Review on the Ball Drop-off Mechanisms of Lead-free Ball Grid Array Package

Missing ball at ppm-level has been an issue in the industry. Several mechanisms are reviewed in this paper. In general, for all surface finishes, fluxing problem is the main cause of missing ball right after BGA assembly process. Massive spalling of Cu₆Sn₅-based IMC is the unique cause for SnAgCu (SAC) solder ball reacting with electrolytic Ni/Au or Electroless Nickel Immersion Gold (ENIG) surface finished pads. Black pad, Kirkendall voids in NiSnP layer, and complicated IMCs stacking at the interface make ENIG a more vulnerable type of surface finish subjecting to ball drop-off problem. Kirkendall voids in Cu₃Sn layer plus two-layer IMCs stacking of Cu₆Sn₅/Cu₃Sn are the main causes of ball drop-off for Organic Solderability Preservative (OSP) pad.