ENEG and ENEPIG surface finish for long term solderability

Abstract

The long-term reliability of flip chip and boardlevel solder joint is significantly affected by the properties of the surface finish. Various surface finishes such as Immersion Sn, Immersion Ag, Organic solderability preservatives (OSP), Electrolytic Ni/Au (ENEG), and electroless Ni/immersion Au (ENIG) surface finish have been widely used on based Cu pads and can be compliant with lead free SAC solder alloys. Recently Electroless Ni/Electroless Pd/Immersion Au (ENEPIG) is being offered as an alternative surface finish with high solder joint quality and wire bondability. It is claimed to be more cost effective as an Au layer of lower thickness can be used. To evaluate the performance of solder joints upon ENEG and ENEPIG surface finishes, extended reflow tests at 245°C were conducted. In the case of ENEPIG surface finish, micro-porosities are found to be present on the Au plating surface. PdO oxide forms as a result of Pd exposure, which causes deterioration of solderability as compared with ENEG plating. Following extended reflow, it is found that columnar Cu-Ni-Sn IMCs with small amount of Pd and P-rich Ni layer have formed at the interface of ENEPIG /solder system. In contrast, layer-type Cu-Ni-Sn IMCs formed at the interface of ENEG/solder system in the absence of Pd. While Ni and Ni(P) layers act as barrier to diffusion, in some pad of the ENEPIG samples, the Ni(P) layer is found to be less than 1um after 120 minutes reflow, which is ineffective as a diffusion barrier. By comparison, ENEG surface finish remains effective with 3.5um of Ni barrier layer remaining on Cu after 120minutes reflow. This serves as a good barrier to prevent the diffusion of Cu atoms from the base Cu underneath. Therefore, it has been demonstrated that solder joint on ENEG surface finish is more reliable and suitable to be used for long-term reliability of electronic products.