Growth behavior of the tin oxide layer on Sn–3.0Ag–0.5Cu solder and its impact on the rework reliability of hybrid BGA packages in HBM applications

In this work, the oxidation behavior of the Sn–3.0Ag–0.5Cu (SAC305) solder alloy was investigated under 85°C/85% relative humidity conditions with varying aging durations. The morphological and compositional evolution of generated oxide layers was characterized via transmission electron microscopy, electron energy loss spectroscopy, and X-ray photoelectron spectroscopy. During the early stage of aging, a SnO layer formed and exhibited linear thickness growth up to 72 h due to the reaction between Sn and O2. With prolonged aging, the oxide layer evolved into a bilayer structure comprising an inner SnO layer and an outer SnO2 layer. The slow growth of the SnO2 layer in this stage was due to the hindered diffusion of Sn cations from the SnO/Sn region to the surface. The outer SnO2 layer induced nonwetting between SAC305 solder balls and the Sn57Bi1Ag solder paste, leading to head-in-pillow (HiP) defects in hybrid ball grid array (BGA) packages. These HiP defects considerably reduced shear strength, drop-impact reliability, and electrical conductivity of the packages. A buffered oxide etchant effectively removed the tin oxide layers and restored the electrical resistance, shear strength, and drop-impact reliability of the hybrid BGA packages to values comparable to corresponding initial ones. These results clarify the oxidation behavior of the SAC305 solder alloy and its effect on the reliability of hybrid BGA packages under humid conditions, demonstrating the feasibility of enhancing rework reliability through selective oxide removal in electronic packaging.