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

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 beta-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 (lambda (1), lambda (2), and lambda (3)) associated with the evolution of the tip cooling rate () during solidification; start and growth of tertiary dendrite branches; yield (sigma (y)) and ultimate tensile strengths (sigma (u)); elongation to fracture (delta); and the morphology of IMCs embedded in the Sn-rich phase. A single ratio between the cooling rate () 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 lambda (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, sigma (u) and sigma (y) are shown to increase with decreasing lambda (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 lambda (2) and the change in the morphology of the Ag3Sn IMC from spheroidal to fibrous.