In recent years, Package on Package (PoP) is increasingly used for high density package solutions. Generally the top package is a stacked memory packaging system connected to a bottom logic packaging system via solder joint: this is representative of PoP configurations. To guarantee the assembly yield and reliability of the solder joint between the top package and bottom package, mechanical compliance between these two packages is crucial during package stacking. Henceforth package warpage needs to be understood and controlled to meet the assembly yield targets. The complexity of the package configuration increases by thinner package thickness, higher number of stacking dies and large package size. Controlling the warpage within the target requirement is very challenging, especially when the material behaviors of substrate, die, molding compound and die attach film are different and also changing as a function of temperature. Certainly, the material properties of key components in top PoP package plays a crucial role in warpage performance. Among various material properties, the chemical cure shrinkage, coefficient of thermal expansion and storage modulus for the molding compounds are determining factors on the temperature dependant warpage control of top PoP package. Warpage variation still exists within parts processed at the same time mainly due to slight material property variation. In this paper, the cause of the warpage variation is investigated. The main cause was found to be filler migration effect in narrow gaps with in the stacked die package during the mold process, which resulted in different filler concentration and distribution, and finally different local molding compound material property among the package unit location in the substrate strip. The findings indicate that mold pressure is not a major modulator of warpage, while filler distribution can dramatically alter the warpage behavior. FEA model results and warpage data are presented to validate the filler migration phenomena and warpage behavior impact. The findings and results provide some clues and design/process guideline for warpage control in Top PoP package, which influence the PoP assembly yield and reliability.
Thickness-shear frequencies of an infinite quartz plate with material property variation along the thickness
