Moeini, Seyed Ali (Ph.D.)
Mesoscale Microstructure Evolution, Reliability and Failure Analysis of High Temperature Transient Liquid Phase Sintering Joints
The continuous increase in application temperature of power electronic devices, due to the growing power density, miniaturization, and functionality in military and commercial applications, requires new packaging technologies with high temperature and reliability capabilities. Currently, the traditional maximum allowable temperature of power electronics (125°C) is a limiting factor for high temperature applications, such as space exploration, drilling, avionics, and electronic vehicles. Substitution of Silicon devices with wide bandgap (e.g., SiC) devices has extended the maximum allowable temperatures to 475°C. However, this created the need for robust high temperature packaging materials, especially interconnects and attachments. High temperature solders are often too expensive, too brittle, or environmentally toxic to be used, leading to increased study of low temperature joining techniques, such as solid phase sintering and Transient Liquid Phase Sintering (TLPS), for producing high temperature stable attachments. TLPS is an emerging electronic interconnect technology enabling the formation of high temperature robust joints between metallic surfaces at low temperatures by the consumption of a transient, low temperature, liquid phase to form high temperature stable intermetallic compounds (IMCs). The performance and durability of these materials strongly depend on their microstructure, which is determined by their processing. The complicated process of IMC formation through eutectic solidification and the extensive number of parameters affecting the final microstructure make it impractical to experimentally study the effect of each factor. In this work, phase field modeling of the microstructure of TLPS materials fabricated by different processing methods will be conducted. Phase-field modeling (PFM) is a powerful thermodynamic consistent method in mesoscale modeling that simulates the evolution of intermetallic compounds during the solidification process, providing insight into the final microstructure. Application of this method facilitates the optimization of influential processing factors. Efforts will also be conducted to identify failure modes and mechanisms experimentally under dynamic, power and thermal cycling loads as a function of critical microstructural features, facilitating the optimization of joining parameters to obtain higher durability TLPS interconnections. The objective of this dissertation is to provide an insight into the processing of a reliable high temperature TLPS and facilitate their application in power electronic industries.
Wu, Bulong (Ph.D.)
Advancement of Moire Interferometry for Rate-Dependent Material Behavior and Micromechanical Deformations
Moire interferometry is an optical technique to map full field in-plane deformations with extremely high resolution and signal to noise ratio. The technique is advanced and implemented to study the rate-dependent thermo-mechanical behavior of Sn-based Pb-free solder alloys and micromechanical deformations. In Part I, the mechanical/optical configuration of moire interferometry for real-time observation of thermal deformations is enhanced to provide measurement capabilities required for the analyses. Two most notable advancements are (1) development of a conduction-based thermal chamber for a wide range of ramp rates with accurate temperature control, and (2) implementation of microscope objectives in the imaging system to observe a microscopic field of view. The advanced system is implemented to analyze the anisotropic behavior of Sn-based Pb-free solder alloys. A novel copper-steel specimen frame is developed to apply a controlled loading to single-grain solder joints. After measuring the grain orientation by electron backscatter diffraction (EBSD), detailed in-situ deformation evolutions and accumulated deformations of solder alloys are documented during a thermal cycle of -40°C to 125°C. The results quantify grain orientation-dependent deformations that can lead more accurate anisotropic constitutive properties of Sn-based Pb-free solder alloys. In Part II, an advanced immersion microscopic moire interferometry system based on an achromatic configuration is developed and implemented for higher displacement sensitivity and spatial resolution. In order to achieve the desired displacement resolution, a high frequency grating (2500 lines/mm) is fabricated on a silicon substrate using lithography first. The square profile is subsequently modified by reactive-ion etching so that it can be used to produce a specimen grating by replication. Secondly, the algorithm of the optical/digital fringe multiplication method is improved to further enhance the measurement resolution of the immersion microscopic moire interferometry. The system and the noise-free grating are used to analyze thermal deformations of micro-solder bumps. With the basic contour interval of 200 nm, the displacement resolution of 25 nm is achieved with the multiplication factor of 8.
Kohani, Mehdi (Ph.D.)
Electrostatic Discharge (ESd) Risks in Wearable Medical Devices: Evaluating The Standard Test Method and Developing a Current Prediction Model
Electrostatic discharge (ESD) is a critical reliability concern for wearable medical devices. In recent years, numerous reports of device malfunction resulting in patient adverse events, and medical device recalls have been attributed to ESD. To mitigate the risk of device malfunction, sufficient ESD immunity standards and accurate ESD prediction models that represent severe discharges during usage are necessary. Thus, ESD test configurations that represent realistic discharges of wearable devices in healthcare applications need to be developed, and the severity of the ESD events need to be compared with the existing ESD immunity standards. The U.S. Food and Drug Administration (FDA) recognizes the IEC 60601-1-2 collateral standards, within which the IEC 61000-4-2 standard is the recommended ESD test method.
The severity of the discharges depends on the electrical impedance of the body and the discharging structure. To identify the realistic discharge scenarios for wearable medical devices, the proper body posture, device location, and the realistic discharge setup need to be determined. A research gap in the literature on electrostatic charging of a human body was that only standing posture was considered. Moreover, current prediction models developed in ESD literature are not based on the physical impedance parameters of the human body and the test setup.
Through conducting surveys, electrostatic measurements in a local hospital, and conducting laboratory studies in a climate chamber, a large set of electrostatic charging activities performed routinely by patients and hospital personnel were identified. ESD measurements for these scenarios showed that the IEC 61000-4-2 is not sufficient for these devices since the peak currents and maximum current derivatives of realistic discharges were up to 1.9 and 2.4 times larger than the standard specifications, respectively. A physics-based model for current waveform prediction was developed using the electrical impedance of the discharging structure and the human body in the identified postures of standing on the floor, sitting and lying down on a hospital bed and two device locations (hand and waist). Discharge waveforms at spark lengths between 100% to 50% of the Paschen’s length were simulated with reasonable accuracy.
Sood, Bhanu (Ph.D.)
The Effect of Epoxy/Glass Interfaces on CAF Failures in Printed Circuit Boards
Reductions in printed circuit board line spacing and via diameters and the increased density of vias with higher aspect ratios (ratio between the thickness of the board and the size of the drilled hole before plating) are making electronic products increasingly more susceptible to material and manufacturing defects. One failure mechanism of particular concern is conductive anodic filament (CAF) formation, which typically occurs in two steps: degradation of the resin/glass fiber bond followed by an electrochemical reaction. Bond degradation provides a path along which electrodeposition occurs due to electrochemical reactions. The path can result from poor glass treatment, from the hydrolysis of the silane glass finish, or from mechanical stresses. Once a path is formed, an aqueous layer, which enables the electrochemical reactions to take place, can develop through the adsorption, absorption, and capillary action of moisture at the resin/fiber interface. This paper describes the concerns with CAF and the methods used for analyzing low-resistance failures. A case study is then given which highlights problems that arose on a commercial circuit board material used by a major telecommunications provider.