L.J. Ernst

Interfacial Delamination Mechanisms During Soldering Reflow With Moisture Preconditioning

This paper first examines the commonly-used thermal-moisture analogy approach in thermal-moisture analogy approach. We conclude that such an analogy using a normalized concentration approach does not exist in the case of soldering reflow, when the solubility of each diffusing material varies with temperature or the saturated moisture concentration is not a constant over an entire range of reflow temperatures. The whole field vapor pressure distribution of a flip chip BGA package at reflow is obtained based on a multiscale vapor pressure model. Results reveal that moisture diffusion and vapor pressure have different distributions and are not proportional. The vapor pressure in the package saturates much faster than the moisture diffusion during reflow. This implies that the vapor pressure reaches the saturated pressure level in an early stage of moisture absorption, even the package is far from moisture saturated. However, the interfacial adhesion degrades continuously with moisture absorption. Therefore, the package moisture sensitivity performance will largely reply on the adhesion strength at elevated temperature with moisture. A specially designed experiment with a selection of six different underfills for flip chip packages was conducted. Results confirm that there is no correlation between moisture absorption and the subsequent interface delamination at reflow. The adhesion at high temperature with moisture is the only key modulator that correlates well with test data. Such a parameter is a comprehensive indicator, which includes the effects of thermal mismatch, vapor pressure, temperature and moisture. In this paper, a micromechanics based mechanism analysis on interfacial delamination is also presented. With the implementation of interface properties into the model study, it shows that the critical stress, which results in the unstable void growth and delamination at interface, is significantly reduced when the effect of moisture on debonding is considered.

A Micromechanics-Based Vapor Pressure Model in Electronic Packages

Polymer materials have wide applications in microelectronic packaging. Some polymer materials are used in bulk form, such as encapsulant molding compound and carrier or printed circuit boards FR4 and BT . Some polymer materials are used as adhesives, such as die-attach, underfill, or other structural and thermal adhesives. Polymers are also used in thin or thick film as an isolation layer, such as a solder mask on a printed circuit board or passivation layer in the wafer level. Despite the diversities of the chemistry and compositions, the polymer materials applied in microelectronics can be either thermoset or thermoplastic materials 1 . Both types of materials have a glass transition temperature around which the material properties, such as the CTE and Young’s modulus, are very sensitive to temperature. Another common feature of polymer materials is the high porosity, which makes the material susceptible to moisture absorption. In order to understand better the capacity of the moisture absorption by a typical polymer material, let us introduce some important parameters, which will be used frequently in subsequent sections, as follows:

Prediction and verification of process induced warpage of electronic packages

Abstract During the manufacturing, testing and service, thermally induced deformations and stresses will occur in IC devices and packages, which may cause various kinds of product failures. FEM techniques are widely used to predict the thermal deformations and stresses and their evolutions. However, due to the complexity of the real engineering problems, various assumptions and simplifications have to be made in conducting FEM modelling. Therefore, the applicability of the predicted results depend strongly on the reliability and accuracy of the developed FEM-based prediction models which should be verified before applications. In this paper, FEM models are developed to predict the thermal deformations of certain electronic packages and naked die samples under packaging and testing loading. For all the package constituents, appropriate material properties and models are used, including temperature-dependent visco-elasticity, anisotropy, and temperature-dependent elasticity and plasticity. To verify the developed FE models, a series of optical metrology tests are performed. A compact 3D interferometry testing system that can measure simultaneously out-of plane and in-plane deformations has been developed. Thermal deformation measurements are performed on samples of both real electronic packages and naked dies attached on a leadframe. Identical deformation patterns were found for the measured fringe patterns in the U -, V -, and W -fields and the simulated ones. Also, quantitatively, the maximum deformation mismatch between the predicted and tested results is within 15%. It is concluded that the thermally induced deformations predicted by the non-linear FEM models match well with measured deformations for both the naked die and the real packages.

Packaging Induced Die Stresses—Effect of Chip Anisotropy and Time-Dependent Behavior of a Molding Compound

