Shear strength of Sn–3.5Ag solder bumps formed on Ni/Au and organic solderability preservative surface-finished bond pads after multiple reflow steps

Sn–3.5Ag solder bumps were formed on electroless Ni/immersion Au (Ni/Au) and organic solderability preservative (OSP) surface-finished bond pads, respectively. The shear strength of the solder bumps was measured as a function of reflow steps. Fracture surfaces and interfacial microstructures were investigated by scanning electron microscope. The shear strength of Ni/Au samples increased up to the seventh reflow step and subsequently decreased after the tenth reflow step. Spalling of Ni 3 Sn 4 intermetallic compounds (IMCs) and the P-rich Ni layer strengthened and weakened the bond, respectively. For OSP samples, although Cu 6 Sn 5 IMCs grew as the reflow step was repeated, no remarkable change in shear strength was observed. Interfacial fractures of OSP samples occurred at the interface between Cu 6 Sn 5 IMC and Cu 3 Sn IMC. Fracture surfaces of OSP samples showed concave pits that consisted of a Cu 3 Sn bottom and an Sn wall. The pits were formed by separation of Cu 6 Sn 5 IMC from Cu 3 Sn IMC and the molten Sn channel between the Cu 6 Sn 5 IMC grains.