Multiaxial Ratcheting Deformation And Low Cycle Fatigue Of Lead-free Solder Sn-3.5Ag

As environment pollution and health problem became more and more seriously concerned, the electronic packaging industry is facing the challenge of changeover to"green", and using the lead-free materials is the trend of the electronic packaging technology. This paper will study the mechanical properties of the lead-free solder Sn-3.5Ag. It will be helpful to improve the accuracy of fatigue life prediction and the reliability of electronic packaging.Uniaxial tensile and pure torsional tests, in addition with some uniaxial and multiaxial ratcheting tests are carried out on lead-free solder Sn-3.5Ag at room temperature. And both torsional fatigue tests and multiaxial ratcheting fatigue tests were conducted. The scanning electron microscope (SEM) was adopted to observe and analyze the morphology of the fracture surface. It is shown that even low second stress can cause high ratcheting strain of Sn-3.5Ag. The trend of shakedown doesn't take place when the ratcheting strain is large. By analyzing the test data, it is also concluded that there is no effect of loading paths on ratcheting strain, and the stress-strain relationship has strong strain rate-dependence. Along with the increase of axial stress and shear strain amplitude, the ratcheting strain increased while the fatigue life decreased. From the microscopical observation and morphology of the fracture surface analysis, it is found that Sn-3.5Ag is a shear mode failure and cyclic softening material. The initiation of fatigue cracks and the propagation of cracks inside the specimen occur intergranularly along Sn-dendrite boundaries. Near the broken surface, Sn-Ag eutectic phases fall to pieces under cyclic loading. Brittlness of Ag3Sn intermetallics is considered to be the cause of this small particle formation and cyclic soften.The Euler backward method is implemented to update the stress. The modified Armstrong-Frederick (A-F) rate-dependent constitutive model is adopted to predict the ratcheting under different shear strain rate and diverse loading paths, including axial/torsional, proportional loading, rhombic path and circle path. The results illustrate that when describing the multiaxial ratcheting the modified rate-dependent A-F model is a suitable choice.Since equivalent strain approach, energy approach and some critical plane approaches exclude the consideration of the ratcheting strain and mean stress, the methods for fatigue life prediction are improper for multiaxial fatigue with ratcheting strain. Coffin model, which combines ratcheting and cyclic strain, presentes the fatigue life predictions on non-conservative side if the ratcheting deformation is large. For this reason, this paper proposes the maximum shear strain range (Δγmax) and axial ratcheting strain rate (ε& r) as a new damage parameters. The new model could not only describe the fatigue life in torsion test, but also predict the multiaxial fatigue life of the lead-free solder with ratcheting. Moreover, the simple definition of parameter and explicit physical significance also make the model more applicable.