A. Xiao

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.

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.

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.

Mixed mode interface characterization considering thermal residual stress

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 setup allows transferring two separated loadings (mode I and mode II) on a single specimen. The setup 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 ldquocurrent crack lengthrdquo is required for the interpretation of measurement results through FEM-fracture mechanics simulations. Therefore, during testing the ldquocurrent crack lengthrdquo 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.

Interfacial strength of Silicon-to-Molding Compound changes with thermal residual stress

As we face higher numbers of material layers in the increasingly complex Microsystems, the rating of layers reliability has to keep pace. Fracture mechanical descriptions are a big qualitative improvement when using simulation for design and reliability support, especially when looking at layer delamination. In order to simulate the interfacial fracture we urgently need to find empirical parameters, because the fracture parameters have to be verified as critical. Such experiments are difficult to carry out at the Silicon-to-Epoxy Molding Compound (EMC) interface. We are now able to do such investigations using the Mixed Mode Chisel (MMC) setup. In this paper we compare results of the Silicon-EMC interface for the in- and exclusion of thermal residual stresses in the simulations. The interface specimens are of package scale and are derived from the embedded wafer level molding process. We find the impact of thermal residual stresses crucial for the validity of fracture toughness values, and show relations to consider when using the MMC setup. We do not find any EMC residuals on the delaminated Silicon surface.

How to fabricate specimens for silicon-to-molding compound interface adhesion measurements

We present a new method to fabricate specimen for interfacial fracture testing. It regards the interface between silicon die and epoxy molding compound (EMC). The crucial element of evaluating interfacial fracture strength is the calculation of critical fracture values. Such values can be obtained analyzing a bimaterial type of sample and by specifically inducing a delamination while monitoring conditions and loads. We use a sandwich type of sample where the epoxy molding compound encloses the silicon die. We give a detailed description of molding specimens using an established transfer molding process that has a sufficiently large cavity available. In order to prevent breakage and related residual stresses we use specific bearings to hold the silicon stripes symmetrically in place. The bearings are made out of cured EMC themselves. The samples did not break and allow for interfacial fracture characterization. For testing the specimens we used the Mixed Mode Chisel (MMC) setup, which is one of the first to at all induce and monitor delamination of the Si-EMC interface.

Characterization and Modeling of Thin Film Interface Strength Considering Mode Mixity

Interfacial delamination is a common cause of failure in microelectronic packages. Characterization and prediction of interface behavior in manufacturing, testing and application conditions is demanded in order to reduce development times and costs of IC packages. In the design processes of microelectronics, possible interface delamination is evaluated by the critical energy release rate to detach the materials. This critical energy release rate can be obtained experimentally using various approaches. However, its measurement is complicated due to the fact that adhesion strength is not only temperature and moisture dependent but also stress state (mode mixity [1]) dependent. This paper describes our efforts on interface characterization as a function of mode dependency. A new test setup is designed and built. It allows transferring two separated loads on a single specimen. The test methodology that is developed in this paper is also able to evaluate the interfacial fracture toughness as function of temperature and moisture. The crack length, necessary for calculation of the energy release rate is measured by means of an optical microscope. Finite element simulation is used to interpret the experimental results and thus to establish the critical energy release rates and mode mixities.

Temperature Moisture and Mode Mixity Dependent EMC- Copper (Oxide) Interfacial Toughness

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. A small-scaled test setup was designed. The test setup is suitable for actualizing both pure mode I DCB (double cantilever beam) loading and pure mode II ENF (end notched flexure) loading and allows transferring two separated loadings (mode I and mode II) on a single specimen. The setup is flexible and adjustable for measuring specimens with various dimensions. For measurements under various temperatures and moisture conditions, a special climate chamber is designed. In this paper, the experiment and simulation procedure for establishing the interfacial fracture toughness from fracture test results at different temperatures, especially in the glass transition temperature region of epoxy molding compound (EMC) will be shown. In order to calculate accurate interface 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.