Experimental Evidence of Underfill Voiding and Delamination during Board Level Assembly of Pb-free Solders

Summary form only given. In this paper, the underfill failure such as void and delamination is shown in the Pb-free flipchip package through solder ball reflow process. Since most of Pb-free solder candidates have similar melting point, Pb-free solder bump can melt again during board level Pb-free BGA reflow cycle. From the phase change of Pb-free solder bump, the volume expansion puts pressure on the surrounding materials, i.e. underfill, chip and substrate. This unique phenomenon is tested by board level solder ball reflow and the changes after reflow are presented by using X-ray and cross-section pictures. In addition, the volume expansion rate of pure tin is measured to confirm the effect of phase change of Pb-free solder