4. Microstructure of SAC305 brazing alloy

4.1 Microstructure after reflow assembly

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 (Ag ₃ Sn), and copper-tin intermetallics (Cu ₆ Sn ₅ ).

Generally, the microstructure observed after reflow soldering consists of tin dendrites, a β-Sn matrix, and Ag ₃ Sn intermetallic compounds located in the interdendritic areas. For electronic assemblies where the copper pad areas are covered with a nickel-containing finish (e.g.,...