Controlling the Interfacial Reactions in Pb-free Interconnections by Adding Minor Alloying Elements to Sn-rich Solders

In microelectronic packaging, the reliability of Pb-free solder joint is critically depending on its interfacial reactions with Under–Bump-Metallurgy (UBM) or pad finishes. Factors such as intermetallic compound (IMC) formation, IMC spalling, UBM dissolution, interfacial void formation, all affects the integrity of solder joints. The failure mechanisms observed in thermal fatigue or electromigration tests of solder joints are often influenced by the interfacial reactions. When Sn-rich solders are used for Pb-free applications, the interfacial reactions become more aggressive since the reflow temperatures of Sn-rich solders are higher than eutectic Sn-Pb solders and the corresponding solubility of barrier metals in a molten Sn-rich solder is expected to be much higher at the higher reflow temperatures. Recently, many studies have been conducted to control the reaction or consumption rate of a Cu layer into Sn-rich solders. However, few studies have been reported on the consumption of Ni barrier layers in Sn-rich solders during reflow. In addition, it has been known that the dissolution rate of a Ni layer varies widely depending on its material deposition processes and parameters, such as electrolytic, electroless or sputtered Ni. In microelectronic applications, an electroless Ni (P) layer is commonly used to provide a reaction barrier layer on Cu pads of laminates or modules, while an electrolytic or sputtered Ni layer is used in a UBM structure of a flip-chip metallization. In this study, the consumption rate of different Ni layers (sputtered, electrolytic and electroless plated) are investigated with several Sn-rich solders, such as Sn-Ag, Sn-Cu, Sn-Ag-Cu in terms of reflow conditions. A method of controlling the consumption of Ni barrier layers is also reported by adding minor alloying elements, such as Ni or Cu into Sn-rich solders. The controlling mechanisms of Ni consumption are discussed for different alloying elements. The effects of minor alloying elements on the mechanical and thermal properties of Sn-rich solders are also reported.