Pasty Ranges and Latent Heat Release Modes for Sn-9Zn-xAg Lead-free Solder Alloys

Abstract

The pasty ranges and latent heat release modes of Sn-9Zn-xAg alloys, where x varies between 0.5 and 3.5, are examined in this study. The effects of alloy composition and cooling rate on the pasty range and latent heat release modes are also investigated. A Computer Aided-Cooling Curve Analysis (CA-CCA) technique is used to determine the pasty ranges and latent heat release modes for the alloys. To comply with the requirements of CA-CCA, the heat transfer conditions of the experimental setup must closely resemble of a lumped system, in which the alloy is cooled in a uniform fashion. To confirm this, two thermocouples are inserted into the crucible where the alloy is melted and subsequently resolidified. The crucible is used with and without a covering of insulating material to obtain different cooling rates. The temperature readings show that the heat transfer conditions in the experimental setup indeed meet the requirements of a lumped system. The solid fraction versus temperature relationship (f_s-T) for the various alloy compositions and cooling rates are obtained from the temperature data through CA-CCA. The experimental results show that as the silver content of the Sn-9Zn-xAg alloy increases, the liquidus temperature rises and the pasty range broadens. The pasty range is approximately 2°C for 0.5Ag, 9°C for 1.5Ag, 14°C for 2.5Ag, and 18°C for 3.5Ag. As long as the silver content is below 0.5 mass%, silver has little effect on the microstructure, which is basically the eutectic Sn-9Zn and the f_s-T relationship is nearly a vertical line. However, as the silver content exceeds 1.5 mass%, the formation of Ag-Zn intermetallic compounds becomes obvious. This causes the alloy composition to deviate from eutectic and to lean towards the tin side of the Sn-Zn phase diagram. This in turn causes the proportion of the primary tin phase to increase and that of the zinc-tin eutectic phase to decrease. This is reflected on the plot of the f_s-T relationship by two distinct vertical regions. One corresponds to the primary tin phase and the other to the eutectic phase. As the silver content further increases, the effects of intermetallic compound formation become even more obvious. As an alternative to CA-CCA, Huang's model can be used to obtain a quantitative f_s-T function. As the silver content increases, the primary solid fraction for Huang's model increases. As the cooling rate increases, the primary solid fraction and the nonlinearity factors n_e and n_p decrease.