Effect of thickness in the SAC 305 plated layer on the interfacial reaction and mechanical properties of the CCSB joint assembled onto the OSP surface finished FR-4 PCB board

The interfacial reaction and the mechanical properties of the Cu core solder balls (CCSB) assembled on the organic solderability preservative (OSP) surface finished FR-4 substrate were investigated with various thicknesses of SAC 305 plated on Cu balls. After electroplating 2 μm thick Ni on a 220 μm Cu core ball, three types of CCSBs (240, 260, and 280 μm) were fabricated by electroplating SAC 305 (Sn 3.0 Ag 0.5 Cu solder) on the top of the Ni-plated Cu core ball. After reflow, (Cu,Ni)₆Sn₅ intermetallic compound (IMC) was formed at the interface of the SAC 305 plated-layer and Ni plated Cu core ball, and Cu₆Sn₅ IMC was formed at the interface of the SAC 305-plated layer and OSP surface finished FR-4 board. The thickness of (Cu,Ni)₆Sn₅ decreased from 6.52 μm to 3.48 μm with increasing thickness of the SAC 305 plated layer, and thickness of Cu₆Sn₅ increased from 4.22 μm to 5.97 μm, respectively. After the third reflow, the shear strength of the CCSB joint decreased from 34.5 MPa to 25.2 MPa as the thickness of the SAC 305-plated layer increased because of the thick Cu₆Sn₅. The crack in the CCSB joints propagated mainly at the interface of the SAC 305-plated layer and Cu₆Sn₅ formed on the OSP surface finished FR-4 substrate.