Intermetallic Formation
Solid-state growth kinetics govern the development of the cu3sn phase during prolonged thermal exposure at the joint interface between a copper substrate and tin-based solder. Thermal aging drives interdiffusion across the boundary, producing this bronze-family compound beneath the more common cu6sn5 layer. Atomic mobility within the copper-tin system controls layer thickness, which expands proportionally with the square root of aging time according to classical parabolic growth laws.
High homologous temperatures accelerate this solid-state transformation, leading to excessive consumption of the underlying copper pad if processing windows remain unoptimized.
Shear Strength
Mechanical integrity degrades when brittle intermetallic growth extends unchecked throughout the interconnection volume during high-temperature operational cycles. Fracture paths propagate preferentially along the boundaries of the cu3sn phase because localized stress concentrations accumulate where thermal expansion coefficients mismatch adjacent metal matrices. Destructive shear testing exposes lower load-bearing capacities in assemblies suffering from thick intermetallic accumulation, distinguishing acceptable ductile solder joints from brittle failures that fracture prematurely under mechanical shock or cyclic bending loads.
Diffusion Barrier
Subsequent thermal excursions alter joint reliability because the cu3sn phase acts as a consumption front that eventually exhausts available copper metallization if protective finishes fail. Plating thickness specifications must restrict initial pad dimensions to prevent complete conversion of the pad during reflow and subsequent thermal storage. Accelerated life testing validates joint performance by measuring electrical resistance shifts and microstructural degradation after standardized environmental exposure cycles.
