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Effects of Fe on the Kirkendall Void Formation of Sn-3.5Ag-xFe/Cu Solder Joints
Keywords: Kirkendall void, segregation, diffusion
Lead-free Sn-based solders / Cu UBM are commonly used solder joint structure of microelectronics packaging, however Kirkendall void formation typically along Cu/Cu3Sn interface or in Cu3Sn phase is thought to reduce the drop reliability significantly. Our previous work showed that residual S coming from the SPS additive of the Cu electroplating bath segregated to the Cu/Cu3Sn interface and facilitated nucleation of interfacial voids, which subsequently grow by residual stress in the film and the Kirkendall effect. In the present work, Sn-3.5Ag-xFe alloy with varying amounts of Fe (0.1, 0.5, 1.0 and 2.0 wt%) were reacted with the Cu films prepared by electroplating deposition with SPS additive (3ⅹ10-5M) over a FR4-PCB substrate. The solder joint microstructure after a reflow and isothermal aging (150℃ for varying times) were investigated using SEM, and EDAX. In all specimens, Cu6Sn5 and Cu3Sn formed a layered structure between the solder and Cu pad, while FeSn2 and FeSn formed in the solder matrix as Fe content increased. With subsequent aging at 150℃, voids nucleated and grew at the Cu/Cu3Sn interface, which is definitely detrimental to the drop impact reliability of the solder joints. Results indicate that the propensity to form Kirkendall voids at the solder joint decreased with the Fe content. Subsequent AES analyses showed that Fe dissolved in the Cu UBM reduced the segregation of S atoms to the Cu3Sn/Cu interface, which suppressed the nucleation of Kirkendall voids at the interface.
Sunghwan Kim, Student
Daejeon 305-701,
South Korea

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