The microstructure features and the IMC kinetics of Cu-Sn and Ni-Sn soldering reaction under space confinement are studied in the present study. Experimentally, Cu/Sn(10 μm)/Cu and Ni/Sn(10 μm)/Ni sandwich structures were prepared through a hot-press bonding. Sn with a thickness of 10 μm was chosen to simulate the joints in 3D IC packages employing a severe space confinement. Microstructure characterisations showed that space confinement does not change the reaction products which are Cu 6Sn 5 and Cu 3Sn for Cu-Sn reactions and Ni 3Sn 4 for Ni-Sn reactions. However, it was found that the interfacial morphology is very different to that of the conventional joints. IMCs in the space confined reactions occupy a very large or even entirety volume in the joint. In addition, voiding occurs in both Cu/Sn/Cu and Ni/Sn/Ni reactions, which having great impacts on the joint strength. According to the kinetics analysis, the IMCs growth rates measured in the present work are in the range of literature results, suggesting that the space confinement behaviour has very little effects on the IMC growth kinetics.