Ibrahim Guven

Analysis of singular stress fields at junctions of multiple dissimilar materials under mechanical and thermal loading

Abstract Traditional finite element analyses of the stress state in regions with dissimilar materials are incapable of correctly resolving the stress state because of the unbounded nature of the stresses. A finite element technique utilizing a coupled global (special) element with traditional elements is presented. The global element includes the singular behavior at the junction of dissimilar materials with or without traction-free surfaces. A hybrid global (special) element is developed utilizing the exact solution for the stress and displacement fields based on the eigenfunction expansion method under mechanical and thermal loading. The global element for arbitrary geometrical and material configurations, not limited to a few dissimilar material sectors, is interfaced with traditional local (conventional) elements while satisfying the inter-element continuity. The coupling between the hybrid global element and conventional finite elements is implemented into ansys , a commercially available finite element program. Also, the global element is integrated into the ansys graphical user interface for pre- and post-processing.

Transient two-dimensional thermal analysis of electronic packages by the boundary element method

The fabrication of electronic packages involves heating and then cooling from high processing temperatures. Because these devices consist of bonded materials with different thermal and mechanical properties, high thermomechanical stresses develop due to thermal and stiffness mismatches of bonded materials at regions with geometric and/or material discontinuities. These high stresses may result in crack initiations, leading to delaminations. Therefore, accurate temperature and flux distributions are critical when computing thermomechanical stresses, knowledge of which is essential for reliable designs. This study presents an analysis method based on the boundary element method (BEM) to investigate the transient thermal response of electronic packages consisting of dissimilar materials while subjected to general boundary conditions. In order to demonstrate its capability, a chip on a substrate configuration subject to convective cooling is considered. The boundary conditions across the interfaces between the chip and the adhesive and adhesive and substrate are matched through exact expressions. The results capture the singular flux field arising from the mismatch in the thermal conduction coefficients and geometric discontinuity. The comparison of the results with those obtained from finite element analysis shows that BEM is rather robust and efficient for this class of transient conduction analyses.

Fatigue failure model with peridynamic theory

This study presents a methodology to predict the fatigue failure of materials due to cyclic loading within the realm of peridynamic theory. This approach incorporates material failure intrinsically without the need for external crack growth criteria and post-processing. Failure occurs when and where it is energetically favorable. Fatigue life prediction is a natural extension of crack initiation and growth while allowing material degradation. The present approach focuses on the crack growth phase of fatigue life rather than initiation. During the crack growth phase, material degrades, and fatigue process is viewed as a quasi-static series of discrete crack growth steps. Crack growth process is controlled by the critical value of material stretch and that cyclic loading causes degradation in the critical stretch. When the critical amount of stretch is reached, stable crack growth occurs.

Crack propagation in multilayer thin-film structures of electronic packages using the peridynamic theory

Abstract This paper presents an application of the peridynamic theory to predict crack paths in multilayer thin-film structures of electronic packages. The peridynamic theory is a nonlocal continuum theory that has an inherent crack initiation and growth criterion. Specifically, the peridynamic theory is employed to model cross-sectional nanoindentation, an experimental technique used for characterizing interfacial adhesion and observing crack paths. Cross-sectional indentation experiments previously conducted on blanket and patterned thin-film structures were simulated. The predicted crack propagation paths in both blanket and patterned multilayer thin-film structures compare well with the results from cross-sectional nanoindentation experiments.

Experimental and Numerical Characterization of Non-Fickian Moisture Diffusion in Electronic Packages

This study investigates moisture diffusion characteristics of electronic packaging materials exhibiting Fickian and non-Fickian behavior. Experimental investigation involves moisture absorption and desorption tests of homogenous underfill materials and inhomogeneous organic substrates as examples of Fickian and non-Fickian solids, respectively. In absorption tests, samples are dried out in an oven prior to testing in a humid environmental chamber. In desorption tests, samples are saturated in the environmental chamber under a specified temperature and relative humidity prior to the moisture desorption inside the oven. Samples in both tests are taken out of the test environments and weighed frequently to obtain moisture weight change data. Using the measurements from testing several different Fickian and non-Fickian materials, diffusivity/moisture concentration relationships are constructed. These relationships are implemented into a customized finite element simulation tool under the ANSYSreg platform. Finally, the experimental tests on multi-material specimens are simulated by using this tool in order to establish its validity.

Peridynamic theory for impact damage prediction and propagation in electronic packages due to drop

In this study, peridynamic theory is used to investigate dynamic response of electronic packages subjected to impact loading arising from drop-shock. First, the theory is briefly described, followed by validation against a fundamental dynamic fracture problem. Finally, peridynamic theory was demonstrated by considering a drop test experiment.

Revisit of life-prediction model for solder joints

A finite element analysis is used in conjunction with failure models for fatigue-life prediction of electronic packages. One of the widely accepted models for predicting the fatigue life of a solder joint is based on the volume-weighted average plastic work density. This model, introduced by Darveaux (1997), utilizes experimental measurements at the package level and relies on the specific values of certain parameters. These parameters were determined through curve-fitting against measurements in conjunction with the finite element simulations using an earlier version of the ANSYS finite element program. However, it was discovered by Anderson et al. (1999) that the previous version of ANSYS computed an incorrect plastic work density, whose value is critical in the determination of these parameters. Therefore, these parameters have been recalculated by Darveaux (2000) using the corrected version of ANSYS. This study investigates the effect of the previous and recent values of these parameters on solder-joint life prediction by considering six different package types provided by leading companies in the electronics industry. The dimensions and material properties, as well as the measured life, of each package were obtained directly from industry.

Predicting crack initiation and propagation using XFEM, CZM and peridynamics: A comparative study

This study presents a comparison of extended finite elements (XFEM), cohesive zone model (CZM) and the peridynamic theory (PD). By comparisons against two experimental benchmark studies, the capability of these techniques to predict dynamic fracture is demonstrated through both qualitative and quantitative observations.

Peridynamic theory for failure prediction in multilayer thin-film structures of electronic packages

This study presents an application of the peridynamic theory to investigate failure propagation in a multi-layer thin- film structure of electronic packages. A brief description of this non-local theory is presented prior to the statement of the problem. The mathematical description of the theory and its validation by considering a well-known dynamic fracture problem are discussed subsequently. Finally, the crack propagation path in a multilayer thin-film structure predicted by the peridynamic theory was compared against measurements from nanoindentation experiments.

Mechanical characterization of nickel nanowires by using a customized atomic force microscope.

A new experimental method to characterize the mechanical properties of metallic nanowires is introduced. An accurate and fast mechanical characterization of nanowires requires simultaneous imaging and testing of the nanowires. However, existing mechanical characterization techniques fail to accomplish this goal due either to the lack of imaging capability of the mechanical test setup or the difficulty of individual alignment and manipulation of single nanowires for each test. In this study, nanowire specimens prepared by an electroplating technique are located on a silicon substrate with trenches. A customized atomic force microscope is located inside a scanning electron microscope (SEM) in order to establish the visibility of the nanowires, and the tip of the atomic force microscope cantilever is utilized to bend and break the nanowires. The ability to visualize the nanowires in an SEM improves the speed and accuracy of the tests. Experimentally obtained force versus bending displacement curves are fitted into existing analytical formulations to extract the mechanical properties. Experimental results reveal that nickel nanowires have significantly higher strengths than their bulk counterparts, although their elastic modulus values are comparable to bulk nickel modulus values.