The Influence of 0–0.1 wt.% Ni on the Microstructure and Fluidity Length of Sn-0.7Cu-xNi

Abstract

Alloys based on the Sn-0.7Cu-xNi system are potential Pb-free solders. In this paper we report on the solidification characteristics and microstructures of Sn-0.7Cu alloys containing 0-1,000 ppm nickel. The microstructural observations show that increasing the nickel content reduces the volume fraction of primary Sn, thus generating a more-eutectic microstructure. In an attempt to better understand the changes in solderability with Ni additions, fluidity tests were carried out using the Ragone method for small incremental increases in nickel content. The maximum fluidity length was found to vary strongly with nickel content. Additionally, the distribution of nickel within samples was investigated using synchrotron micro X-ray fluorescence.

Effects of Phosphorus on Microstructure and Fluidity of Sn-0.7Cu-0.05Ni Lead-Free Solder

“…At the higher measured levels of 0.021 and 0.025 mass% P, however, the microstructure is clearly hypoeutectic, containing a significant volume fraction of Sn dendrites. Previous research by the authors 5,14) has shown that, as the Ni content in Sn-0.7Cu-xNi decreases from 0.05 mass%, the volume fraction of primary Sn dendrites in the microstructure increases. This result was shown to be consistent with the phase equilibria of the Sn-Cu-Ni system, 15) where Sn-0.7Cu-0.05Ni is closer to a eutectic valley than Sn-0.7Cu.…”

“…This result was shown to be consistent with the phase equilibria of the Sn-Cu-Ni system, 15) where Sn-0.7Cu-0.05Ni is closer to a eutectic valley than Sn-0.7Cu. 4,5) However, the increased volume fraction of Sn-dendrites with increasing P content in Figs. 5 and 6 cannot be solely explained by the decreased Ni content, as the P containing samples in Figs.…”

“…Next, the dross was removed from the surface of the melt and two samples of the alloy were collected concurrently in boron nitride coated tapered stainless steel moulds using the method outlined in reference. 5) One of the samples was used to record the cooling curve using an N-type thermocouple. The other sample was left to solidify and was later used for metallographic examination.…”

“…4). The L f of Sn-0.7Cu-xNi alloys has previously been shown to be highly sensitive to the Ni content, 4,5,17) and it is therefore important to assess the role of the changing Ni content on the change in L f with P additions. Figure 7(b) shows the influence of Ni content on L f in P-free Sn-0.7Cu-xNi using data from reference.…”

“…4,5) The improved capacity of Sn-0.7Cu-(0.04-0.1)Ni to flow as it solidifies is thought to be beneficial during wave soldering, when it is important that the solder is able to completely fill joints and cover connections, and for excess solder to drain away to leave tidy fillets.…”