Impact of Cooling Rate and Nickel Addition on the Dendritic Spacing, Ag3Sn Morphology and Cytotoxicity of Sn-2Ag Solder

Binary Sn-Ag alloys have been investigated extensively, as they are the base components of commercial alloys for mediumtemperature applications in the electronics industry. However, Sn-Ag alloys have low toughness due to the Ag3Sn intermetallic morphology and low wettability on metallic substrates such as copper and nickel. Furthermore, there are no studies on the toxicity of lead-free alloys that qualify such alloys to replace traditional Sn- Pb alloys. In this sense, this work aims to study the effects of Ni addition (0.2% in weight) on the solidification thermal parameters such as cooling and growth rates (T-L/V-L-liquidus isotherm and T-E/V-E-eutectic front), microstructure, macrosegregation, and cytotoxicity of a hypoeutectic Sn-2wt.%Ag alloy. The characterization was carried out using optical microscopy (OM), scanning electron microscopy (SEM), x-ray diffraction (XRD), and x-ray fluorescence (XRF). The CALPHAD (CALculation of PHAse Diagrams) method was used to generate property diagrams and isopleths. Cytotoxicity analyses (cell viability) were performed using alloys of the Sn-Pb system (Pb-10 wt.% Sn and Sn-20 wt.%Pb) as control samples. The addition of 0.2 wt.%Ni in the hypoeutectic Sn-2 wt.%Ag alloy caused an increase in the cooling rate and growth rate. The results show that the as- cast microstructures of the Sn-2 wt.% Ag and Sn- 2 wt.%Ag-0.2 wt.%Ni alloys are completely dendritic, with a tin-rich matrix surrounded by eutectic ss-Sn +Ag3Sn and ss-Sn +Ag3Sn +Ni3Sn4) mixtures, respectively. The Ni addition refined the dendritic arrangement but did not change the eutectic length scale. Ni stabilized the fibrous morphology of the Ag3Sn intermetallic in the Sn-Ag-Ni ternary alloy, exhibiting a mixture of Ag3Sn fibers + spheroids to T-L < 42.0 degrees C/s and VL < 2.45 mm/s. Cytotoxicity was controlled by chemical composition and incubation time and was not affected by the microstructural scale. The Sn-Ag and Sn-Ag-Ni alloys showed higher levels of cell viability than the Pb-containing alloy control samples. Ni addition did not increase cytotoxicity according to the measurements obtained.

Automated summary

This study investigated the effects of Ni addition on Sn-Ag alloys and found that the addition of Ni improved the cooling and growth rates of the alloy and stabilized the morphology of Ag3Sn intermetallics.