During the encapsulation of electronic components stresses are generated due to curing effects and the difference in thermal shrinkage between molding compound and die.@en

Warpage is a critical issue for a QFN panel molding process. Much work was done in the past to predict the warpage of a package during cooling down from molding temperature. However, until now, warpage could not always be predicted well, even if the viscoelastic behavior of the molding compound is taken into account. It was for example observed that the cooling velocity affected the warpage after cooling down. Because of this reason, the mechanical behavior of the molding compound was investigated in more detail. In this research, the mechanical properties of the molding compound are determined. It turned out that the properties are highly dependent on time and temperature. A complete viscoelastic model of the model compound is achieved by combining DMA and dilatometric test results. The model is implemented in the finite element software ABAQUS. In this study, our advanced model is compared with elastic calculations which are normally done. A validation experiment is performed in which simulation results are compared with experimental warpage data of a double layered beam, consisting of a layer of molding compound and a layer of silicon. This beam is cooled down from a temperature above Tg to room temperature with different cooling rates. In the meantime warpage is measured and compared to simulation results. Finally, the advanced material model is used for calculations on a QFN-panel.@en

During the encapsulation of electronic components stresses are generated due to curing effects and the difference in thermal shrinkage between molding compound and die. These residual stresses add up to the stresses generated during thermal cycling and mechanical loading and may eventually lead to product failure. In this paper we focus on three commercial molding compounds and analyze in detail the increase in elastic modulus and the change in viscoelastic behaviour during cure. This was done with a special shear tool which allows to measure mechanical properties with sufficient accuracy in the liquid as well as in the solid state. The cure dependent viscoelastic material behaviour was modeled using a cure dependent shift factor and rubber modulus. The visoelastic behaviour of the molding compounds is also shown not to be stable. During postcure the materials slowly continue to crosslink thereby systematically changing their viscoelastic behaviour. The material models presented here therefore only account for the initial curing stage and do not include postcure.@en

Reliability calculations of the microelectronic packages require cure dependent viscoelastic constitutive relationship for the packaging polymers in order to predict residual stress and strain fields in the final product. The residual stresses can result in the product failure due to warpage, interfacial delamination, thermal fatigue etc. The previous work in the same group (Ernst et al. 2006, 2003) towards complete viscoelastic model development was done using "approximate fully cure dependent" model in order to predict warpage of the QFN package. The relaxation shear modulus was accurately established but due to measurement limitation only estimated values of bulk modulus could be used. In the present work, a high pressure dilatometer (Gnomix PVT apparatus) was used to establish the time, temperature and pressure dependence of bulk modulus. The bulk modulus shows negligible time dependence which suggests that bulk modulus is not a viscoelastic but merely a temperature dependent linear elastic parameter. A material model for time, temperature and pressure dependency of the bulk Modulus is developed.@en