Modelling Interfacial Package Creep in High Temperature Electronics
Interfacial package creep modeling requires viscoplastic constitutive equations and temperature-dependent stress mapping to prevent long-term field drift.
Microstructural Coarsening
Phase Evolution
Increase in the average size of grains or precipitates occurs to reduce the total interfacial energy of a solid system. Metallurgists study microstructural coarsening in lead free solder alloys to understand the reduction in mechanical strength over time. This process involves the growth of larger particles at the expense of smaller ones through a diffusion controlled mechanism.
The driving force is the curvature dependent chemical potential described by the Gibbs Thomson effect.
Kinetic Path
Atomic diffusion along grain boundaries or through the bulk material facilitates the transport of solute atoms. During microstructural coarsening, the intermetallic layers at the interface of a solder joint thicken and become more brittle. The rate of growth follows a power law relationship with time, typically increasing as the temperature approaches the melting point of the alloy.
Alloying elements like nickel or cobalt are added to suppress the mobility of the diffusing species. The evolution of the grain structure follows the Lifshitz Slyozov Wagner theory for diffusion limited growth in a solid matrix.
Analysis Method
Scanning electron microscopy provides the resolution necessary to track the change in particle size distribution. Experts measure microstructural coarsening by analyzing backscattered electron images of cross sectioned samples. Quantitative metallography software calculates the mean intercept length and the volume fraction of the phases.
Sample preparation must avoid introducing mechanical damage that could bias the observations.
Property Impact
Structural integrity of the interconnect depends on the fine distribution of strengthening precipitates. When microstructural coarsening proceeds, the material undergoes softening and shows a decreased resistance to creep deformation. Solder joints with coarse structures are more susceptible to crack propagation under vibration.