Study On Reliability Of SnAgCu Based Lead-free Soldered Joint And Related Theory

Increasing global concern about the environment is bringing regulatory (European directives) and consumer ("green products") pressure on the electronics industry in Europe and Japan to reduce or completely eliminate the use of lead (Pb) in products. Among all lead-free solder alloys, SnAgCu solder system, which has better thermo-mechanical properties compared with those of SnPb solder, is proven to be one of the promising candidates for electronic assembly. Many problems with SnAgCu solders still exist, such as the formation of large brittle intemetallic compounds and the short creep-assistance. Moreover, under the drives of increasingly finer pitch and severe service conditions, SnAgCu solders with better creep and thermal fatigue performances are needed sometimes. Rare earth (RE) elements have been called the"vitamin"of metals, therefore, adding minute amount of RE to SnAgCu alloys is considered to be an effective way to improve their soldered joints reliability. In this paper, under the temperature loadings, the properties and microstructures evolution of SnAgCu/SnAgCuCe were investigated, the constitutive models, fatigue life of soldered joints and failure mechanism were studied systematically.For quad flat packages (QFP256), mechanical properties of SnAgCu/SnAgCuCe soldered joints were analyzed and compared with SnPb soldered joints, it is found that the mechanical properties of SnAgCu solder joints are enhanced with the addition of Ce, and the tensile forces of SnAgCuCe soldered joints are 12.7% higher than that of SnAgCu soldered joints. With the addition of rare earth Ce, the microstructure of SnAgCu can be refined, and the size of intermetallic compound (IMC) particles (Cu₆Sn₅ and Ag₃Sn) can be reduced, which are the reasons for the mechanical properties enhancement of SnAgCu soldered joints. In addition, during the solidification of SnAgCuCe solders, the (IMC) particles (Cu₆Sn₅, Ag₃Sn and CeSn₃) can precipitate with small sizes, therefore, these precipitated particles will undoubtedly strengthen the mechanical properties of SnAgCuCe soldered joints. Under the same temperature loading the growth rate of SnAgCuCe/Cu interface layer is lower than that of SnAgCu/Cu interface layer. The growth dynamics of interface layer was investigated, it is found that the ripening flux of SnAgCu/Cu is the 2.1 times more than that of SnAgCuCe/Cu, the results indicate that the coarsening of SnAgCu/Cu interface is stronger than that of SnAgCuCe/Cu interface to some extent. Comparison with the results during aging, during thermal cyclic loading, because of large difference in the coefficient of thermal expansion (CTE) of the different constituents in the packaged assembly, the shear stresses and strains vary with temperature concentrating in the solder joints, the shear increases the crystal lattice disfigurement to promote Cu atom diffusion and the growth of IMC layer, therefore, the growth rate of IMC layer at both interface during thermal cyclic loading is higher than that during aging loading. Moreover, under both temperature loadings, it is found that the addition of rare earth Ce can reduce the thickness of IMC layer (total thickness, Cu₆Sn₅ and Cu3Sn).Uniaxial tension was utilized to investigate the unified viscoplastic constitutive model of SnAgCu/SnAgCuCe solders. The Anand parameters of the constitutive equations for SnAgCu and SnAgCuCe solder were determined from separated constitutive relations and experimental results. Nonlinear least-squares fitting were selected to determine the model constants. And comparisons were then made with experimental measurements of the stress-inelastic strain curves: excellent agreement was found. It is found that the Anand constitutive equations are suitable to represent the stress-strain response of SnAgCu/SnAgCuCe soldered joints. Based on the Anand model, the investigations of thermo-mechanical of SnAgCu and SnAgCuCe soldered joints in fine pitch quad flat package by finite element code have been done under thermal cyclic loading, it is found that the reliability of SnAgCuCe soldered joints is better than that of SnAgCu soldered joints. Moreover, the effect of height of soldered joints on the reliability of soldered joints was studied, it is found that the height of soldered joints has a optimum value. Based on the experiments, it is found that fracture sites are near interfaces, and many small second phase particles can be observed in the fracture microstructure, when the height is short. However, when the height is high, the fracture occurs in the soldered joints, no second phase particles are found in the fracture microstructure due to the fracture inβ-Sn.The Dorn and Garofalo-Arrhhenius creep models of SnAgCu/SnAgCuCe solders were researched, respectively. The results show that the creep activation energy of SnAgCuCe solder is higher than that of SnAgCu solder, so the creep-resistance of SnAgCuCe alloy is