Wetting and Interfacial Chemistry of New Pb-Free Sn-Zn-Ag-Al-Li (SZAAL) Solder with Cu, Ni, and Al Substrates

Abstract

This paper presents the results of the Ni substrate wetted with the liquid Sn-Zn eutectic alloy with the addition of Ag, Al, and Li (84.3 at.% Sn, 13.7 at.% Zn, 1 at.% Ag, 0.5 at.% Al, and 0.5 at.% Li-SZAAL). The wetting tests were performed using two methods: the wetting balance tests (WBT) and the sessile drop (SD) method at 250 °C, in the presence of an ALU33® flux. The wetting times were 5, 20, 60, 180, and 1800 s. Next, the microstructure of selected solidified solder joints was investigated using scanning electron microscope. The Ni-Zn system's intermetallic phases (IMCs) were identified at the solder–Ni substrate interface. The kinetics of the formation and growth of the IMC layer was determined. Interfacial tension and contact angle (CA) values were calculated from WBT measurements in the presence of the ALU33® flux. Interfacial tension was compared to surface tension from the maximum bubble pressure and CA values to those obtained using the SD method. The value of the contact angle of SZAAL on Cu (39°) substrate is lower than on Ni (43°). For the comparison, also the interaction between SZAAL and Cu substrate was measured. After 1800 s, the IMCs thickness is significantly reduced for Ni pad than for Cu substrate. The study found that the addition of Al, Ag, and Li improved the tensile strength and wettability of Sn-Zn eutectic-based alloys.