Meaning
Material deformation models that account for rapid temperature changes and high strain rates describe behaviors that deviate from steady-state conditions. Non-equilibrium viscoplasticity addresses how solder joints respond when thermal and mechanical loads change faster than the microstructural state can adapt. It represents the transient microstructural evolution under dynamic loading conditions, which differs from behavior under constant strain rates.
This model describes materials where dislocation density and grain boundary configurations do not stay in immediate balance with the applied stress.
Microstructural Evolution
Solder alloys experience rapid grain coarsening and phase rearrangement when subjected to thermal fluctuations. When temperature changes occur swiftly, the internal structure of the metal is unable to maintain an equilibrium state. The material exhibits a time-lag in its mechanical response as the dislocation structure reorganizes.
This lag causes the flow stress to depend not just on the current temperature and strain rate, but on the history of the deformation. Models must use evolutionary equations for internal state variables to capture this transient phase.
Mechanical Simulation
Finite element programs incorporate these dynamic laws to improve the accuracy of fatigue predictions in harsh environments. Simulating thermal shock tests requires these advanced formulations to avoid overestimating the stress generated during rapid transitions. By modeling the transient state, the analysis correctly predicts where stress relaxation occurs during the short cycle times of aggressive testing.
Standard viscoplastic models that assume steady-state flow tend to predict higher stresses and shorter lives than actually occur.
Experimental Validation
Testing protocols run cyclic shear deformation at multiple frequencies to measure the lag between stress and strain. The gathered data calibrate the state evolution equations for different solder formulations.
