MEASUREMENT OF THE HIGH STRAIN RATE MECHANICAL BEHAVIOR OF SAC305 ALLOY AT PRODUCT OPERATING TEMPERATURE AND DERIVATION OF ANAND VISCOPLASTICITY CONSTANTS

摘要

Predictive life models for assessment of the electronics survivability need the high strain rate mechanical properties to capture the interconnect deformation during transient-shock. Electronics products are often at a high or low operating temperature when subjected to shock-impact during accidental drop. Leadfree solders have been used as interconnects in electronic packaging, due to its environmental friendly chemical property. Interconnects during shock may be subjected to strain rates in the neighborhood of 1-100 sec~(-1). However, the high strain rate mechanical properties of SAC leadfree alloys at operating temperatures are scarce. Anand viscoplasticity is often used to model the constitutive behavior of the leadfree solders under thermo-mechanical deformation at strain rates in the neighborhood of 10~(-5) to 10~(-3) per sec. However, Anand viscoplasticity constants are not available for the high strain rates encountered by electronic interconnects under shock and vibration. In this paper, the nine material parameters to fit the Anand viscoplastic model at high strain rates have been presented. In order to develop the constants for this model, uniaxial tensile tests at several strain rates and temperatures have been completed. A constant strain rate impact hammer which enables attaining strain rates around 1 to 100 per sec has been employed to implement tensile tests and a small thermal chamber is applied to control testing temperature. High speed cameras operating at 70,000 fps have been used to capture images of specimen and then digital image correlation method is used to calculate tensile strain. Uniaxial stress-strain curves have been plotted over a wide range of strain rates (ε=10, 35, 50, 75 /sec) and temperatures (T = 25, 50, 75, 100, 125°C). Anand viscoplasticity constants have been calculated by non-linear fitting procedures. In addition, the accuracy of the extracted Anand constants has been evaluated by comparing the model prediction and experimental data.