Comparison between ENA and ENEPIG surface finish for high density TBGA package

Abstract

Recently Electroless Ni/Electroless Pd/Immersion Au (ENEPIG) is being offered as an alternative surface finish to established processing such as Immersion Sn, Immersion Ag, Organic solderability preservatives (OSP), Electrolytic Ni/Au (ENA), and electroless Ni/immersion Au (ENIG) 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. The performance of solder joints upon ENA and ENEPIG surface finishes are evaluated by extended reflow tests at 245°C followed by ball shear testing. Following extended reflow, the ENEPIG/solder system provides lower shear strengths than the ENA/solder system. 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, causing brittle fracture with lower shear strength in the ball shear test. In contrast, layer-type Cu-Ni-Sn IMCs formed at the interface of ENA/solder system in the absence of Pd which caused ductile fracture in ball shear test. Therefore, it has been demonstrated that solder joint on ENA surface finish is more reliable and suitable to be used for long-term reliability of electronic products.