Fully Cure-Dependent Modeling and Characterization of EMC's with Application to Package Warpage Simulation

Abstract

In order to establish the possible influence of residual stressand strain fields after cure on the failure prediction of electronic packages, cure-dependent viscoelastic constitutive relations for the applied polymers are required. This paper gives an overview of progress in the field of viscoelastic modeling and characterization of applied polymers over the past few years. It also discusses limitations and needs for future development.1. Cure dependent visco-elastic modeling Processing induced stresses are more and more recognized as an important factor for IC-package yield and reliability. Previously, a stress-free state after curing at the curing temperature was commonly assumed, merely because adequate modeling of polymer behavior during cure was not feasible, rather than that evidence of such a stress-free state was ever found. Using the "stress-free" assumption, failure initiated during the fabrication process such as excessive package warpage, die cracking and interface delamination, can not be explained.During the cure, thermosetting polymers gradually transform from the more or less liquid state into a viscoelastic solid with a relatively high glassy modulus, accompanied with increasing chemical shrinkage. Due to these phenomena stresses will arise in areas where shrinkage is (partly) prevented by geometrical constraints. Furthermore, if the cross-linked product is cooled down to ambient temperature, the difference in thermal expansion between matrix and adjacent materials will cause additional stresses and deformation.In the past decade, various efforts have been made to understand the occurrence of residual stress and/or processinduced warpage in packages. Many researchers just apply linear elastic models combined with some estimate of initial strain due to chemical shrinkage, in their simulations of the residual stress state. Only for cure at elevated temperatures, far above the ultimate glass transition temperature, where the polymer merely is in its rubbery state during cure, such simulations have proper meaning. However, in many cure procedures, curing temperatures are within the visco-elastic region such that proper visco-elastic modeling and characterization is required.Pioneering work in visco-elastic modeling and characterization of curing polymers was reported by Adolf et al. [1,2]. Here a deviatoric stress relationship is proposed, that includes a shear relaxation modulus that is dependent of the momentary degree of cure:A more or less similar description, but extended to fully isotropic behavior was proposed by Kiasat [3]. This model requires both a cure dependent shear modulus and a cure dependent bulk modulus. Since in these models temperature history effects and curing history effects are not involved, in the sequence these models are referred to as "partly cure dependent" models.On the basis of the Kiasat model, in previous work, Ernst et al. [5] applied a "partly cure-dependent" viscoelastic model to describe the evolution of the stress/strain state in a flip chip package during isoth...

Prediction Models for Distortions and Residual Stresses in Thermoset Polymer Laminates: An Overview

“…Some researchers have assumed a linear relationship between the stiffness and degree of cure [15] neglecting the glass transition temperature, viscoelastic relaxation, and temperature dependency. All the latter effects have been taken into account in some other works [55,73,74].…”

“…Based on the characterisations, they developed models to predict the warpage and residual stresses. For the details of such modelling and experimental achievements, the reader is recommended to have a look at their published papers [69,74,80,95,96,[105][106][107][108][109][110][111][112][113]. For example, Jansen et al, in 2012 and 2013, developed an analytic model based on classical laminate theory (CLT) for single and multi-layered coating layers used in electronic packaging and predicted the resulting warpage due to the curing process [75,114].…”