CSP (Chip Scale Package) and/or FCA (Flip Chip Attach) are becoming the important new high-density packaging technology for the consumer electric products. In addition, the use of build-up boards for mounting the packages has been tried. However, the reliability of the solder connection depends on the structure of the package, the motherboard, and the material properties. In this paper, the solder connection reliability for CSP and FCA packages, mounted on the build-up board, is assessed. The results based on optimizing each of the parameters are reported. The most reliable packaging design and a compact FEM (Finite Element Method) model for the reliability assessment are suggested. The verification of the numerical analysis results using tests on the actual hardware is also shown. On these results, this FEM model is applied to the reliability assessment of MCM with the evaluation of the warpage and Encapsulant volume.