of both the inelastic-strain history and hydrostatic constraint. The model described well the evolution of the eutectic microstructure and the predictions of the model agreed reasonably well with experimentally observed trends.
Due to miniaturization of solder joints in flip chip assembly, a micro-joint of SAC solder may contain very few grains. In such a case, the traditional approach based on the isotropic constitutive description for bulk materials is inadequate in describing the mechanical behavior of SAC solder joint. An anisotropic crystal plasticity model was adopted for the micro-joint. The analysis found: 1) stress strain distributions were highly non-uniform in the beta-tin grain aggregate; 2) the shape of solder joint was still an important factor which determines the distribution of stresses in a micro-joint in cases of the multi-crystal; 3) the stress strain state was sensitive to grain orientation
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