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 Cu
6
Sn
5
-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 Cu
3
Sn layer plus two-layer IMCs stacking of Cu
6
Sn
5
/Cu
3
Sn are the main causes of ball drop-off for Organic Solderability Preservative (OSP) pad.