Meaning
Numerical frameworks describing the time-dependent and permanent deformation of materials under stress combine the characteristics of both viscous flow and plastic yielding. These models are essential for accurately predicting the behavior of solder at temperatures above half its melting point. Using a viscoplastic constitutive model allows engineers to simulate how joints will relax or creep during the dwell times of a thermal cycle.
Rate Sensitivity
Application of this specific model type involves parameters that account for the rate sensitivity and the hardening of the material. It treats the deformation as a unified process which is the primary advantage of a viscoplastic constitutive model. This approach simplifies the numerical integration required for complex simulations while maintaining high accuracy for lead-free solder alloys.
Constant Extraction
Accuracy of the simulation depends on the experimental data used to calibrate the model for a specific alloy composition. Lab technicians perform tensile tests at different temperatures and strain rates to extract the constants for the viscoplastic constitutive model. These constants reflect the internal resistance of the grain structure to flow under the applied mechanical load.
Fatigue Simulation
Simulation software enables the prediction of stress distribution in microelectronic packages. Designers use the viscoplastic constitutive model to evaluate how different lead frame materials or board thicknesses will affect the long-term durability of the solder joints. This predictive capability reduces the need for multiple rounds of physical prototyping and accelerated life testing.