higher than that of SnAgCu solder. At higher stresses, it is believed that dislocation creep is the dominate creep mechanism in solders, involving the dislocation climb and moving away from barriers. At lower stresses, the lattice diffusion is believed to be the dominate mechanism for solder alloys, involving the diffusion and migration of interstitial atoms and lattice vacancies along the grain boundaries due to the applied tension stresses. Grain boundary sliding can be accompanied with the above mechanisms at any stress level. Furthermore, the dispersed nano-Ag₃Sn in solder matrix can enhance the creep properties of SnAgCuCe solder, and the CeSn₃ particles can be found in the grain boundaries.Based on tensile testing, it is found that the fatigue lives of SnAgCu/SnAgCuCe soldered joints are 1158 and 1304.6 respectively. Without thermal cycling tests, fracture morphology of soldered joints exhibits the characteristic of toughness fracture, and the second phase particles distribute in the fracture microstructure uniformly. Fracture morphology of soldered joints subjected to 1500 thermal cycles indicates the brittle intergranular fracture on the fracture surface, and no intense plastic deformation appears before fracture. Based on Weibull analysis, it is found that the fatigue lives of SnAgCu/SnAgCuCe are 1150 and 1290, respectively, the results coincide well with the data tested by tensile experiments. From these investigations, it is seen that the fatigue life of SnAgCu soldered joints can be increased up to 112.17% based on Weibull analysis, 112.66% based on tensile testing. With the fatigue data and the results calculated based on three constitutive models, the parameters of fatigue life equation can be obtained, it is computed that the parameters of fatigue life equation based on plastic strain are 0.514 (SnAgCu: 2ε′f ), 0.486(SnAgCuCe: 2ε′f ) and -0.5708(SnAgCu/SnAgCuCe: c ). Moreover, the parameters C′calculated based on creep strain are 0.241(SnAgCu: Garofalo-Arrhenius), 0.256(SnAgCuCe: Garofalo-Arrhenius), 0.261(SnAgCu: Dorn) and 0.267(SnAgCuCe: Dorn), the parameters W′computed based on creep strain density are 0.00391(SnAgCu: Garofalo-Arrhenius), 0.00723(SnAgCuCe: Garofalo-Arrhenius), 0.00387(SnAgCu: Dorn) and 0.00697(SnAgCuCe: Dorn).By thermal cycling experiments, in the QFP device, it is found that crack appears in the SnAgCuCe soldered joints, and the relationship can be found between the bulk Cu₆Sn₅ IMCs and crack appears in the heel of SnAgCuCe soldered joints. Moreover, due to the different distribution of Cu₆Sn₅ IMCs, the crack propagates through the soldered joints in linear, curve-like and mesh ways. And the creep strain response of fine pitch QFP device lead-free soldered joints were analyzed using finite element method based on Garofalo-Arrhenius model. The simulated results indicate that the creep distribution is not uniform, the heel of SnAgCuCe soldered joints is the maximum creep strain concentrated sites, it is found that the shape of creep curves change like a ladder-shape with time goes by. With the further increasing of temperature loading, the crack propagates along the Cu₆Sn₅/Cu3Sn interface, meanwhile, bulk Ag₃Sn can be found near the interface, it is evident that the growth of Ag₃Sn phase attaches to the Cu₆Sn₅ phase. Furthermore, nanoindentation is utilized for probing the mechanical behavior (Young's modulus and hardness) of Cu₆Sn₅, Cu3Sn, Sn Ag₃Sn andβ-Sn at small length scales, The indentation in Cu₆Sn₅ IMCs, shows cracks at the indentation corners, presumably due to its relatively poor toughness. And the value of Cu₆Sn₅ hardness is largest of all, the hardness of Cu₆Sn₅ and Cu3Sn is much higher than that of Ag₃Sn andβ-Sn。During thermal cyclic loading, the size of bulk Cu₆Sn₅ increases with the development of cycles, it is found that Cu₆Sn₅ particles distribute regularly in the solder matrix. By tensile testing, the linear relationship between increase amplitude of tensile strength and square root of increase amplitude particles diameter can be found:ΔP = ?3 0.285Δd12+ 10.15.Moreover, different morphologies (rod, lump, dot, fork, etc.) of Cu₆Sn₅ IMCs were enumerated and described. And the thermal cycling has a trifling effect on the size of Ag₃Sn, the small Cu₆Sn₅ particles disappear during temperature cycling, the size of large Cu₆Sn₅ particles increases with the advance of cycles. In addition, the kinetics exponent of Cu₆Sn₅ particles growth is determined to be 3.2, it indicates volume diffusion as the controlling mechanism of Cu₆Sn₅ growth. Furthermore, based on finite element simulation, it is found that the von Mises stress concentrate around the bulk Cu₆Sn₅ IMCs inside the SnAgCuCe soldered joints during thermal cyclic loading. From the stress distribution, the failure site was predicted to fracture near the bulk Cu₆Sn₅/Sn interface. Particles aggregation region is the concentrated stress area, the stress caused by CTE mismatch will arise at the interface between Cu₆Sn₅ and Sn matrix.