Ultra thin PoP top package using compression mold: Its warpage control

Abstract

Package on Package (PoP) is one of the increasingly used high density package solution for package stacking technology in various device manufacturing applications. Normally PoP configurations use a top package with a stacked memory packaging system connected to a bottom logic packaging system via solder joint. 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. The industry is constantly working toward reducing the total stack height and current road maps point toward a thinner total stack Z height of PoP configuration. It is necessary to reduce the package height by reducing the mold cap thickness as well as reduce the mold cap clearance, the distance between the top silicon die to the mold top. However, it is one of the increasing challenges to reduce top package thickness maintaining its low warpage behaviors during the reflow process and to avoid bonding wire exposure after laser mark process due to thin mold cap clearance. In this paper, we used the compression mold process to fabricate PoP top package test vehicle with different mold cap thickness at around 80 um mold cap clearance on 14×14 package size, and investigated the effects of various molding compound materials and substrate material properties on PoP top package warpage behaviors during solder reflow process. The substrates are all 100 um thickness and 2 layer structure. As the mold cap thickness gets thinner, the warpage variation during reflow process gets larger. The material property of key component in top PoP package plays a crucial role in warpage performance. The thermo-mechanical properties of molding compounds after compression molding process and substrate materials are very important and mainly determine the warpage level of the top package at room temperature and the warpage behaviors during the reflow profile for the board level assembly with bottom package. The compression mold compound material properties and substrate core materials were optimized to improve the ultra thin PoP top package. The findings and results provide a guideline of the material selection for warpage control in the PoP top package to improve the PoP assembly yield and reliability.