Electromigration Performance of Fine-Line Cu Redistribution Layer (RDL) for High-Density Fan-Out Packaging

The width of the copper (Cu) redistribution layer (RDL) for High-Density Fan-Out (HDFO) packaging gets smaller and smaller to achieve higher input/output (I/O) capability. The downsizing trend generates high current and brings electromigration (EM) concerns. This occurs because electromigration is a diffusion-controlled mechanism that results in momentum transfer of electrons through the conductive layer. Recently, it has been reported that EM failures in Cu RDL are attributed to not only electrical potential but also thermal energy. In this investigation, electromigration characterization was conducted under various current and temperature conditions to understand the electromigration reliability performance of fine Cu RDL with ≤10 µm in an HDFO package. Also, the impact of current and temperature on Cu RDL and passivation were observed through focused ion beam (FIB)/field emission scanning electron microscopy (FESEM) analysis. Lastly, the maximum current capacity of a fine-line Cu RDL was estimated under actual use conditions such as operating temperature and lifetime using Black's model with experimental activation energy (Ea) and current density exponent (n factor) parameters.