CREEP, STRESS-RELAXATION, AND PLASTIC-DEFORMATION IN SN-AG AND SN-ZN EUTECTIC SOLDERS

Because of the high homologous operation temperature of solders used i n electronic devices, time and temperature dependent relaxation and cr eep processes affect their mechanical behavior. In this paper, two eut ectic lead-free solders (96.5Sn-3.5Ag and 91Sn-9Zn) are investigated f or their creep and stress relaxation behavior. The creep tests were do ne in load-control with initial stresses in the range of 10-22 MPa at two temperatures, 25 and 80 degrees C. The stress relaxation tests wer e performed under constant-strain conditions with strains in the range of 0.3-2.4% and at 25 and 80 degrees C. Since creep/relaxation proces ses are active even during monotonic tensile tests at ambient temperat ures, stress-strain curves at different temperatures and strain rates provide insight into these processes. Activation energies obtained fro m the monotonic tensile, stress relaxation, and creep tests are compar ed and discussed in light of the governing mechanisms. These data alon g with creep exponents, strain rate sensitivities and damage mechanism s are useful for aiding the modeling of solder interconnects for relia bility and lifetime prediction. Constitutive modeling for creep and st ress relaxation behavior was done using a formulation based on unified creep plasticity theory which has been previously employed in the mod eling of high temperature superalloys with satisfactory results.