Microstructure of SAC305 brazing alloy
Thermomechanical durability of lead-free electronic assemblies

The Sn-3.0Ag-0.5Cu (SAC305) solder alloy is a hypoeutectic ternary compound whose microstructure has three distinct phases: dendrites and a matrix of tin in its β allotrope form (β-Sn), silver-tin intermetallics (Ag₃Sn), and copper-tin intermetallics (Cu₆Sn₅).

Generally, the microstructure observed after reflow soldering consists of tin dendrites, a β-Sn matrix, and Ag₃Sn intermetallic compounds located in the interdendritic areas. For electronic assemblies where the copper pad areas are covered with a nickel-containing finish (e.g., Au/Ni), very few Cu₆Sn₅ intermetallic compounds are observed in the solder, as the nickel layer acts as a diffusion barrier. Figure