Avaliação microestrutural da liga Sn-2%Ag-0,7%Cu (SAC207) submetida à tratamento térmico de envelhecimento

Abstract

Although there are still few studies on lead-free alloys (Pb), in recent years some alternative alloys have been studied, since the presence of this element poses a threat to human health due to its high toxicity. In the search for new Pb-free alloys for electronic assembly applications, Sn-Ag-Cu (SAC) alloys are emerging as one of the main alternatives to substitute Sn-Pb (Sn-37 or 40wt.% Pb) alloys due to its wetting level, medium melting point and superior mechanical properties. However, the assessment of the effects of thermal cycles during the life of these soldered joints has not yet been properly performed. In this context, the present study focused on microstructural analysis of the tertiary Sn-2.0wt.%Ag-0.7wt.%Cu alloy (SAC207) directionally solidified under transient heat flow conditions and subjected to aging heat treatment for 10, 20 and 30 days at 100°C. The effects of aging treatments were associated with the solidification thermal parameters such as growth rate (VL) and cooling rate (ṪL), so that microstructural characterization considered both dendritic and eutectic regions, taking into account size, distribution and morphology of phases. The results showed a microstructure composed of a Sn-rich dendritic matrix (β-Sn) surrounded by a eutectic mixture Sn, Ag3Sn e Cu6Sn5 phases, ternary eutectic. Ag3Sn intermetallic displayed a morphological transition from spheres to fibers, with spheres related to higher values of VL and ṪL (positions 5 and 20mm), while fibers associated with lower values of VL and ṪL (position 90mm). However, Cu6Sn5 showed no morphological change along the SAC207 alloy casting. The increase in treatment time promoted an increase in the size of the dendritic arrangement and of the Ag3Sn intermetallic, and a reduction in the hardness of the SAC207 alloy.