The stringent and conflicting requirements imposed on optomechanical instruments and the ever-increasing need for higher resolution and quality imagery demands a tightly integrated system design approach. Our aim is to drive the thermomechanical design of multiple components through the optical performance of the complete system. To this end, we propose a new method combining structural-thermal-optical performance analysis and topology optimization while taking into account both component and system level constraints. A 2D two-mirror example demonstrates that the proposed approach is able to improve the system’s spot size error by 95% compared to uncoupled system optimization while satisfying equivalent constraints.

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Warpage of flat packages is partly due to chemical shrinkage of the molding compound during cure and partly due to differences in thermal contraction in the subsequent cooling stage. The latter effect is relatively easy to incorporate in numerical simulations but the cureinduced shrinkage effect is not and is therefore often neglected in warpage simulations. Recent validation studies however showed that it is essential that both effects are taken into account. In this paper we develop explicit analytical expressions for the cure-induced residual stresses and warpage of a simple bilayer construction. We do this by assuming that curing always takes place above the molding compounds glass transition temperature such that the material is in its rubbery state and that viscoelastic effects are absent. The analytical warpage model was shown to give results comparable to numerical calculations using a fullly cure dependent viscoelastic material model. Furthermore, for the first time accurate analytical expression for the Stress Free Temperature and Stress Free Strain are obtained. With these expressions the effect of cure shrinkage on residual stresses can easily be incorporated in existing (numerical) stress analyses without the need of using extensive cure dependent viscoelastic material models.@en

Copper based wire bonding technology is widely accepted by electronic packaging industry due to the world-wide cost reduction actions (compared to gold wire bond). However, the mechanical characterization of copper wire differs from the gold wire; hence the new wire bond process setting and new bond pad structure is required. It also refers to the new intermetallic compound (IMC) will form at the interface of wire and bond pad. This paper will present the finite element analysis of the copper wire bond process and IMC forming and results in the stress pattern shift during the processes. Keywords: Cu wire bond, IC BE structure (BEOL), epoxy curing, IMC grow@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

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

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

An ongoing root cause of failure in microelectronic industry is interface delamination. In order to explore the risk of interface damage, FE simulations for the fabrication steps as well as for the testing conditions are generally made in the design stage.@en

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

Commercial FEM software codes do not directly support the cure dependency of polymers behavior. In electronics packaging, for example, cure shrinkage of the molding compound is consequently treated as a surplus to thermal contraction and its changes in visco-elastic behavior during curing are usually ignored. This gives rise to inaccurate results and may cause difficulties even in recognizing factors important for further optimization. This work introduces a cure dependent visco-elastic material model being implemented into ANSYS by USERMAT, the user material subroutine. The results obtained when simulating the warpage of bi-material strips during and after curing with the full scale material model were compared to those obtained with simplified material models. Varying the geometric configuration different changes in the deformation results have been found caused by cure dependency anywhere between -10%, 0%, and +20%. Hence, correct results can only be achieved in general case with models accounting for cure dependency directly.@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

Interfacial delamination has become one of the key reliability issues in the microelectronics of portable devices and therefore is getting more and more attention. The analysis of delamination of a laminate structure with a crack along the interface is central to the characterization of interfacial toughness. Due to the mismatch in mechanical properties of the materials adjacent to the interface and also possible asymmetry of loading and geometry, usually the delamination propagates under mixed mode conditions. In this study, a modified mixed mode bending test using production line interface samples is proposed. The critical fracture properties are obtained by interpreting the experimental results through dedicated finite element modeling. The interface types being considered in the present work are between EMC's and copper lead frame. Keywords-fracture; delamination; testing@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

Interfacial delamination has become one of the key reliability issues in the microelectronic industry and therefore is getting more and more attention. The analysis of delamination of a laminate structure with a crack along the interface is central to the characterization of interfacial toughness. Due to the mismatch in mechanical properties of the materials adjacent to the interface and also possible asymmetry of loading and geometry, usually the crack propagates under mixed mode conditions. The present study deals with delamination toughness measurements of an epoxy molding compound - copper lead frame interface as directly obtained from a real production process. As a consequence the specimen dimensions are relatively small and therefore a dedicated small-size test set-up was designed and fabricated. The test set-up allows transferring two separated loadings (mode I and mode II) on a single specimen. The set-up is flexible and adjustable for measuring specimens with various dimensions. For measurements under various temperatures and moisture conditions, a special climate chamber is designed. The "current crack length" is required for the interpretation of measurement results through FEM-fracture mechanics simulations. Therefore, during testing the "current crack length" is captured using a CCD camera and a micro deformation analysis system (MicroDac). The critical fracture properties are obtained by interpreting the experimental results through dedicated finite element modeling.@en