Considerable amounts of voids are often observed in micro joints used in microelectronic industry. To devise proper strategies to suppress the formation of these voids requires an understanding of the void formation mechanism. The Ni/Sn/Ni system is used in this study to determine the effects of surface diffusion and reaction-induced volume shrinkage. It is confirmed out that the reaction-induced volume shrinkage plays an important role. Surface diffusion has a strong effect on the surface morphology of the micro joints. Additionally, for the first time the three dimensional void distribution in a micro joint is uncovered by using sequentially ion beam cross-sectioning. (C) 2017 Elsevier B.V. All rights reserved.