A Study on Combination of ENEPIG Surface Finish and Solder Ball for Solder Joint Reliability

An electroless nickel (Ni)/electroless palladium (Pd)/immersion gold (Au) (ENEPIG) surface finish is widely applied to substrates used in various semiconductor packages. However, many issues have been arising in terms of solder joint reliability (SJR) during the assembly process. A number of factors can affect SJR such as thickness of each layer of ENEPIG, reflow profile, plating bath conditions, chemical composition of Pd and Ni layer, and chemical composition of solder balls. This paper focuses on investigating the influence of the factors on SJR and formation of intermetallic compounds (IMCs). The present study investigated the effect of the number of reflows (1, 3 and 5), Ni and Pd thickness of ENEPIG (Ni: 3, 6 and 9 µm, Pd: 0.02, 0.06 and 0.1 µm), chemical composition of Pd layer (pure Pd and Pd-phosphorus (P)) and chemical composition of solder balls (tin (Sn)0.7copper (Cu), SAC305 (Sn3.0 silver (Ag) 0.5Cu), SAC302 (Sn3.0Ag0.2Cu) and Sn3.5Ag). Special focus involved the combination of solder balls (SAC305 and Sn3.5 Ag) and chemical composition of the Pd layer (pure Pd and Pd-P). SJR that were evaluated through pull test and shear test, and then IMCs were observed through scanning electron microscope (SEM) analysis. The effect of Pd and Ni thickness with Pd-P and pure Pd was confirmed. Compared to Pd-P ENEPIG, a relatively high risk of SJR was found when using solder ball with no or low Cu content (e.g., SAC302 and Sn3.5Ag) and pure Pd ENEPIG. As a result of SEM analysis, Kirkendall voids were observed in the Ni
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SnP IMC layer only on pure Pd ENEPIG with Sn3.5Ag solder balls and the fracture occurred along the void line. For Sn3.5Ag, a thicker P-rich layer (around 700 µm) was formed compared to SAC305 (300µm).