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. And 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 m 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 Ni3P phase in the P-rich Ni layer which was verified by the X-ray analysis. And 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, 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 growth kinetics of Ni-Sn IMC and P-rich Ni layer followed 3 steps: there was a rapid initial growth during the first 1 minute of solder reflow followed by a reduced growth step, and finally a diffusion controlled growth. During the diffusion process, there was a linear dependence between the IMC and P-rich Ni layer thickness and time . Flip chip bump shear test was performed to measure the effects of the IMC and P-rich Ni layers on bump adhesion property. Most failure occurred at the inside of soft solder and partly at the Ni3Sn4 IMC. Exposed portion of the Ni3Sn4 IMC area after shearing bump surface increased, as solder reflow time increased. 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 result in a decreased bump adhesion strength. Introduction Recently, in micro-electronic packaging application, electroless Ni plating has been applied for flip chip bumps and under bump metallurgies (UBM) because of its many advantages, such as maskless selective metal deposition, low cost processing, good solder wettability, and good solder diffusion barrier compared with the conventional solder bumping and UBM processes [1,2,3,4]. However, issues like improvement of the reliability of the electroless Ni and studies of the interactions with solders have to be investigated in order to apply the electroless Ni extensively. Unlike the Ni layer prepared by vacuum deposition, the electroless Ni plated by hypophosphite (H2PO2) has been known that phosphorus (P) in the electroless Ni greatly influences the interfacial reactions with solders. And solder joint reliability must be also closely related to the P in the electroless Ni [5,6,7,8]. Consequently, the behavior of P at the electroless Ni/solder interface has to be investigated. In the case of electroless Ni, not only IMCs but also P rich Ni layer was detected during the solder reflows [5,8]. This P rich Ni layer forms because of the phosphorus accumulated at the interface between the electroless Ni and the IMC layer. Jang reported that P rich Ni layer was composed of Ni3P phase by TEM (Transmission electron microscopy) analysis [5]. Apparently, solder joint failure is closely related to the growth of these products at the electroless Ni/solder interface. Ni-Sn IMCs are very brittle. And P rich Ni layer is also considered to have poor solderability [1,2,4,8]. In present study, interfacial reactions between the electroless Ni and solders (96.5 Sn-3.5Ag and 62.5 Sn-37.5Pb) and the shear strength of the solder bumps was measured. The P rich Ni layer was also investigated in detail: its composition, growth, and phenomena at adjacent layers due to the generation of the P rich Ni layer. 0-7803-7038-4/01/$10.00 (C)2001 IEEE 2001 Electronic Components and Technology Conference Experimental Electroless Ni-P UBM fabrication Test chips which have 100 μm×100 μm area Al pad and 400 μm pitch were fabricated after a 1 μm Al sputtering on Si wafer. Table 1 shows the process flow of the electroless Ni plating. Before the electroless Ni plating, double zincate process was performed to remove the Al2O3 layer and to activate the Al surface. Zincate solution was made of NaOH and ZnO, whose ratio was optimized to create fine and numerous Zn particles on Al pad [2,3]. After the double zincate process, a 5 μm electroless Ni UBMs were formed. Table 1. Electroless Ni deposition processes on an Al surface Process Solution Time Thickness Cleaning HNO3 50% 20 sec N/A Zincate pretreatment ZnO, NaOH 20sec N/A Acid dipping HNO3 50% 5sec N/A Double Zincate ZnO, NaOH 20sec N/A Electroless Ni plating NiSO4⋅6H2O NaPH2O2⋅6H2O CH3COONa Lactic acid