Corrosion of Aluminum Pad in Fan-Out Panel Level Packaging (FOPLP)

In chip-first fan-out panel-level packaging (FOPLP), redistribution layers (RDLs) are formed on top of the die, with the initial interconnection between the RDL and die established on aluminum (Al) pads through lithography, seed metal deposition, and copper electroplating. However, Al pads are susceptible to pitting corrosion under both acidic and alkaline conditions. In this study, we investigated the corrosion behavior of Al pads from various suppliers before and after deionized water (DIW) rinsing and treatment with 2.38% tetramethyl ammonium hydroxide (TMAH), a common solution used in the RDL formation process. The microstructure and chemical composition of the Al pads were analyzed using focused ion beam (FIB), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and electron backscatter diffraction (EBSD) techniques. Our results indicate that pitting corrosion of Al pads is influenced by multiple factors, including fluorine concentration, alloying elements, crystallographic orientation, heat treatment history, and storage conditions. Notably, Al pads with higher fluorine content and significant grain boundary disorientation exhibited more severe pitting, especially after alkaline treatment, highlighting the role of both chemical composition and microstructural properties in the corrosion susceptibility of Al pads.