Influence of aging on microstructure and hardness of lead-free solder alloys

CARINA MORANDO AND OSVALDO FORNARO

SOLDERING & SURFACE MOUNT TECHNOLOGY

EMERALD GROUP PUBLISHING LIMITED

Año: 2020 vol. 33 p. 57 - 64

0954-0911

Purpose: The objective of this work is to carry out a study of the evolution of the microstructure and the microhardness of Sn-Cu-Ag alloys from as-cast condition and under artificial isothermal aging at different temperatures (100º and 180º C) for a treatment time up to 500hs. A comparison with Sn-37%Pb eutectic solder samples was also made. Design/methodology/approach: Sn-3.5%Ag, Sn-0.7%Cu, Sn-3.5%Ag-0.9%Cu were poured in two different cooling rate conditions and then aged at 100ºC (373ºK) and 180°C (453ºK) during 500h. Microstructural changes were observed by Optical Microscopy (OM), Scanning Electron Micrograph (SEM) and Energy Dispersive X-Ray Microanalysis (EDAX). Differential Scanning Calorimetry technique (DSC) was also used to confirm the obtained results. Findings: A decrease up to 20% in microhardness respect to the value of the as-cast alloy was observed for both aging temperatures. These changes can be explained considering the coarsening and recrystallization of Sn dendrites present in the microstructures of all the systems studied.Originality/value: There is no evidence of dissolution or precipitation of new phases in the range of studied temperatures that could be detected by DSC calorimetry technique. The acting mechanisms must be the result of coarsening of Sn dendrites and the residual stresses relaxation during the first stages of the isothermal aging.