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

摘要

The electroless deposited Ni-P (Phosphorus) under bump metallurgy(UBM) layer was fabricated for Sn containing solder bumps. The amount ofP in the electroless Ni film was optimized by controlling complexingagents and the pH of plating solution. The interfacial reaction at theelectroless Ni UBM/solder interface was investigated in this work. Theintermetallic compound (IMC) formed at the interface during solderreflowing was mainly Ni₃Sn₄, and a P-rich Ni layerwas also formed as a by-product of Ni-Sn reaction between the NiSn IMCand the electroless Ni layer. A 1-4 μm ofNi₃Sn₄ IMC and a 1800-5000 Å of P-rich Nilayer were formed in less than 10 minutes of solder reflowing dependingon solder materials and reflow temperatures. However, less than 1 μmthickness of the electroless Ni layer was consumed. It was found thatthe 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 theP-rich Ni layer which was verified by the X-ray analysis. No Sn wasdetected at the electroless Ni layer located just below the P-rich Nilayer. Therefore, the P-rich Ni layer, a by-product layer of Ni-Sninterfacial reaction, is not appropriate for a Sn diffusion barrier atthe electroless Ni UBM and Sn containing solders. Because of the fastdiffusion of Sn into the P-rich Ni layer, a series of Kirkendall voidswere found in the Ni₃Sn₄ IMC, just above theP-rich Ni layer during extended solder reflowing. The amount of theKirkendall voids appeared to be proportional to the growth of the P-richNi layer determined by solder reflowing and subsequent annealingprocesses. Because the Kirkendall voids are considered to be the maincause of the brittle fracture, it is recommended to restrict the growthof the P-rich Ni layer by choosing proper processing conditions. Thebrittle characteristics of the Ni-Sn IMC and the Kirkendall voids at theelectroless Ni UBM-Sn containing solder system cause brittle bumpfailure which results in a decreased bump adhesion strength