Analysis of extensive wetting angle vs. cooling rate data in Bi-, Zn- and Sn-based solder alloys

Abstract

Adequate wetting is essential to generate high-quality solder connections. The wetting angle (θ) is an important technical element in alloy selection and associated joining operations. In order to find appropriate θ, several alloys have been tested for low, intermediate, and high temperature soldering, particularly in the previous ten years. A wide range of alloys have been investigated and produced, with alternatives available among Bi-, Zn-, and Sn-based alloys. The most accepted concept is that θ may influence the ability of heat to flow across the solder/substrate contact, which has a direct impact on the evolution of solidification through altering the cooling rates (Ṫ). However, latent heat of fusion (L), mushy thermal diffusivity (aSL) and solidification interval (ΔT) may change with solute content, from one alloy system to another and must be verified, since there is the possibility of impacting solidification. In the current study, 23 alloys from various systems were investigated, with solute content varying in each tested alloy system. Microaddition appeared to show that the wetting angle is the primary cooling rate controlling factor. The larger the resultant cooling rate, the smaller the wetting angle. Moreover, two groups were complained for alloys with effective alloying elements, one in which the contact angle controlled the process (i.e., SnNi and ZnSn) and was decisive for the cooling rates, corresponding to Stefan (Ste) numbers from 1.4 to 2.2. Another group of alloy systems (Sn-Ag-Cu, BiSb, SnSb and Sn-Sb(Cu, Ag)) in which θ was ineffective, with Ste numbers ranging from 3.7 to 109.7. A parametric χ factor that agrees with the Ste number while also allowing the most important factors changing cooling rate to be verified is proposed. It is understood that depending on the alloy system of interest, θ or the properties (L, aSL and ΔT) regulating the microstructure of the soldered joint should be prioritized.