Mechanical property and plated solder volume effect of Cu core ball

Mechanical property of Cu core solder ball (CCSB) was investigated with different plated SAC305 solder volume and thickness at the ball-grid-array (BGA) after reflow. Cu core ball size was $220\mu m$ and plated SAC305 on Cu core ball was tested the 3 kinds of thickness such as $8\mu m$, $18\mu m$, $28\mu m$ with $Ni\mu m$ on the Cu ball surface. The (Cu, Ni)6 Sn5 interfacial IMCs were formed at interface between Cu/Ni core ball and plated solder layer after reflow. In addition, (Cu, Ni)6 Sn5 IMC was also observed at interface between plated solder and Cu-OSP pad after reflow. And these interfacial IMCs were increased after multiple reflow. Additionally, thicker SAC plating layer has higher IMC growth rate than thinner plated CCSB. In the Cu core ball joint property, thicker SAC plated layer is, the higher ductile deformation length by ball shear test (BST). And thinner SAC plated CCSB showed larger reduction rate of ductile deformation length than thicker SAC plated CCSB at BST after multiple reflow. In SMT test, minimum plated solder volume and thickness was conformed and calculated to get the stable SMT yield after flip chip process. So, when targeting gap size between PKG and interposer and solder resist opening (SRO) size were designed for PoP PKG development, it is possible to calculation of Cu core ball size and needed minimum plated volume to get good workability yield at flip chip bonding process.