Herman F. Nied

Mechanics of interface with applications in electronic packaging

The purpose of this paper is to present a brief review of the mechanics of interface fracture, with a focus on applications in electronic packaging. From a structural mechanics perspective, electronic devices can be thought of as composite structures fabricated from highly dissimilar materials. Often, the interfaces between these materials are where failure is most likely to occur when the device is subjected to thermomechanical loading. The mechanics of interface fracture is a specialized subtopic within the discipline of fracture mechanics and the nonspecialist may be unaware of some of the subtle differences encountered in applying interface fracture concepts. The mechanics associated with interface fracture introduces certain mathematical concepts that may seem to be unnecessarily complicated, but are essential for its proper application. It is important that the electronic packaging reliability engineer be aware of these concepts, understanding the most important implications. This review will focus on the mechanics and computational aspects of interface fracture in electronic structures, with a particular emphasis on some details that the nonexpert could only obtain after an extensive review of the available literature. Numerical results are presented for the important problem of corner cracking between silicon and epoxy materials subjected to thermomechanical loading. These new results provide insight into the three-dimensional nature of interfacial crack propagation at bimaterial corners.

Microwave intermodulation technique for monitoring the mechanical stress in RF MEMS capacitive switches

For the first time, a microwave intermodulation technique is used to measure the mechanical resonance directly on packaged and unpackaged RF MEMS capacitive switches with quality factors approaching unity due to air damping. The result is validated by similar measurements in vacuum with much higher quality factors. From the measured resonance frequencies, the residual mechanical stress of the fixed-fixed membrane of the switches is derived and its temperature dependence is analyzed and correlated with that of the pull-in voltage. The present technique offers a convenient means for monitoring the residual stress in RF MEMS devices in both manufacturing and operation. It also allows mechanical and electrical degradation effects to be conveniently separated during life testing of the switches.

Numerical Simulation of Welding Induced Residual Stresses in a Circular Hollow Section T-Joint

Heavily welded circular hollow cross sections (CHS) are a common feature in civil structures such as draglines used in the mining industry and other off-shore structures. The sheer mass of the weldment and the application of intense heat generated during the welding process give birth to significant residual stresses in the structure. Often, residual stresses are high enough to act to accelerate factors such as corrosion, crack growth and fatigue. The objective of this research investigation was to predict welding generated residual stresses in a typical CHS T-Joint using Sysweld+, a welding Finite Element Analysis software. The T-joint is the first of the four lacings welded on to the main chord of a BE 1370 mining dragline cluster (designated All) of a type which is often used in the mining industry in Australia. This work examines a massive 3-dimensional geometry, which is on a much larger scale than those examined in existing studies. The paper presents the results of the simulation of residual stresses generated during the welding process in a single weld pass and compares them with the approach used in the commonly used document R6-Revision 4, Assessment of the Integrity of Structures Containing Defects.Copyright

Mechanical Behavior of Highly-Flexible Elastomeric Composites with Knitted-Fabric Reinforcement

This paper examines critical issues associated with the fabrication and forming of highly-flexible polymeric composites, reinforced with knitted-fabric structures. Knitted-fabric reinforcements have not generally been preferred over more traditional woven reinforcements in high-performance composites, mainly because of their lower stiffness/strength performance when embedded in a rigid, thermosetting matrix material. However, with their unique formability, knitted fabrics promise great potential in applications where large deformation of the structure is desirable; such as energy/impact absorption and forming applications. One very attractive feature of knitted composite materials, is the large displacements that the underlying knitted fabric can potentially undergo before exhibiting a significant increase in stiffness. The unusual extensional behavior of knit fabric is attributed to the fact that the fibers are more-or-less free to slide over each other before the yarns become highly oriented, eventually “locking” in a packed formation. When the loops become highly elongated, the knit fabric achieves its maximum resistance to in-plane deformation, and exhibits a stiffness closely related to the elastic stiffness of the straightened fiber/yarn bundles. The unique formability of knitted fabrics is mainly due to this yarn movement. The highly “stretchable” behavior of knitted textile reinforcement materials can be used to great advantage in thermoforming composite structures. In order to fully utilize the exceptional stretch properties of the knitted-fabric, the matrix material should be able to deform at least as much as the fabric, and the knitted yarn movements need to be restricted by the matrix as little as possible. In this study, a multi-level finite element procedure was developed to analyze and control the deformation characteristics of plain weft knit reinforced composites. A database of mechanical properties for various knit geometries was obtained. Using these results, it is shown that carefully “tailored” knit fabric reinforcement can be used to improve mechanical performance and facilitate polymer forming processes, such as thermoforming. In this study, elastomeric materials such as polyurea and thermoplastic elastomer (TPE) were used to fabricate composites with knitted-fabric. Two different types of arrangements were experimentally studied: knitted fabric embedded in the elastomer and a sandwich of knitted fabric between elastomeric skins. It is shown that by fully utilizing the high stretchability of the knitted fabric reinforcements, attractive material properties can be obtained especially for energy/impact absorption and forming applications. The improvement of thermoforming process stability with the use of carefully tailored knitted fabric reinforcements is also presented.