Studies on Ni-Sn intermetallic compound and P-rich Ni layer at the electroless nickel UBM-solder interface and their effects on flip chip solder joint reliability

The electroless deposited Ni-P (Phosphorus) under bump metallurgy (UBM) layer was fabricated for Sn containing solder bumps. The amount of P in the electroless Ni film was optimized by controlling complexing agents and the pH of plating solution. The interfacial reaction at the electroless Ni UBM/solder interface was investigated in this work. The intermetallic compound (IMC) formed at the interface during solder reflowing was mainly Ni₃Sn₄, and a P-rich Ni layer was also formed as a by-product of Ni-Sn reaction between the NiSn IMC and the electroless Ni layer. A 1-4 μm of Ni₃Sn₄ IMC and a 1800-5000 Å of P-rich Ni layer were formed in less than 10 minutes of solder reflowing depending on solder materials and reflow temperatures. However, less than 1 μm thickness of the electroless Ni layer was consumed. It was found that the P-rich Ni layer contains Ni, P and a small amount of Sn (~7 at%). The atomic ratio of 3Ni:1P indicates that there is Ni,P phase in the P-rich Ni layer which was verified by the X-ray analysis. No Sn was detected at the electroless Ni layer located just below the P-rich Ni layer. Therefore, the P-rich Ni layer, a by-product layer of Ni-Sn interfacial reaction, is not appropriate for a Sn diffusion barrier at the electroless Ni UBM and Sn containing solders. Because of the fast diffusion of Sn into the P-rich Ni layer, a series of Kirkendall voids were found in the Ni₃Sn₄ IMC, just above the P-rich Ni layer during extended solder reflowing. The amount of the Kirkendall voids appeared to be proportional to the growth of the P-rich Ni layer determined by solder reflowing and subsequent annealing processes. Because the Kirkendall voids are considered to be the main cause of the brittle fracture, it is recommended to restrict the growth of the P-rich Ni layer by choosing proper processing conditions. The brittle characteristics of the Ni-Sn IMC and the Kirkendall voids at the electroless Ni UBM-Sn containing solder system cause brittle bump failure which results in a decreased bump adhesion strength