The wettability and microstructure of Sn-Zn-RE alloys

Rare earth (RE) elements with concentrations of 0.05 wt.% and 0.1 wt.% of primarily Ce and La were added to the Sn-9Zn eutectic alloy to produce Sn-9Zn-RE alloys. A small amount of rodlike Zn-rich phases, which decreases in amount as the RE content increases, is distributed in the Sn matrix under slow cooling. The microstructure is refined with RE additions, and particulate-shaped Sn-RE compounds begin to appear when the RE content reaches 0.1 wt.%. There is no change in the liquidus temperature after RE additions. Of the rosin-activated (RA), rosin mildly activated (RMA), rosin-nonactivated (R), and volatile organic compounds (VOC)-free types of fluxes used for the wetting balance tests in ambient air at 245°C, 260°C, and 290°C, only the RA flux can provide wetting. The addition of RE elements or the increase in soldering temperature reduces the wetting angle and increases the wetting force. The microhardness of the Sn-Zn-RE alloy system was found to be higher than that of Sn-9Zn.