In 3D IC technology, the vertical interconnection consists of through-Si-vias (TSV) and micro solder bumps. The size of the micro-bump is approaching 10 μm, which is the diameter of TSV. Since joule heating is expected to be the most serious issue in 3D IC, heat flux must be conducted away by temperature gradient. If there is a temperature difference of 1 °C across a micro-bump, the temperature gradient will be 1000 °C/cm, which can cause thermomigration at the device operation temperature around 100 °C. Thus thermomigration will become a very serious reliability problem in 3D IC technology. We review here the fundamentals of thermomigration of atoms in microbump materials; both molten state and solid state thermomigration in solder alloys will be considered. The thermomigration in Pb-containing solder joints is discussed first. The Pb atoms move to the cold end while Sn atoms move to the hot end. Then thermomigration in Pb-free SnAg solder joints is reviewed. The Sn atoms move to the hot end, but the Ag atoms migrate to the cold end. Thermomigration of other metallization elements, such as Cu, Ti and Ni is also presented in this paper. In solid state, copper atoms diffuse rapidly via interstitially to the cold end, forming voids in the hot end. In molten state, Cu thermomigration affects the formation of intermetallic compounds.