Thermo-setting polymers are widely used as underfill materials to improve the reliability of electronic packages. In the design phase, the influence of underfill applications on reliability is often judged through thermal and mechanical simulations, under assumed operating conditions. Because of lacking insight into the mechanical processes due to polymer curing, the impact of processing induced residual stress fields is often neglected. To investigate the evolution of stress and strain fields during the curing process it is important to assume a more appropriate starting point for subsequent process modeling. Furthermore, study of possible damage originating from the fabrication process then comes within reach. To facilitate future analysis of stress and strain fields during the curing process a cure dependent constitutive relation is assumed. An approximate investigation method for the process-dependent mechanical properties, based on Dynamic Mechanic Analysis (DMA), is developed. As an illustration the parameter identification is performed for a selected epoxy resin.
Moulding compounds are used as encapsulation materials for electronic components. Their task is to protect the components from mechanical shocks and environmental effects such as moisture. Moulding compounds are epoxy resins filled with inorganic (silica) particles, carbon black and processing aids. They shows a clear viscoelastic behaviour which is not only temperature but also cure dependent. Due to both thermal and reaction shrinkage moulding compounds introduce residual stresses which may eventually result in product failure.Therefore they can be considered as key materials for the overall thermomechanical reliability. This paper deals with the characterization and modeling of the mechanical behaviour of such moulding compound. The focus will be on the effects of the degree of cure and the filler concentration.
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...
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