Assessment of Tertiary Dendritic Growth and Its Effects on Mechanical Properties of Directionally Solidified Sn-0.7Cu-xAg Solder Alloys

Abstract

Although commercial SAC107/207/307 alloys are being used as alternatives to traditional Sn-Pb solder alloys, there is a lack of studies emphasizing some metallurgical aspects, for instance, the different morphologies of Cu₆Sn₅ and Ag₃Sn intermetallic compounds (IMCs) as well as conditions for the launch of tertiary dendritic β-Sn branches and their effects on localized mechanical properties. A wide range of cooling rates during solidification is normally associated with quite different microstructural length parameters. Hence, Sn-0.7 wt.%Cu-x wt.%Ag (where x = 1.0, 2.0, and 3.0) alloys were directionally solidified under transient heat flow conditions, undergoing cooling rates varying from 0.1 K/s to 32.0 K/s. This experimental study encompasses: primary, secondary, and tertiary dendrite arm spacings (λ ₁, λ ₂, and λ ₃) associated with the evolution of the tip cooling rate ($\dot{T}$) during solidification; start and growth of tertiary dendrite branches; yield (σ _y) and ultimate tensile strengths (σ _u); elongation to fracture (δ); and the morphology of IMCs embedded in the Sn-rich phase. A single ratio between the cooling rate ($\dot{T}$) and the alloy silver content (C _0-Ag) of 0.45 seems to be the parametric factor associated with the beginning of the growth of tertiary dendritic branches. SAC307 is shown to be the only alloy examined having λ ₃ along the entire casting length. Despite the presence of some tertiary branches in part of the SAC207 alloy casting, for both the SAC107 and SAC207 alloys, σ _u and σ _y are shown to increase with decreasing λ ₂, while the opposite trend is exhibited by the SAC307 alloy. It seems that the well-defined array of tertiary branches in such alloy, and consequently the more complex dendritic network, allowed the strength to increase despite the associated increase in λ ₂ and the change in the morphology of the Ag₃Sn IMC from spheroidal to fibrous.