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 mu m thick Ni on a 220 mu m Cu core ball, three types of CCSBs (240, 260, and 280 mu m) 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 mu m to 3.48 mu m with increasing thickness of the SAC 305 plated layer, and thickness of Cu6Sn5 increased from 4.22 mu m to 5.97 mu m, 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.
