A metastable phase of Sn has been found to co-exist with ß-Sn in Pb-free SnAg microbumps in 3D integrated circuit (3D IC) technology. Synchrotron microbeam x-ray diffraction, high-resolution TEM imaging and selected-area electron diffraction were used to confirm the metastable phase, which has a orthorhombic lattice, with lattice parameter a = 0.635 nm, b = 0.639 nm, and c = 1.147 nm. Its composition is Sn containing about 10 percent of Ni, a few percent of Au and Pd. The structure is probably isostructural with those of Sn4Au, and Sn4Pd with Ni replacing the noble metal. The formation of this phase may be because of Au and Pd in surface finishing for Ni. With only one reflow, this new phase could form almost continuously in microbumps solder when the thickness of solder layer shrinks to 2 μm. The formation of this phase implies the well adopted Au/Pd surface finishing structure may cause potential reliability problems in 3D IC.