Via plugging process has been demonstrated to be beneficial to improve the overall hardness of PBGA substrate due to the IR-6 material properties. This increase of hardness helps in molding process due to the nature of mold that clamp at high pressure. A harder material reduces the potential of solder resist crack. A simplified composite model using the "Rule of Mixture" successfully established to understand the characteristic of the substrate hardness as more IR-6 pillars included into the substrate. Addition of copper dummy traces on the air venting area improve the solder resist evenness.