Anisotropic Creep
Modern microelectronic assembly relies on a set of mechanical and constitutive behaviors that govern how non leaded solder alloys deform under thermal and mechanical loading. In particular, lead free solder mechanics dictates the rate of stress relaxation and strain accumulation in electronic joints subjected to thermal cycling. This behavior is dominated by creep and microstructural evolution under high homologous temperatures.
Microstructural Coarsening
Grain coarsening and intermetallic compound growth occur continuously in these alloys because electronic operating temperatures are often above half the absolute melting point of the solder. SnAgCu alloys, which represent the most common lead free formulations, develop coarse tin grains that can make the joint anisotropic. This anisotropy leads to localized stress concentrations that promote crack initiation and propagation.
Relaxation Modeling
Thermal mismatch between the silicon die and the printed circuit board generates high shear stresses that decay over time through creep. Measuring this relaxation rate provides the constitutive equations required for finite element modeling of joint reliability. These models help design solder geometries that minimize stress accumulation.
Fatigue Formulation
Fatigue life is typically characterized using the Coffin Manson relationship or strain energy density criteria. These formulations estimate when the accumulated plastic work will cause the joint to rupture.
