Effect of Surface Finish on Electromigration Reliability of Line-Type Sn-3.0Ag-0.5Cu Solder Joints

The electromigration (EM) reliability of line-type Sn-3.0Ag-0.5Cu (SAC305) solder joints with surface finishes of organic solderability preservatives (OSP), electroless nickel/electroless palladium/immersion Gold (ENEPIG), and Electroplated Ni was systematically investigated under a current density of 1.0 × 10⁴ A/cm² at 150 °C. In the as-soldered state, scallop-shaped Cu₆Sn₅ grains formed at both interfaces of the OSP joints. In contrast, driven by the Cu-Ni interaction, (Cu,Ni)₆Sn₅ grains formed at both interfaces in the ENEPIG and Electroplated Ni joints, with their morphology varying with Ni content and exhibiting polyhedral, prismatic, or scallop-like shapes. When the surface finishes served as the cathode, the failure rate of the OSP joints (100%) was 1.2 times that of the ENEPIG joints (83%) and 2.0 times that of the Electroplated Ni joints (50%). Extensive dissolution of the Cu substrate caused the rapid failure of the OSP joints, whereas both Ni-P and Ni layers effectively blocked the diffusion of Cu atoms. In the ENEPIG joints, the consumption of Ni atoms transformed the amorphous Ni-P layer into a porous Ni₃P layer, leading to joint failure. Electroplated Ni possesses a more stable crystalline structure and stronger atomic bonding, so more energy is required to release Ni atoms, resulting in a slower dissolution rate. Compared with the OSP joints, the EM lifetimes of the ENEPIG joints and Electroplated Ni joints were improved by 30% and 53%, respectively.