Balasubramanyam, R.S. (M.S. Mechanical Engineering)

Conductive Filament Formation in Printed Wiring Boards

Experiments were conducted on three printed wiring board materials under accelerated conditions and temperature and relative humidity, and a failure mechanism called "conductive filament formation" was observed under an applied bias. This thesis presents the experimental data and the empirical model developed. Failure analysis was performed on the data and the dependence of conductive filament formation on various parameters including conductor spacing, geometry of the electrodes, surface and inner layers of the multilayer board, surface coating present on the board, board material, applied voltage, temperature, and relative humidity is discussed.

Chandra, Vikram (M.S. Mechanical Engineering)

Computer Tools for Reliability Modelling and Assessment of Microelectronic Packages

Reliability prediction in the past has been based on empirical models developed from curve fits of filed failure data. This methodology does not give any insight to the designer over the actual cause of failure. A new methodology called physics-of- failure for reliability assessment and computer tools that incorporate this approach have been developed. The approach involves identifying the key failure mechanisms and models in microelectronic devices. The models are based on physics of the damage mechanisms, geometry, architecture and materials. The sensitivity of each failure mechanism to the environmental and test stresses, and architecture is then evaluated. Thus the root cause of failure can be found and reliability is viewed as an iterative process.

Gupta, Vineet K. (Ph.D. Mechanical Engineering (Solid Mechanics)

Modeling Solder Joint Reliability For Surface Mount Compliant Leaded Electronic Components

It is a challenge to determine the reliability of solder joints in modern electronic industry. Traditionally the reliability of solder joints have been assessed by either conducting extensive experiments, using a simple empirical equation, or by doing a detailed numerical finite element simulation. Both experiments and finite element simulation are time consuming, expensive and need lots of resources while the accuracy of simple empirical equations are seriously questioned This study aims to develop a more accurate analytical-cum-empirical model to predict fatigue life so as to avoid experiments and finite element simulation. A generic elastic-visco-plastic energy model is obtained based on theoretical stress analysis solutions and by using a design of experiments technique with FEA simulations. This model includes the contribution to damage from both local and global coefficient of thermal expansion (CTE) mismatch to the damage in the solder joint. Stress obtained from this model is used to get the partitioned elastic and plastic energies. These stresses acts as initial conditions to get the creep work dissipated in a cycle. Once energy densities are determined fatigue life is calculated using power law relationships between life and energy densities. Results from the model are in good agreement with FEA simulation results.

Huang, Choupin (Ph.D. Materials and Nuclear Engineering - Dec. 1994)

Degradation Analysis and Modeling of Bi-Sb-Te(Se) Semiconductor Thermoelectric Power Modules

Close-packed array (CPA) thermoelectric module technology has been recently developed for use in terrestrial and space thermoelectric power generators. In order to achieve high efficiencies in converting thermal energy into electricity, compound semiconductors have been extensively used for thermoelectric modules. Since most thermoelectric devices operate in hostile environments for remote and unattended applications, highly reliable thermoelectric modules are required. Accordingly, reliability issues that arise from thermally induced degradation have attracted much attention from researchers for several decades.

To study the performance degradation and reliability problems encountered in semiconductor thermoelectric modules, two different module types are characterized and modeled using both experimental and analytical approaches. The objective of this research is to investigate the thermally induced degradation of (Bi,Sb)2(Te,Se)3 semiconductor thermoelectric modules caused by chemical inhomogeneity (segregation, precipitation, coarsening and impurity diffusion) and microstructure change (microcracking, oxidation and the formation of dark band) , as well as to understand the mechanism of the formation of dark band caused by diffusion of impurities from insulator into semiconductor. In the experimental investigation, surface characterization of thermoelectric modules and materials has been systematically conducted using both scanning electron microscopy and energy dispersive x-ray spectroscopy. In mathematical models, exactly analytical solutions to partial differential equations based on the dynamic thermal stress assisted diffusion cracking model and a new modified Whipple's diffusion model for high diffusivity paths have been obtained to elucidate the mechanism of the observed dark bands. This modified Whipple's diffusion model considers the effect of the thermomigration driving force perpendicular to a microcrack as the high diffusivity path on impurity diffusion. The diffusion model established in this study is also applicable to semiconductor solid state electronic devices for understanding the effect of the thermomigration and electromigration driving forces on grain boundary diffusion, which usually results in the most prevalent failure mechanisms in microelectronic devices and circuits.

Malhotra, Anupam (M.S. Mechanical Engineering)

Standard Program for Reliable Product Development

A methodology for reliable product development is presented in this standard. The methodology is based on the identification, understanding, and elimination of the root causes of product failure. The standard provides the fundamental requirements for a reliability program geared toward precluding the weaknesses in the product development process which damage the reliability of the product. The advantages of this approach include higher product reliability and quality, greater customer satisfaction, lower life cycle costs, improved liabilities and warranties, lower product development times with the obviation of non value-added activities a reputation for the producer, a wider share of the product market, and an edge over the competition.

Prabhu, Ashok S. (M.S. Mechanical Engineering)

Stress Analysis of the G.E. High Density Interconnect

Finite Element stress analysis was conducted on the local model of the G.E. high density interconnect. The stress distribution along the via-polymer interfaces were studied. The total strain range when the model was subjected to a temperature cycle of -65 to 150øC was determined and the number of cycles to failure determined.

Sridhar, Sundaram (Ph.D. Mechanical Engineering)

Heat Transfer and Pressure Drop in Flows Perpendicular to an Offset Fin Structure

Liquid-cooled compact heat exchangers with offset fin structures are widely used in high power electronic cooling applications. The offset fin geometry has a long history with numerous studies using air as the heat transfer fluid. However, very little work has been reported using higher Prandtl number fluids even in the traditional parallel flow configuration. No reported studies have been found with flow perpendicular to the fin structure. The objective of this research is to investigate the heat transfer and pressure drop characteristics of perpendicular flow in offset fin arrays. A secondary objective is to compare the augmentation for this flow configuration relative to the more conventional configuration of flow parallel to the offset fin structure. A key aspect of the present research is the development of design correlations for heat transfer and pressure drop as a function of Prandtl number, reynolds number and fin geometry.

Vikram, Seshadri (M.S. Mechanical Engineering)

Correlation of Natural Convection Heat Transfer from a Horizontal Printed Circuit Board in an Enclosure

Temperature variations within electronic equipment are often a primary source of undesirable stresses and may impact performance of critical components. Conventional thermal analysis methods for printed circuit boards (PCBs) based on conduction solvers end up with large errors in estimating the operating temperature of the components. This leads to overdesigning and hence increased cost of the equipment, indicating the need for performing a conjugate heat transfer analysis. However, solving the same problem using a computational fluid dynamics/heat transfer (CFD/CHT) technique is considerably more complex and time consuming. This can be overcome by providing the correct heat transfer coefficients at the surface of the solid regions as would be given by the CFD/CHT analysis, to a PCB level solver. A parametric study was performed initially to determine the variables which would most affect the flow and the temperatures within an enclosure containing a heated horizontal PCB. Design of experiments was then used to define numerical experiments to study the interactions between the variables. The required CFD/CHT runs were performed and correlated to describe the surface heat transfer coefficients by a single equation which could then be incorporated within a conduction solver. The correlation was tested for a number of different power levels, and was found to give excellent results. All the predicted maximum temperatures were within 5% of the values found through a CFD/CHT simulation. A genetic algorithm based optimizer for thermal control was implemented using the developed correlation as a proof of concept demonstration of its practical significance.