J. de Vreugd

Prediction of cure induced warpage of micro-electronic products

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.

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.

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].

Effect of postcure and thermal aging on molding compound properties

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. Also weight loss during aging is reported. Since thermomechanical properties determine mainly the reliability of electronic devices it is essential to have knowledge on 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.

Establishing Fracture Properties of EMC-Copper (-Oxide) Interfaces: Test Procedures and Simulations for Establishing the Interface Toughness, Depending on Temperature, Humidity and Mode Mixity

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 EMCs and copper lead frame.

Comprehensive material characterization and method of its validation by means of FEM simulation

Numerical simulation plays an important role in product design. Its accuracy relays on a detailed description of geometry, material models, load and boundary conditions. This paper focuses on a new approach of FEM material modeling of three commercially available molding compounds. Curing shrinkage, modulus of elasticity and coefficient of thermal expansion were measured and implemented into commercially available FEM code Ansys. Fringe pattern technique has been used to measure warpage of bimaterial strips. Then FEM simulation of bimaterial strips were done and compared with experimental results. Curing shrinkage has been modeled in an effective way. Its accuracy has been checked on one of the materials by creating bimaterial strips with three different geometrical dimensions, that is varied thickness of mold and copper substrate.

Influence of cure dependency of molding compound properties on warpage and stress distribution during and after the encapsulation of electronics components

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 give 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.

Effects of Molding Compound Cure on Warpage of Electronic Packages

Warpage is a critical issue for an electronic package molding process. Much work is done in the past to predict the warpage of a package after the cooling down from the molding process. However, there are many material models for the molding compound to predict warpage: Elastic and viscoelastic models are used, and there are even groups which use cure-dependent viscoelastic models. In this paper, we compare results obtained with the different models with each other. In our group we investigated the mechanical properties of a molding compound in detail. From this research it is concluded that the viscoelastic properties highly depend on the chemical conversion level. Furthermore, dilatometric studies showed that the bulk modulus and the coefficient of thermal expansion are independent on time. The thus obtained model which includes cure shrinkage is implemented in the finite element software ABAQUS by making use of user-subroutines (UMAT). The model is used in a theoretical study on the warpage of a double layered beam consisting out of a layer of molding compound and a layer of copper. This beam is cooled down form a temperature below Tg to room temperature with different cooling rates. Warpage of the beam is calculated with different models and a comparison is made. It turned out that there is a big difference between the different approaches.

Cure induced warpage of micro-electronics: Comparison with experiments

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 mechanical 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.