Rate-Dependent Transition from Thermal Softening to Hardening in Elastomers

The thermal-mechanical properties of the materials currently used in packaging are being reexamind as the electronic packaging industry moves towards chip scale packages and wafer scale packages. The rate-dependent transition of elastic modulus and viscosity from thermal softening to thermal hardening with rising temperature, which does not involve any phase change, has been observed in certain elastomers. An explanation about this interesting phenomenon is given based on thermodynamic considerations. A theoretical analysis is performed to show the limitation of existing viscoelastic models in predicting the transition. It appears that macroscopic material properties should be reexamined based on the physics behind the interaction between ordinary elasticity and entropic elasticity.