Effects of material properties on PoP top package warpage behaviors

Abstract

Top package with stacked memory die connected to a bottom logic package via solder balls is a representative PoP configuration. This POP configuration has become very popular in mobile phones. and the future trend in mobile phone systems is targeting one mm as the combined thickness of logic and memory packages. To guarantee the assembly yield and reliability of the solder joint between such thin top package and bottom package, mechanical compliance between these two packages is crucial during package stacking. Therefore package warpage needs to be understood and controlled to meet the assembly yield target. In this study we focus our efforts studying the effect of material properties on the top memory package. As the complexity of the package configuration increases by thinner package thickness, higher number of stacking dies and large package size, the warpage control within the target requirement is very challenging, especially when the material behaviors of substrate, die, molding compound, die attach film and substrate materials are different and also changing as a function of temperature. The thermo-mechanical properties of molding compounds and substrate materials are very important and mainly determine the warpage level of the Top package at room temperature as well as the warpage behaviors during the reflow profile for the board level assembly with bottom package. Among the material properties, the chemical cure shrinkage, CTE and storage modulus for the molding compound; CTE and modulus for the substrate core materials and solder mask layer; are determining factors on the temperature dependant warpage of top PoP package. In this paper, the effect of molding compound material and substrate material properties on PoP top package warpage behaviors during solder reflow process are investigated in detail through comprehensive experiment and modeling work. The test vehicles are used for different package Z-height and different molding compound/substrate materials. The findings form this study provide some clues and guideline for material property selection for warpage control in Top PoP package, which influence the PoP assembly yield and reliability.