Early stages of wetting of copper by Sn–Zn eutectic alloy

The manuscript presents studies of the interaction of Sn–Zn eutectic solder alloy with a solid copper substrate, in the early times of the wetting process. The wetting tests were performed using the sessile drop method, with flux, in air, at 230 °C, 250 °C and 280 °C and reflow time of 2 s, 5 s, 10 s, 20 s, 40 s, 90 s and 180 s. The wetting time was measured from the moment of contact of the liquid solder with copper. It was noted, that during the reaction of copper with the liquid solder, an IMC (intermetallic compound) layer formed in the interface. Solidified solder/substrate couples were studied using scanning electron microscopy (SEM) and synchrotron measurements. The intermetallic phases of Cu–Zn system that formed at the solder/substrate interface were identified as epsilon and gamma. The kinetics of the formation and growth of the intermetallic layer were determined. The model describing the sequence of events in the early time of wetting was proposed. The earliest stage of wetting the copper substrate by liquid solder Sn–Zn included: adsorption of Zn atoms on the Cu surface, surface diffusion of Zn atoms on the copper surface, dissolution of the copper pad by the liquid solder and diffusion of Cu atoms into the liquid. In the liquid solder, the copper atoms combined with zinc atoms to create clusters of the epsilon phase. Those clusters coagulated and a short-range ordered (SRO) Cu–Zn structure formed in the liquid. After short heating and cooling, the liquid solidified and the CuZn$_5$ border-layer appeared in the interface. Further heating caused the saturation of the reaction zone with Cu atoms and the crystallization of two intermetallic phases Cu$_5$Zn$_8$ from the solid support side and CuZn$_4$ one from the solder side occurred