This work reports a systematic study that addresses the evolution of the solder and intermetallic bond layer microstructure and resulting properties in microbump based assemblies during reflow and subsequent aging. So far, pure Sn as well as alloys with two different initial concentrations of Ag were considered. Thicknesses down to 5μm were deposited and reflowed on Cu, Ni, and Ni/Au pads of different thicknesses and diameters ranging from 150μm to 11μm. Flip chips with these structures were finally assembled onto different substrate pads using a fine pitch bonder and aged at different elevated temperatures for various lengths of time. Solder and intermetallic layer microstructures were characterized by cross polarizer microscopy and scanning electron microscope (SEM). Different combinations led to size and composition dependent effects including different solder composition and microstructure as well as much faster growth of intermetallic layers, and different ratios of intermetallic layer thicknesses. Copyright © 2012 by ASME.