K.M.B. Jansen

Prediction of process-induced warpage of IC packages encapsulated with thermosetting polymers

One critical issue for manufacturing of map-molded packages is the warpage induced during the molding process. A cure-dependent viscoelastic constitutive model has been established to describe the evolution of material properties during the curing process of a thermosetting polymer. The evolution of the rubbery moduli is described by a model based on scaling analysis and is measured with a new method. The relaxation behavior of the transient part is described by the cure-dependent relaxation amplitude and reduced relaxation times which are based on the time-conversion superposition principle. The cure-dependent parameters are characterized by using an combinational approach of DMA and DSC measurements. The predictions agree well with the experimental results. FEM is conducted for QFN map molding processes, and prediction of the warpage induced during the curing process and the cooling down is made. The results show that warpage induced during the curing process has a significant contribution on the total warpage. Furthermore, when decreasing the number of maps, the contribution of curing-induced warpage significantly increases.

Cure, temperature and time dependent constitutive modeling of moulding compounds

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 show 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 overall thermomechanical reliability. This paper deals with the characterization and modeling of the mechanical behaviour of such moulding compounds. The focus is on the effects of the degree of cure and the filler concentration.

Modeling of cure-induced warpage of plastic IC packages

The accurate prediction of warpage induced during manufacturing processes is important for the optimal design of both package structure and process conditions. In this paper, a cure-dependent viscoelastic constitutive model is established to model the cure-induced warpage after the map-mould manufacturing process. In the model, the relaxation moduli of the silica particle-filled polymer during the curing process are considered to be the sum of two parts, i.e. the cure-dependent equilibrium moduli and the transient parts. The equilibrium moduli are modeled with a model based on scaling analysis. The relaxation behavior of the transient part is described by the cure-dependent relaxation amplitude and reduced relaxation times, which are based on the time-conversion superposition principle. The cure-dependent parameters are characterized by using an integrated approach of DMA and DSC measurements. The chemical shrinkage strain is measured with an online density measuring setup. The viscoelastic parameter-functions of the resin, measured by DMA and DSC, have been incorporated in the MARC finite element code. Finite element modeling is carried out for three configurations of a carrier package map-mould and the warpage induced during the curing process and cooling down is predicted. The results show that warpage induced during the curing process has a significant contribution on the total warpage of the map.

Warpage minimization of the HVQFN map mould

A FEM model for the calculation of the HVQFN 100 warpage after moulding and post mould curing (PMC) was created. A viscoelastic model for the compound is required, since time and temperature history play a major role. In the production process the HVQFN 100 is forced to remain flat during PMC by applying of a dead weight. The good agreement between warpage measurments and FEM warpage results shows that fully cured viscoelastic compound properties can be used. Also adding curing shrinkage had only little effect on the end warpage. However in case of no dead weight, i.e. map mould free to warp, the curing shrinkage had a large effect on the end warpage and then curing effects can absolutely not be neglected. After validation (with dead weight) the model was used to minimize the warpage. To do so first the viscoelastic 1 Hz curve of the compound needs to be characterized by some parameters. Therefore the curve is described by three lines from which a viscoelastic spectrum can be derived. The input variables were die thickness and the compound properties: thermal expansion, glass transition temperature and stiffness. A design of experiments was created and used as input for the FEM model. The results were described by a so called response surface model, which showed that there is a lot of room to further reduce the warpage by choosing a compound with different properties and/or by variation of the die thickness. The response surface model was implemented in a spread sheet tool so that the effect of compound selection on warpage can be calculated in a split second.

Delamination and combined compound cracking of EMC-copper interfaces

The present study deals with experimental investigation of the delamination toughness of EMC (epoxy molding compound) and Copper-leadframe interfaces. Test samples were directly obtained from the production line. EMC is attached on copper substrates with various surface treatments. Mixed mode bending experiments were performed under various temperature and moisture environments. The test procedure and some results were reported previously in ECTC08 and ECTC09 [1–2]. Recently, we studied the effect of delaminated surfaces in order to get better understanding of the established fracture toughness. Therefore, after the delamination experiments, some of the delaminated samples were subjected to various surface analyses (SEM, FIB, EDX). Two types of failure patterns are found depending on the loading mode mixture, and the environmental conditions. Firstly, depending on the type of copper surface treatment, pure interface delamination is observed for some of the interfaces. Here, we observed clean delaminated copper surfaces. The second type of failure is a combination of interface delamination and compound cracking. Here, it is found that after the separation of interfaces, some EMC remains on the copper surface. In this case the experiment results showed that the interface delamination and molding compound cracking combined failure occurs at relatively high force values.

