Warpage of micro-electronics caused by the curing process and thermal cycling is of major importance in electronic packaging. Industry is availed by good methods to be able to predict warpage accurately. The main difficulty for prediction of warpage is caused by the complicated material behavior of molding compound. It turns out that the mechancial behavior of molding compound is dependent on time, temperature and degree of conversion. Since molding compound is available in large variabilities, for each type the model parameters should be established experimentally. In our group an efficient method is developed for experimentally determining the model parameters [1, 2, 3]. In this approach, different experimental results have to be combined in order to achieve the final material model. Since this material model is not standard, user-subroutines are used for implementation in finite element software (ABAQUS). In this paper we present validation experiments which are done in order to verify the material model. A TDM (Topography and Deformation Device) is used to measure the curvature of a mold-map at different temperatures. Good agreement between experiment and simulations results is achieved.@en

Reliability and functionality of microelectronic devices is highly influenced by their packaging material. For reliable predictions of stresses in microelectronics, properties of the packaging material have to be known. In this work the moisture absorption and desorption characteristics of epoxy molding compound is investigated by making use of accurate sorption equipment. It is found that the diffusion coefficient is dependent on humidity level. Moisture induced swelling is measured by making use of a DMA at which a humidity generator is connected. By clamping a sample strip in the DMA and applying a humidity step, the sample will deform. This deformation is measured by the DMA and is recalculated to a strain. In this way, moisture absorption experiments could be done at different temperature and humidity levels. It is found that the swelling coefficient is increasing at higher temperatures. Results of both absorption and moisture induced swelling experiments are modeled by modifying Fick's law. Finally creep experiments are performed in order to measure the mechanical behavior of molding compounds at different temperature and relative humidity levels. These results and the obtained models are discussed in this paper. Index terms-Creep measurements, hygroscopic swelling, moisture absorption, viscoelasticity.@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

An electronic device cannot perform its designed functions until it is packaged such that it is interconnected with the rest of the system and protected. As an encapsulation material, thermosetting polymers are widely used. It is well known that properties of polymerbased composites like molding compounds are highly affected by the influence of temperature, relative humidity and degree of conversion. The effect of above mentioned internal and external circumstances are investigated extensively in the past. Surprisingly the effect of high temperature storage on the mechanical properties is scarcely studied. From literatures research it is concluded that high temperature storage and postcure treatments increases the glass transition temperature. Also a weight loss during high temperature storage is reported [1], [2]. Since thermal treatments are very common in micro electronic industry, it is relevant to understand in more detail the influence of thermal aging on the mechanical properties of molding compounds. In this research the effect of postcure and thermal aging on the mechanical properties of molding compounds is systematically studied. It turns out that postcure and thermal aging cause a broadening of the viscoelastic region, an increase in Tg, and increase in rubbery modulus and ongoing shrinkage of the molding compound. It is concluded that the change in properties can be attributed to a reaction of the molding compound with oxygen. This paper presents the observed change in properties caused by oxidation.@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

Prediction of residual stresses in micro-electronic devises is an important issue. Virtual prototyping is used to minimize residual stresses in order to prevent failure or malfunction of electronic products. Already during encapsulation stresses build up due to polymerization induced shrinkage of the molding compound. Differences in coefficient of thermal expansion of the involved materials cause additional stresses during cooling down from molding to ambient temperature. Since industry is availed by reliable prediction methods, detailed material models are required. In electronic packaging, mechanical properties of most of the involved materials have constant mechanical properties. However, the viscoelastic properties of the encapsulation material depends highly on temperature and degree of cure. Reliable predictions of residual stresses require simulation models which take into account the effect of temperature and conversion level. In this paper, properties of molding compound are discussed which are relevant for the prediction of warpage of micro-electronics products. The models for the individual properties are combined to one single model suitable for finite element simulations. The numerical implementation in finite element code is not standard and is done by using user-subroutines. Validation experiments are performed in order to verify the developed material model which is done by measuring and predicting the warpage of a mold map. A Topography and Deformation Measurement (TDM) device is used to measure the deformations at elevated temperatures in a non-intrusive way such that the developed material model could be validated in a broad range of temperature. Finally, simulations are carried out with simplified material models of molding compound. The results of these simulations are compared with results obtained with the cure dependent viscoelastic model and real warpage data. From these comparisons it is concluded that for reliable prediction of warpage, the cure dependent viscoelastic model is has to be used.@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.@en

The mechanical properties of polymer encapsulating materials change considerably when packages are exposed to elevated temperatures for longer times. Here we report a systematic study on the change in viscoelastic properties of a commercial molding compound which is stored at 175°C for periods of time ranging up to 4 weeks. It is shown that instead of the expected degradation, the modulus and the glass transition temperature increased and that these effects are accompanied with a relatively large amount of shrinkage. All effects can be interpreted in terms of the formation of an oxidized layer where extra crosslinking occurred.@en

Thermosetting polymers are widely used in electronic industry as encapsulants of electronic devices. It is well known that properties of polymers and polymer-based composites like molding compounds are highly dependent on conditions like: temperature, time, humidity, degree of cure etc. These effects are investigated extensively in the past. Surprisingly, the effect of postcure and thermal aging on the thermomechanical properties of molding compound is scarcely studied. Some studies are devoted to this topic but are not systematically carried out. The main conclusion of previous research is that the glass-transition temperature increases at increasing postcure time [1]. Also weight loss during aging is reported [2]. Since thermomechanical properties determine mainly the reliability of electronic devices it is essential to have knowledge of the effect of post mold cure treatment and thermal aging. In this research the influence of postcure and thermal aging is studied in a systematic way. It turns out that postcure and thermal aging treatment causes an increase in Tg, a change in viscoelastic behavior, an increase of the rubbery modulus, and ongoing shrinkage of the molding compound. The change of these properties is attributed to the oxidation of the molding compound at high temperatures.@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