Ductile-to-brittle transition in fracture behaviors of common solder alloys over a temperature range down to −150 °C

Abstract

Detailed comprehensive comparison of temperature variations of ductility over common solders forms the sound basis of designing reliable solder joints that operate across a wide range of low temperatures, but the cryogenic fracture mechanism needs further investigation. Here, we investigate fracture behaviors of typical solders at different temperatures from −150 to 20 °C. Using standardized Charpy impact tests and fractography, we discover that all Pb-free Sn-based solders (Sn, Sn0.7Cu and SAC305) make abrupt transitions from ductile to brittle with decreasing temperature. Instead, Pb-containing solders (Sn37Pb and Sn36Pb2Ag) gradually change their fracture mode across the broad temperature range; and furthermore, a Pb-based solder (Pb10Sn) undergoes ductile deformation even at −150 °C. Sn exhibits deformation twinning below the DBTT and the cryogenic intergranular fracture is due to the strain incompatibility. Intermetallics in Sn matrix facilitate the transgranular cleavage fracture and the cleavage plane is {211}. Pb-rich phase can block cleavage crack propagation to prevent brittle failure. Estimated toughness together with presented cryogenic fracture mechanisms would be useful to inspire the design of robust joints that survive cryogenic temperatures.