High density packaging is developing toward miniaturization and integration,which causes many difficulties in designing,manufacturing,and reliability testing.Package-on-Package(PoP) is a promising three-dimensional highdensity packaging method that integrates a chip scale package(CSP) in the top package and a fine-pitch ball grid array (FBGA) in the bottom package.In this paper,in-situ scanning electron microscopy(SEM) observation is carried out to detect the deformation and damage of the PoP structure under three-point bending loading.The results indicate that the cracks occur in the die of the top package,then cause the crack deflection and bridging in the die attaching layer.Furthermore,the mechanical principles are used to analyse the cracking process of the PoP structure based on the multi-layer laminating hypothesis and the theoretical analysis results are found to be in good agreement with the experimental results. High density packaging is developing toward miniaturization and integration, which causes many challenges in designing, manufacturing, and reliability testing. Package-on-Package (PoP) is a promising three-dimensional high density packaging method that integrates a chip scale package the top package and a fine-pitch ball grid array (FBGA) in the bottom package. In this paper, in-situ scanning electron microscopy (SEM) observation is carried out to detect the deformation and damage of the PoP structure under three-point bending loading the results that the cracks occur in the die of the top package, then cause the crack deflection and bridging in the die attaching layer. Hotter, the mechanical principles are used to analyze the cracking process of the PoP structure based on the multi-layer laminating hypothesis and the theoretical analysis results are found to be in good agreement with the experimental results.