This paper investigates the effect of the anisotropic behavior of the die and the time- and temperature-dependent behavior of epoxy molding compound on the packaging induced stresses for a quad flat package. Finite element (FE) simulations using isotropic and anisotropic properties of the die are carried out, respectively, and the results are compared. Creep experiments were performed at different temperatures ranging from 265°C to 230°C to obtain the long-term master curves and the related shift factors for the creep compliance of the molding compound. FE models which incorporate the viscoelastic constitutive relation of the material are constructed to simulate the thermo-mechanical stresses caused by the packaging processes. The influences of both the chip anisotropy and the viscoelastic behavior of the molding compound on the packaging induced stresses are discussed. @DOI: 10.1115/1.1604153# At present, thermo-mechanical reliability of integrated circuit ~IC! packages is still one of the major concerns in the electronic industry. Critical stress levels may be induced in the package constituents during the thermal processing due to mainly the mismatch in thermal-expansion coefficients of the materials. The prediction of these packaging induced thermo-mechanical stress levels can only be true if reliable material properties for each constituent are taken into account in the modeling. In most of the publications addressing the thermo-mechanical behavior of IC packages, the silicon die was modeled as temperature independent and isotropic. However, given the nature of the silicon material, which is a crystal, the assumption of isotropy is not true. In fact, due to the orientation of the silicon crystal, a diamondlike crystallographic structure, anisotropic material behavior is to be expected. Ultrasound measurements ~20/40 MHz! were used to obtain the stiffness values in the different directions for the silicon crystal @1‐3#. It was reported that the stiffness values at temperature 273 K range from 170 GPa in the @110# plane to 130 GPa in the @100# plane @2#. These values show that an isotropic approach of the silicon material may not be valid. The temperature dependence of the stiffness values was found to be negligible, for instance the in plane value at 473 K is only 0.5% lower. Previous research work also showed that the temperature independence of the thermal-expansion coefficient ~CTE! for the silicon crystal is valid within a certain temperature range @3‐6 #. Thermosetting resins, like other polymeric materials, have strong time- and temperature-dependent mechanical properties even if they are filled with a high percentage of filler. The creep and relaxation of the packaging material during packaging processes and/or testing will cause a redistribution of stress and strain levels in the chip. However, for the reason of simplicity, in most of the thermo-mechanical packaging simulations the viscoelasticity of the molding compound is totally or partially neglected. As a consequence, the predicted stress levels and its evolution during packaging processes and/or testing may not be representative for the reality. In Refs. @7‐9# it was reported that by using a viscoelastic model for the molding compound, the predicted stresses and deformations are closer to the real situations. The present study focuses on the investigation of the influence of chip anisotropy and the time- and temperature-dependent behavior of a molding compound on the stress levels during packaging processes and testing conditions. In the finite element ~FE! simulations, the anisotropic and isotropic properties of the silicon die are used, respectively, and a comparison of their results is made. Creep experiments were performed at different temperatures ranging from 265°C to 230°C to obtain the long-term master curves and the related shift factors for the creep compliance of the molding compound. FEM models, representing a quad flat package ~QFP! package, in which the viscoelastic model of the material is implemented, are constructed and used to simulate the thermo-mechanical stresses caused by the packaging processes.

Response Surface Modeling for Nonlinear Packaging Stresses

The present study focuses on the development of reliable response surface models (RSMs) for the major packaging processes of a typical electronic package. The major objective is to optimize the product/process designs against the possible failure mode of vertical die cracks. First, the finite element mode (FEM)-based physics of failure models are developed and the reliability of the predicted stress levels was verified by experiments. In the development of reliable thermo-mechanical simulation models, both the process (time and temperature) dependent material nonlinearity and geometric nonlinearity are taken into account. Afterwards, RSMs covering the whole specified geometric design spaces are constructed. Finally, these RSMs are used to predict, evaluate, optimize, and eventually qualify the thermo-mechanical behavior of this electronic package against the actual design requirements prior to major physical prototyping and manufacturing investments.

Driving Mechanisms of Delamination Related Reliability Problems in Exposed Pad Packages

Exposed pad packages were introduced in the late 1980s and early 1990s because of their excellent thermal and electrical performance. Despite these advantages, the exposed pad packages experience a lot of thermo-hygro-mechanical related reliability problems during qualification and testing. Examples are die lift, which occurs predominantly after moisture sensitivity level conditions, and die-attach to leadframe delamination leading to downbond stitch breaks during temperature cycling. In this chapter, nonlinear finite element (FE) models using fracture mechanics based J-integral calculations are used to assess the reliability problems of the exposed pad package family. Using the parametric FE models any geometrical and material effects can be explored to their impact on the occurrence diepad delamination, and dielift. For instance the impact of diepad size is found to be of much less importance as the impact of die thickness is. Using the fracture mechanics approach, the starting location for the delamination from thermo-hygro-mechanical point of view is deducted. The results indicate that when diepad delamination is present, cracks are likely to grow beneath the die and dielift will occur. The interaction between dielift and other failure modes, such as lifted ball bonds, are not found to be very significant. The FE models are combined with simulation-based optimization methods to deduct design guidelines for optimal reliability of the exposed pad family.

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.

Numerical modeling of warpage induced in QFN array molding process

Warpage is a critical issue for QFN array molding process. In this paper, a cure-dependent viscoelastic constitutive model is established to model the cure-induced warpage in array molding process. For the relaxation modulus functions of the packaging polymer, the equilibrium moduli are modeled with a model based on scaling analysis and 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 dynamical mechanical analysis (DMA) and differential scanning calorimetry (DSC) measurements. 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 significant contribution on the total warpage of the map.

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.