Combined effects of Ag content and cooling rate on microstructure and mechanical behavior of Sn-Ag-Cu solders

Sn-0.7 wt%Cu-1.0 wt%Ag and Sn-0.7 wt%Cu-2.0 wt%Ag alloys were directionally solidified under transient conditions undergoing cooling rates varying from 0.1 to 25 K/s. The microstructure was characterized along the castings lengths and the present experimental results include the secondary dendrite arm spacing (lambda(2)) and its correlation with: the tip cooling rate ((T) over dot) during solidification and microhardness (HV), yield tensile strength (sigma(y)), ultimate tensile strength (sigma(u)) and elongation to fracture (delta). The aim is to examine the effects of Ag content and tip cooling rate on both the microstructure and mechanical properties. The initiation of tertiary branches within the dendritic arrangement, as well as the distinct morphologies of the intermetallic compounds (IMC) related to the solidification cooling rate was also assessed for both examined alloys. While the Cu6Sn5 phase appeared as large faceted crystals along the entire casting length, very fine Ag3Sn spheroids prevailed at higher cooling rates (>7.5 K/s and > 4.0 K/s for 1.0 wt%Ag and 2.0 wt%Ag alloying, respectively) with a mixture of Ag3Sn coarser spheroids and fibers predominating at lower cooling rates. The Sn-0.7 wt%Cu-2.0 wt%Ag alloy exhibited smaller dendritic spacings and HV of about two times higher than the corresponding values of the Sn-0.7 wt%Cu-1.0 wt%Ag alloy. A single Hall-Petch equation is proposed relating delta to lambda(2) for both alloys, which means that the increase in Ag content from 1.0 to 2.0 wt% does not affect the elongation. It is shown that delta decreases with the increase in lambda(2). (C) 2012 Elsevier Ltd. All rights reserved.