Development of Cu-Core Pb-free solder bumps

Cu-core Pb-free solder bumps were developed by electrodeposition in combination with photolithography. A bump with a pitch of 100 μm and a diam of 50 μm was successfully achieved with dry film resist (80 μm thick) under optimum process conditions, and the bumps reached the target height of 50 ± 5 μm. The adoption of a dummy pattern improved the height uniformity of the bumps. Pull test results showed that the shear strength of the Cu bump (around 45 gf) was obtained by the soft-etching pretreatment of the Cu foil surface. Sn-Zn, Sn-Ag, Sn-Cu, and Sn-Bi platings were deposited as Pb-free solders with near-eutectic compositions. In particular, the interface properties of Sn-Zn solder plating/Cu were investigated after reflowing in this study, as well as the effect of Ni plating as an under ball metallurgy layer on compound formation at the interface. It was found that a compound of Y-Cu ₅Zn_g layer existed in the interface of the reflowed Sn-Zn plating/Cu. The Ni plating layer inhibited the formation of the interface Y-Cu₅Zn_g, compound with reflowing.