Barrier Properties of Electroless Ni-B UBM for Solder Joining

Abstract

As demands accelerate for high density, high speed transmission and low power integrated circuits, 3D-ICs with through-silicon via (TSV) is pursued. In the structure of 3D-ICs, the first die is attached to the second die with micro bump, and the second die is attached to the circuit substrate with a C4 solder bump. The electrode structure of the second die is Cu/Ni UBM. The stress of the Ni-B layer is less than that of the Ni-P layer, and the Ni-B layer can suppress stress and die warpage. The purposes of our study are to clarify the difference in the barrier properties of the Ni-B UBM and Ni-P under bump metal (UBM) and the relevance of the barrier properties of Ni UBM and intermetallic compound (IMC) growth. It was found that an electroless Ni-B plating layer is superior to a Ni-P plating layer for UBM in liquid phase diffusion and in solid phase diffusion, and that a segregated B layer is formed under the IMC layer of a Ni-B land due to reflow soldering. It was estimated that this B layer plays the role of being a barrier layer for solder diffusion.