Effect of sample size on the solidification temperature and microstructure of SnAgCu near eutectic alloys

The degree of undercooling of Sn in near eutectic, SnAgCu solder balls upon cooling at a rate of 1 °C/s from the melt was examined and found to increase linearly with inverse nominal sample diameter (for balls of radius between 100 and 1000 μm). The mean undercooling for SnAgCu solder balls in a flip chip assembly was 62 °C. The microstructures of these different samples were examined by means of scanning electron microscopy. The Sn dendrite arm width was observed to monotonically increase with ball diameter, indicating a possible dependence of the mechanical response of such solder balls upon size.