Optimization of FC-CSP and MLCC Soldering Process Using Type 7 Solder Paste

Abstract

To confirm the possibility of simultaneous bonding between Si chip and multilayer ceramic capacitors (MLCCs), vacuum reflow soldering is performed between a Si chip and each of 2012, 1608, 1005, 0603, and 0402 MLCCs using Type-7 Sn-3.0Ag-0.5Cu (SAC305) and Sn-0.7Cu solder pastes. To enable an unbiased comparison with the vacuum reflow process, the Si chip and MLCCs are bonded via thermo-compression (TC) bonding and hot air reflow soldering, respectively. The maximum void content of Si chip solder joints bonded via vacuum reflow is 8%. At the MLCC solder joints, the void content yielded by vacuum reflow is lower than that yielded by hot air reflow. The bonding strength of the MLCC solder joint afforded by vacuum reflow is higher than that afforded by hot air reflow. Vacuum reflow is superior to hot air reflow in terms of the void content and bonding strength of the MLCC solder joint. After performing a thermal cycle test (TCT), a crack appeared at the Si chip solder joint bonded by TC bonding. The Cu pillar/Sn-2.5Ag bump is susceptible to cracks after the TCT owing to the insufficient bonding layer thickness yielded; however, the SAC305 sample from vacuum reflow is stable. Meanwhile, (Cu,Ni)6Sn5, Ni3P, and Ag3Sn intermetallic compounds are formed at the MLCC solder joints and then further developed after the TCT. The results of this study indicate the possibility of co-bonding Si chip with MLCCs for semiconductor packaging.