The Effect of Nickel Oxidation Formed in the Interface of ENEPIG Structure for Flip Chip Technology

Abstract

Abstract In flip chip assembly, the smaller solder ball is used in chip and substrate joint. This may have an adverse influence on the reliability of electronic products. Thus, the mechanical strength of solder joint is an important topic for the major reliability in electronic assembly technology. We found the intermetallic compound (IMC) crack issue for Electroless Ni(P) / Electroless Pd(P) / Immersion Au (ENEPIG) Interposer product. In this study, we analyzed the failure samples, for ENEPIG processes verified to find out the IMC crack root cause. The interposer pad structure for these samples was manufactured by ENEPIG process, in which the formation of the oxidation layer is affected by the duration that the sample is exposed on top of Ni bath. These samples print solder paste were performed multi-times reflow for ENEPIG and solder interfacial reaction observation to clarify the nickel oxidation influence on crack issue. These samples were observed and analyzed by Scanning Electron Microscope (SEM), Focused Ion Beam (FIB), Transmission Electron Microscope (TEM), and Energy-Dispersive X-ray Spectroscopy (EDX). The interface strength between ENEPIG and solder were tested by hot bump pull test (HBP), and also has the adhesion analysis. The results show that with the shorter exposure on top of nickel bath is no nickel oxidation formed and the interface has a higher pull strength, with a longer exposure on top of nickel bath had more nickel oxidation formed in the ENEPIG interface and also reveal lower pull strength. Therefore, the IMC crack was induced by the void at ENEPIG interface, which was the reaction of nickel oxidation and Au chemical, and the control of the exposure time on top of nickel bath can inhibit the oxidation formation to reduce the IMC crack risk.