Torsion Fatigue And Multiaxial Ratcheting Fatigue Of Lead-free Solder

As the lead solder will affect the environment and people's health seriously, so the lead-free solders are widely used in the electronic packaging industry. Two types of lead-free solders, Sn-0.7Cu and Sn-3Ag-0.5Cu, which are used mostly and has best future, are adopted in our experimental tests. Some uniaxial and multiaxial tests are carried out on these two lead-free solders to get mechanical properties. All the tests were carried out with a minitype material tester developed by CARE Lab at room temperature.Uniaxial tensile and pure torsional tests under different strain rates were carried out on Sn-3Ag-0.5Cu. The results have shown that the stress-strain relationships, yield stress, shear yield stress, tensile strength, shear strength of the solders have strong strain rate-dependence. But the elastic modulus and shear modulus have not apparent strain rate-dependence. Multiaxial ratcheting tests were carried out on Sn-3Ag-0.5Cu. It is shown that the ratcheting strain rate increased with shear stain amplitude and axial stress, while the life decreased. The shear strain amplitude had no effect on the ratcheting strain when the solder failed. Pure torsional fatigue tests were conducted on Sn-0.7Cu and Sn-3Ag-0.5Cu. It was discovered that the load drop had great influence on plastic strain vs fatigue life, but had no influence on elastic strain vs fatigue life. Sn-0.7Cu had shorter fatigue life than that of Sn-3Ag-0.5Cu. Besides multiaxial low fatigue tests with axial ratcheting train were conducted on Sn-3Ag-0.5Cu, the tests result appeared that shear tress peak kept a constant at different axial stress. The ratcheting strain rate increased with shear train amplitude and axial tress while the final ratcheting strain changed between 20% to 30%.Four methods for multiaxial low fatigue life prediction were applied to the multiaxial low fatigue tests of Sn-3Ag-0.5Cu. Without considering the ratcheting strain, equivalent strain approach was improper for multiaxial fatigue life prediction. Critical plane approaches gave better prediction results than equivalent strain approach, however, it gave dangerous results when observed lives were more than 103. Energy approach also gave improper results. Coffin model, which combines ratcheting and cyclic strain, can predict fatigue life with small ratcheting, but presents the fatigue life predictions on non-conservative side if the ratcheting deformation is large. The Gao-Chen model ,which adopted the maximum shear strain range (Δγmax) and axial ratcheting strain rate (ε& r) as a new damage parameters, gave the best prediction.