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Package-on-Package(PoP) technology is being used for high density designed and miniaturized electronic applications. In order to develop these small and complicated structured packages, it is important to predict accurately the structural behavior under various thermal conditions. For example, the package warpage control at solder melting temperature is required for the card or sub-package assembly and also the mechanical stress of the joint needs to be taken care of for the jointing reliability. The FEM analysis has been used to understand these behaviors. To get a good model correlation with an actual package, it is most important to use the faithful material property data on the analysis model. On the other hand, if the interposer card is modeled in detail, it must have huge number of mesh and much calculation time is required to solve. The simplified model is preferable to avoid that situation. In general, each interposer card has a different property by application even the package structure to be used is same. The property is calculated to have good accuracy of the rule of mixture (ROM) that can estimate the behavior as a composite material consists of copper wiring, several resins and FR4. In this paper, the ROM was confirmed by measurement with DIC equipment on different circuitry patterned interposer cards, and the behavior dependencies on the wiring density and the warpage are discussed.