A huge shift in classical system integration composed of heterogeneous and homogeneous substances from 2D to 2.5D or even 3D assembly is a promising solution to satisfying the requirements of electronic packages, such as high operating speed, multifunctionality, and low form factor, under the physical limits of nanoscaled transistors and bottlenecks in related fabrication technologies. However, considerable dimension mismatch is introduced into the entire packaging by numerous microbumps ( ${\mu }$ -bumps) with complicated compositions in existing frameworks. In addition, failure location and precise reliability estimation are difficult to achieve when the layout of high-density ${\mu }$ -bump arrays is considered. To solve this urgent issue, an equivalent material characteristic methodology for ${\mu }$ -bump arrays based on finite element analysis (FEA) is developed in this study. A test vehicle for chip-on-chip packaging with wafer-level underfill is set up to obtain the mechanical characteristics of equivalent materials for the portion with internal ${\mu }$ -bump arrays. The equivalent plastic strain generation of the tin layer is simulated by systematically comparing the numerical convergence of FEA under the assumption that the lead-free solder forms ${\mu }$ -bumps within after the assembly process. Analytic results indicate that at least four rows of actual ${\mu }$ -bumps from the outermost edge of the ${\mu }$ -bump array are required to ensure numerical accuracy in contrast with the result obtained from the actual model of the present packaging structure.