The interactions of lead (Pb) in lead free solder (Sn/Ag/Cu) system

The solder interaction of Pb within the Sn/Ag/Cu system was characterized using Differential Scanning Calorimetry (DSC). Components were then assembled with the Pb-free solder. Cross-sectioning and fine polishing were performed on the solder joints at the as-soldered stage and after temperature cycle readouts at 250, 500, 750, and 1000 cycles. The microstructure of the solder joints was examined using Scanning Electron Microscopy (SEM), and solder elements were mapped and identified by Energy-Dispersive X-ray (EDX) analysis. DSC detected Pb reaction with Sn/Ag at 179°C and ternary compound formation. SEM/EDX found that Pb diffused into the Sn/Ag/Cu matrix during reflow soldering to form different microstructures, namely CuSn, SnAg, SnPbAg, and Pb-rich phases. The SnAg structure was found as a rod/needle morphology that was detrimental to solder joint reliability. During temperature cycling, this structure loosened its embedding effects due to grain-boundary sliding in the solder matrix, which accelerated solder fatigue crack propagation. Solutions to this problem are discussed.