Brittle to ductile fracture transition in bulk Pb-free solders

The fracture toughness of bulk Sn, Sn–Cu, Sn–Ag, and Sn–Ag–Cu lead-free solders was measured as function of the temperature by means of a pendulum impact test (Charpy test). A ductile to brittle fracture transition was found, i.e., a sharp change in the fracture toughness. No transition was found for the eutectic Sn–Pb. The transition temperature of high purity Sn, Sn–0.5%Cu and Sn–0.5%Cu(Ni) alloys is around –125 °C. The Ag-containing solders show a transition at higher temperatures: in the range of –78° to –45 °C. The increase of the Ag content shifts the transition temperature towards higher values, which is related to the higher volume fraction of SnAg₃ particles in the solder volume. At fixed volume fraction, smaller particle size shifts the transition temperature towards higher values. Therefore, a careful microstructure control is needed during the solder solidification after reflow in order to decrease the low temperature brittleness hazard.status: publishe