Interfacial fracture parameters of silicon-to-molding compound

The rapid diversification in microelectronics forebodes more complex system integration, be it for denser function integration or a span of dimensions between various technologies. Products may include more features, perform faster and be cheaper. With these trends the amount of material layers is increasing. This challenges development to a faster rating of material pairings. Delamination is a major issue among the related reliability aspects. When the design or testing steps are accompanied by simulation, fracture mechanical descriptions are increasingly proving helpful. The parameters needed for simulation have to be measured and should be available for different fracture mode mix angles. We investigated the interfacial fracture toughness of the Epoxy Molding Compound (EMC) to Silicon interface. Although difficult to delaminate we could carry out measurements using the Mixed Mode Chisel setup (MMC) that allowed us to induce different stress states at the crack tip at various external load angles. The samples we derived from the molding process of embedded wafer level ball grid arrays. Therefore we were able to use samples made with the same process as in real packaging. The crack tip position was determined by analysis of displacement results by digital image correlation. In order to interpret the sample reaction for extracting fracture mechanical parameters, adequate numerical modeling and simulation was required. The experiments provided the parameters for the models. Establishing the residual stress state in the materials preceded the interface delamination simulation: a two step interpretation. Residual stresses cannot be neglected; indeed they are part of the challenges to delaminate this interface at all. We found energy release rates increasing with fracture mode mix, and such values close to pure tensile opening at the crack tip. We recommend to exclude data from short crack lengths and to carefully expose the sample flanks. The results promise to extend the available interfacial fracture data soon.

Advanced viscoelastic material model for predicting warpage of a QFN panel

Warpage is a critical issue for a QFN panel molding process. Much work is 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 out 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.

Establishing fracture properties of EMC-Copper interfaces in the visco-elastic temperature region

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. In order to be able to judge the risk for interface fracture, the critical fracture properties of the interfaces being applied should be available, for the occurring combinations of temperature and moisture preconditioning. As a consequence there is an urgent need to establish these critical interface fracture parameters. For brittle interfaces such as between epoxy molding compound (EMC) and metal (-oxide) substrates the critical energy release rate (or delamination toughness) can be considered as the suitable material parameter. This material parameter is strongly dependent on the temperature, the moisture content of the materials involved and on the so-called mode-mixity of the stress state near the crack tip. The present study deals with experimental investigation of the delamination toughness of EMC-Copper lead-frame interfaces as can directly be obtained from the production line. The experimental set-up as designed for this purpose was previously reported, together with some measurement results and toughness evaluations for room temperature fracture tests. This study deals with experiment and simulation procedure of establishing the interfacial fracture toughness from fracture test results at high temperatures, especially in the glass transition temperature region of epoxy molding compound (EMC). In order to calculation accurate fracture toughness, the material property of molding compound is characterized as a function of temperature. A detailed discussion of how EMC responses at its glass transition region will be provided. The influence of the material property on interfacial fracture toughness will be given.

Establishing mixed mode fracture properties of EMC-Copper (-oxide) interfaces at various temperatures

Interfacial delamination is known as one of the root causes of failure in microelectronic industry. 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. In order to be able to judge the risk for interface fracture, the critical fracture properties of the interfaces being applied should be available, for the occurring combinations of temperature and moisture preconditioning. As a consequence there is an urgent need to establish these critical interface fracture parameters. For brittle interfaces such as between epoxy molding compound (EMC) and metal (-oxide) substrates the critical energy release rate (or delamination toughness, Gc) can be considered as the suitable material parameter. This material parameter is strongly dependent on the temperature, the moisture content of the materials involved and on the so-called mode mixity of the stress state near the crack tip. The present study deals with experimental investigation of the delamination toughness of EMC-Copper lead-frame interfaces as can directly be obtained from the production line. The experimental set-up as designed for this purpose was previously reported [1], together with some measurement results and toughness evaluations for room temperature fracture tests. This study deals with the experimental and simulation procedures to establish the interfacial fracture toughness from fracture test results at different temperatures, especially in the glass transition temperature region of epoxy molding compound. In order to calculate accurate fracture toughness, the viscoelastic material properties of molding compound are measured and considered. A special test procedure used to investigate the fracture properties in the glass transition temperature region of EMC will be introduced. The FE model used to simulate the viscoelastic material behavior will be discussed. The delamination toughness as a function of mode mixity at different temperatures will be given in the result section.

High temperature storage influence on molding compound properties

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 polymer-based 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].