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 -125degC. The Ag-containing solders show a transition at higher temperatures: in the range of -78 to -45degC. The increase of the Ag content shifts the transition temperature towards higher values, which is related to the higher volume fraction of SnAg 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.