Different conservation laws utilized for warpage prediction of MUF FCCSP with 4L ETS

Abstract

While the density of copper line pitch of substrate has faced a significant challenge, such as photo resist mold of 20μm pitch line, for manufacturers over the past decade, the subtractive method can be overcome by using semi-additive method. However, subtractive method is cost effective and most widely used on copper line formation, and then an emergent need of a significant change at substrate level is developed to improve both production yield and capability. As a result, embedded trace substrate (ETS) structure can replace the semi-additive method due to increase the adhesion between copper trace and prepreg (PP) material, especially for next generation requirement on further fine design of line / space (L/S). Thermal induced stress utilized to design warpage of ETS package in manufacturing process, especially for unsymmetrical structure, are becoming increasingly important due to every more stringent electronic product requirements. Although the effect of time-dependent properties can be further aggravated in glass transition temperature (Tg) neighborhood especially for epoxy molding compound (EMC), sometimes it can be neglected due to many process temperatures of component assembly is not time-dependent process and difference in a few seconds in general. In recent years, the temperature-dependent properties based on strain-, strain-stress, and stress-conservation laws has become a vital and effective methodologies for electronic package design to offer sufficient insight and understand about the warpage behavior during reflow process. Furthermore, the FEA (Finite Element Analysis) is capable of mathematically simulation irregular, complex geometry, thus an accurate and rapid methodology are thirsted for engineers in manufacturing factory due to less time-consuming and manpower-loading. In this paper, a non-incremental solution based on stress conservation law has been developed, and then processing model can been derived continuously in non-incremental formula. Different to past methodologies which non-incremental and incremental solutions are only responsible to strain and strain-stress conservation laws, respectively. In FEA for the former, element birth and death utilized in processing model can be treated as non-increment solution by assigning different reference temperatures (Tref) associated to different materials, where mean of CTE (CTEmean) can be obtained by average dimension change integrated from Tref to uniform temperature (Tuni). For the later, incremental solution utilized to perform temperature-dependent properties associated to package composite in each temperature span, and then to superpose nodal displacement of each temperature span as global and local analysis. Compare to incremental solution, although non-incremental solution based on strain conservation is not popular but also save time calculated by professional computer, especially for high density or fine increment of data described in temperature-dependent properties. Different to stress conservation law, non-incremental solution based on strain conservation law only considers the target modulus E(Tuni) regardless of the different path of modulus from E(Tref) to E(Tuni), where the same result will be obtained if E(Tuni) of these two materials are the same. Thus the warpage in Tg neighborhood would cause cutthroat change for strain conservation law, but not always has apparent variation for strain-stress conservation, where usually presented smooth deviation for stress conservation law. Finally, the closer of this study provided the products of MUF FCCSP (Molded underfill Flip-Chip Chip Scale Package) with two different compound types for validation on warpage from 25°C to 260°C, and the results showed that stress conservation law has good agreements with measured data by technique of phase-shifting shadow moiré for these two products in spite of strain conservation law has only a good correlation for which one product.