Study of Ni3P growth due to solder reaction-assisted crystallization of electroless Ni-P metallization

Abstract

Nickel (Ni) has been widely used for advanced electronic applications to replace Cu or act as a diffusionbarrier layer in the conventional Au/Cu metallization because of a slower intermetallic compound (IMC) formation rate [1] and a relatively low diffusion rate through Au and Cu [2, 3]. Amorphous electroless nickel-phosphorus (Ni-P) has attracted much interest and widespread uses in the PCB fabrication and UBM for flip chip technology because it offers a lower cost alternative to a more expensive physical Ni deposition methods, good corrosion resistivity, strong adhesion, and good solder wetting [4–6]. It has been found that the amorphous Ni-P alloy will only undergo a selfcrystalline transformation to Ni and Ni3P at temperature above 300 ◦C [7]. Recently, Jang et al. [5] have found that the solder reaction will assist crystallization of electroless Ni-P UBM in flip chip packages even if the reflow temperature is well below the self-crystallization temperature. However, the details of the Ni3P growth in electroless Ni-P are still unclear. In this letter, some results of Ni3P growth in electroless Ni-P with chemical composition of Ni77P23 are presented and then compared with those results obtained in Ref. [5] in which the electroless Ni-P has a composition of Ni85P15. Electroless Ni-P was deposited on Cu pad substrate and a gold (Au) flash was deposited on the top of that electroless Ni-P in order to avoid oxidation of nickel surface. Eutectic Pb-Sn solder balls were placed on the prefluxed Au/Ni-P/Cu pad substrates and reflowed at temperature of 220 ◦C for different annealing time from 0.5 to 90 min. For interfacial microstructural examination, samples were mounted in epoxy and then sectioned using a slow speed diamond saw. The crosssectioned samples were ground, polished and etched with 10% conc. HCl+ 90% water. The chemical composition of electroless Ni-P was found to be Ni77P23 by using Philips XL 40 FEG scanning electron microscope (SEM) equipped with energy dispersive X-ray analysis (EDX). The structure of electroless Ni-P was confirmed to be amorphous by X-ray diffraction (XRD). Fig. 1 shows the SEM image of cross-sectioned sample after reflow at 220 ◦C for 0.5 min. The IMC layer is confirmed to be Ni3Sn4 by XRD and EDX analyses. It is found that both chunky-type and needle-type Ni3Sn4 IMCs are present. It is interesting to note that there is a thin dark layer between the Ni3Sn4 IMC and the electroless Ni-P. This dark layer is confirmed to be Ni3P, which is consistent with the findings of Jang et al. [5]. Fig. 2 shows the relationship between the thickness of Ni3P and the annealing time at 220 ◦C. During the first 15 min of annealing, the Ni3P grows rapidly due to the solder reaction-assisted crystallization of electroless Ni-P. It is found that the thickness of Ni3P is linearly proportional to the square root of the annealing time during the first 15 min. This means that the Ni3P growth is controlled by a diffusion process during the first 15 min of annealing. However, it is interesting to note that the growth of Ni3P is stopped or even dropped after the first 15 min of annealing. For the sake of comparison, Fig. 2 also shows the results of Ni3P thickness growth in electroless Ni85P15 [5]. It shows that the Ni3P layer in electroless Ni85P15 continued to grow throughout the entire annealing range from 0 to 40 min. If we assume all the Ni and P elements in electroless Ni-P react into Ni3P and Ni3Sn4 without any loss into the solder, a general solder reaction-assisted crystallization equation in electroless NiaPb will be as follows: