Investigation On The Microstructure And Properties Of Sn-Bi Based Solder Alloys

Because of the environmental and health issues, legislation restricting the use of lead products had been passed in many countries. Many researches have been focused on potential lead-free solder, including Sn-Ag, Sn-Zn, Sn-Cu, and Sn-Bi alloys in the field of electronic packaging. Among them, Sn-Bi based solder has received much attention as an important alternative for its lower melting temperature, good wettability and well tensile strength. But the frangibility and reliability are still the major problems restraining the application of Sn-Bi solder alloys in industry. Therefore, the development of Sn-Bi alloy with excellent performance has become an urgent need in the field of lead-free solder.Sn-Bi based alloys were studied by optical metallographic analysis (OM), X-ray analysis (XRD),scanning electron microscope (SEM), X-ray energy spectrum analysis (EDX), transmission electron microscopy (TEM), etc. The effects of Bi content, strain rate and temperature on the deformation characteristics of Sn-Bi binary alloys were systematically researched. In situ observation method was employed to investigate the fracture mechanism of Sn-Bi alloys. Combined with the nanoindentation experimental method, the deformation mechanism of the alloy was discussed. Based on this, the effects of alloy elements, strain rates and temperatures on the mechanical properties of Sn-Bi based alloys were studied by the technology of Ag and In micro-alloying. The effects of elements on the melting point,melting range and wettability were analyzed. Moreover, the growth dynamics of the IMC layer and its influence on mechanical properties of the joints during aging process were also investigated in this paper.The microstructure of Sn-Bi eutectic alloy is consists of Sn-rich phase and Bi-rich phase continuous network distributing in the matrix. With the decreasing Bi content, the ?-Sn primary phase formed and expanded leading to the isolation of the eutectic structure. As to Sn-6Bi alloy, small Bi particles precipitated near the grain boundary of P-Sn solid solution. The tensile strength and elongation increases first and then decreases with the Bi content decreases, and the maximum elongation is obtained in the Sn-42Bi alloy. The mechanical properties under different strain rates and temperatures show that, the tensile strength of all alloys showed a growth trend with the increase of strain rate, and the growth range enlarges with the increase of Bi content. High and medium bismuth content alloys have the highest sensitivity to the strain rate and temperature. The elongation increases with the decrease of strain rate or the increase of temperature. While the low bismuth alloy have less sensitivity to the strain rate.The stress exponent n of Sn-58Bi alloy is 14.7 in the high strain rate range (?>0.002s-1) indicating that the deformation is controlled by dislocation glide mechanism. The dislocation tangles and the stress concentrates,leading to the significant increase of strength and the crack through matrix followed by rapid failure. Under low strain rate (?<0.002s-1), n is 3.26 indicating that the deformation is controlled by the phase boundary sliding and dislocation climb mechanisms. Fracture observation shows that the mode of high bismuth alloy is attributed to cleavage fracture. The initial cracks form in the ?-Sn phases and expand along the grain boundary in the middle bismuth alloy. Low bismuth alloy exhibits intergranular fracture with initial crack forming and extending in the grain boundary.The microstructure analysis shows that adding small amount of Ag attributes to the formation of fine?-Ag3Sn improving the ductility of the alloys. Under the condition of high strain rate, adding Ag element can remarkablely improve the plasticity and the elongation increases by about 67%. But the improvement on ductility is not obvious under higher temperature. With excess Ag content, ?-Ag3Sn primary phases formed. The coarsening and agglomeration of there phases caused the reduction of elongation. DSC analysis shows that, the Ag element has definite influence on the melting point, melting range and spreading rate, but no significant difference.The decrease of Bi solid solubility in the Sn matrix caused by the In solid solution contributes to the improvement of the plasticity of the Sn matrix. When the In content reaches saturation, BiIn particles precipitate in the Sn matrix leading to the decrease of plasticity. The maximum elongation is obtained adding 2.5% In, showing an increase of 121%. In element has a significant effect on improving the ductility at lower strain rate, while the improvement is no longer obvious under higher strain rate and higher temperature. In element has a significant influence on the phase transition temperature. The addition of 1%In leads to about 5? drop of the initial melting temperature. Wetting results show that, the spreading rate has a gradually decline increasing the In content and rises again when the In content is more than 4%.The interfacial reaction layer of the Sn-Bi/Cu and Sn-Bi-Ag/Cu joints is Cu6Sn5 compounds. The interface reaction layer of the Sn-Bi-In/Cu joint is Cu6(Sn,In)5 compounds. In the process of isothermal aging, a small amount of Ag addition suppresses the coarsening of Bi phase. Aging at 80?, there is a certain influence of restraining the growth of IMC adding 0.4% Ag and 2.5% In. When increasing the aging temperature, Ag and In elements all promote the growth of the IMC layer. The results of calculated growth factor n suggest that the growth of interface reaction layer of all joints is controlled by volume diffusion.The calculated activation energy Q of five kinds of joints reveals that Ag has little effect on the activation energy. The activation energy of 42Sn-Bi-2.5In/Cu joint is the highest and the IMC diffusion coefficient is not sensitive to the temperature. Due to a higher diffusion constant, the growth rate of interface reaction layer becomes faster aging at higher temperature.Addition of Ag element improves the strength of joint before and after aging. Addition of In element reduces the shear strength, but the shear strength declines more slowly after higher temperature aging. In the process of aging, the fracture position of 42Sn-Bi/Cu and 42Sn-Bi-0.4Ag/Cu joints shifts from alloy to the IMC showing lower plasticity. Because the strength of Cu6(Sn,In)5 layer increases, fracture position of 42Sn-Bi-2.5In/Cu joint gradually shifts from IMC to alloy and the plasticity of the joints improves.