Authors: Young, Bi-Lian; Duh, Jenq-Gong; Jang, Guh-Yaw

Source: Journal of Electronic Materials, Volume 32, Number 12, 1 December 2003 , pp. 1463-1473(11)

Publisher: Springer

Abstract:

The Ni-based under-bump metallurgies (UBMs) are of interest because they have a slower reaction rate with Sn-rich solders compared to Cu-based UBMs. In this study, several UBM schemes using Ni as the diffusion barrier are investigated. Joints of Sn-58Bi/Au/electroless nickel (EN)/Cu/Al₂O₃ and Sn-58Bi/Au/electroplated nickel/Cu/Al₂O₃ were aged at 110°C and 130°C for 1-25 days to study the interfacial reaction and microstructural evolution. The Sn-Bi solder reacts with the Ni-based multimetallization and forms the ternary Sn-Ni-Bi intermetallic compound (IMC) during aging at 110°C. Compositions of ternary IMC were (78-80)at.%Sn-(12-16)at.%Ni-(5-8)at.%Bi in joints of Sn-58Bi/Au/Ni-5.5wt.%P/Cu, Sn-58Bi/Au/Ni-12wt.%P/Cu, and Sn-58Bi/Au/Ni/Cu. Elevated aging at 130°C accelerates the IMC growth rate and results in the formation of (Ni,Cu) Sn₄ and (Cu,Ni)₆Sn₅ adjacent to the ternary Sn-Ni-Bi IMC for the Sn-58Bi/Au/Ni-12wt.%P/Cu and Sn-58Bi/Au/Ni/Cu joints, respectively. The Cu content in the (Cu,Ni)₆Sn₅ IMC is six times that in (Ni,Cu)₃ Sn₄. Electroplated Ni fails to prevent Cu diffusion toward the Ni/solder interface as compared to EN-based joints. Cracks are observed in the Sn-58Bi/Au/Ni-5.5wt.%P/Cu/Al₂O₃ joint aged at 130°C for 25 days. It is more favorable to employ Ni-12wt.%P for the Sn-58Bi/Au/EN/Cu joint. Electroless nickel, with the higher P content of 12 wt.%, is a more effective diffusion barrier during aging. In addition, P enrichment occurs near the interface of the EN/solder, and the degree of P enrichment is enhanced with aging time. The Au(Sn,Bi)₄, with pyramidal and cubic shape, is observed in the Sn-58Bi/Au/Ni/Cu/Al₂O₃ joint.

Keywords: BALL GRID ARRAY BGA; UNDER-BUMP METALLURGY UBM; SN-BI; SOLDER; INTERMETALLIC COMPOUND; INTERFACIAL MICROSTRUCTURE

Document Type: Research article

Publication date: 2003-12-01

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