A Pb-free solder alloy based on Sn as matrix is provided, which is a metal that has no toxicity and is environmental friendly. The Pb-free solder comprises a tetra-nary composition consisting essentially of about 99.0 weight % Sn, 0.3 to 0.4 weight % Ag, and 0.6 to 0.7 weight % Cu, with off-eutectic melting temperature of 217 to 227 degree Celsius. The fourth component is a non-metallic phosphorus (P), and 0.01 to 1.0 weight % of phosphorus is added into said composition to improve better micro-structural stability and hence reduce the formation of dross when hand soldering, wave soldering and reflow soldering during the electronic assembly installation process.