CALCE EPSC Graduate Student Theses (2002)

Becker, Daniel V. (M.S. Mechanical Engineering)

Development and Application of Yielded Cost in Electronic Manufacturing Process Improvement

In this work, yield is defined as the probability that a part is non- defective. When cost is accumulated and divided by yield, a metric called yielded cost results, which represents the effective cost incurred per non- defective part completing a manufacturing process. Although yielded cost is a not a new concept, it has no consistent definition in engineering literature, and several different formulations and interpretations exist in the context of manufacturing and assembly.

This thesis reviews existing yielded cost formulations and presents a new definition of the yielded cost associated with an individual process step (step yielded cost) as the change in yielded cost when the step is removed from its process. This definition is preferred because it enables consistent measurement of effective step costs that incorporate upstream and downstream process information. Simple and complex assembly process examples are presented to demonstrate this interpretation. Furthermore, conventional wisdom dictates that processes are best improved by increasing the lowest step quality, which is not always the case. To address this inconsistency, this thesis formulates and applies a distribution array of step yielded cost terms to several examples, including the general sequential model for a manufacturing process. This array is used as part of a methodology to determine the most efficient quality to increase in reducing yielded cost. This methodology is developed and mathematically verified with several examples.

Bhaskaran, Harish (M.S. Mechanical Engineering)

Die Shear Experimental and Modeling Verification of Chip-to-Chip Bonded Microelectromechanical Systems

Microelectromechanical systems (MEMS) are finding applications in products or systems that require reliable operation over extended periods of time. The reliability requirements for the final product encompass both the mechanical behavior and the electrical characteristics of the overall system. One critical element in many MEMS applications is chip-to-chip bonding (component bonding), for which long-term operation and storage reliability needs to be understood.

MEMS packages are likely to have a large number of bond layers because of multiple interfaces inside the package. A primary indicator of failure (or impending failure) in a chip-to-chip bonded system is delamination between the chip and the material used to bond the chips together. In spite of its importance in MEMS packaging, previous work on bonding in MEMS structures is limited. Very little MEMS-specific work on the reliability of the chip- to-chip bonds exists, let alone, non-destructive methods for determining the reliability of chip-to-chip bonded MEMS.

This thesis presents the results of the die-shear experiments used to determine the bond strength in samples subjected to environmental testing. A Finite Element Model is used to simulate the die-shear experiment using inputs from scanning acoustic microscopy. The die shear experimental results coupled with simulation serve to validate a previously developed delamination measurement methodology that enables non-destructive evaluation of chip-to-chip bonded MEMS.

Cunningham, Jeremy (M.S. Mechanical Engineering)

Demonstration of Physics-of-Failure Assessment of an Avionic Controller

The physics-of-failure product assessment procedure is used for evaluating the reliability of electronic products based on the wear-out failure mechanisms that will be precipitated during the life of the product. The procedure predicts the potential failure modes, mechanisms, and sites of the product based on: first, the product^s design and the life-cycle environment, and second the physical test load conditions. Physical testing is then used to verify the failure prediction and the life-cycle reliability of the product. A demonstration of the physics-of-failure product assessment is provided for an avionic electronic controller, where the physical testing was performed off-site. The accelerated thermal and vibration tests confirmed the reliability prediction of high-cycle fatigue of solder interconnects as the life limiting failure mechanism. The physics-of- failure assessment determined that the electronic controller would not reach the reliability goals. The costs of performing the physics-of-failure demonstration is compared to a previous controller and reliability test method to provide a return on investment. The physics-of-failure assessment procedure proved to cost substantially less and be less time intensive than the previous reliability test method. Finally lessons learned are provided from performing the physics-of-failure assessment.

Mager, Brent M. (M.S. Mechanical Engineering)

Analytic Characterization of Interconnect Shear Force Behavior

Electronic devices are often subjected to thermal loads as a result of environmental exposure and power cycling. Temperature cycling can result in an electrical open in an interconnect and cause the device to fail. An electrical open is often due to forces in the interconnects that develop from the global coefficient of thermal expansion (CTE) mismatch between a component and the printed wiring board (PWB). The fundamental concern is the determination of the device life under a given loading.

In order to address the critical issue of product life, the forces that lead to failure need to be characterized. The forces directly relate to the device life, indicated in terms of cycles to failure, through a series of relationships. The intermediate relationships that bridge the connection between force and device life are that force relates to an average stress and thus a strain, which relates to cycles to failure through a damage model.

An analytic approach to determining the forces is desirable because of the fast computation speed, straightforward derivation, and programming platform versatility of analytic models. The analytic approach used in this thesis to characterize the shear forces in the interconnects of electronic devices is based on strength of materials principles. Conclusions are made regarding the influence of the number of interconnects, device dimensions, and material properties on the resulting shear forces. Guidelines for efficiently designing a successful electronic package are presented, which can save both time and money in the design process.

Moores, Kevin A.(Ph.D. Mechanical Engineering)

Thermal and Hydrodynamic Investigation of Shrouded Pin Fin Heat Sinks With Liquid Cooling

Today's power semiconductor modules, which can generate 3 W/mm2 or more of heat at the die level, typically employ metallic base plates to aid in efficient spreading of heat between the die package and a liquid cooled metal heat sink. Due to large differences in the coefficient of thermal expansion (CTE) between the base plate and die packaging, this approach often results in long term problems in mechanical reliability and thermal performance. To rectify this, a new generation of base plates is under development, that integrates the functionality of a heat sink directly into the base plate through the use of metal matrix composite (MMC) materials to produce a plate with low CTE, high thermal conductivity, and an dense array of integrated pin fins in direct contact with the cooling fluid. However, due to the complex geometry of these structures, it is generally impractical to analyze thermal and hydrodynamic performance using standard three-dimensional computational fluid dynamics (CFD) codes. A more computationally efficient means of determining design performance is needed.

In this study, experimental characterization of the heat transfer and hydrodynamic pressure drop is performed for flow through a shrouded array of pin fins with a thermally active bounding wall. Array temperature is to be measured both at discrete locations using thermocouples, and full field using thermochromic liquid crystals. Differential pressure drop across the pin array as a function of Re is also determined. Detailed numerical CFD based simulations are also performed on a unit cell bases to characterize the heat transfer and fluid flow over a wider range than that covered through experiment. The results of this work will be used to develop a new correlation for the prediction of pin fin array heat transfer and pressure drop performance. A porous media based CFD model will be developed, using input parameters supplied by the new correlation. The CFD model will be applied to a simulation of the complete base plate and verified by the experimental results.

O'Conner, Casey (Ph.D. Mechanical Engineering)

Influence of Rapid Altitude Cycling on the Reliability of Plastic Encapsulated Microcircuits

A major concern regarding the use of plastic encapsulated microcircuits (PEMs) in rapid-climbing-jet applications is possible failure related to moisture ingress associated with rapid altitude changes. The concern is that at high altitudes, the PEM will become depressurized, reaching an equilibrium with the high altitude environment. Upon descent to the humid ground environment, the lower pressure inside may draw excess moisture into the package. The excess moisture combined with ionic contaminants at the die surface may lead to subsequent corrosion of the elements inside the package.

A theoretical model was explored to describe the movement of moisture into a PEM under the influence of pressure cycling. The model predicted that if there was no delamination, moisture ingress into dry, evacuated packages would not be greater, and would not occur at a faster rate than moisture ingress into dry, non-evacuated packages.

A series of experiments were designed to validate the theoretical model. Results of one of the experiments showed that the rate of moisture ingress into dry, evacuated packages was not different from moisture ingress into dry, non-evacuated packages, thus confirming the model and invalidating the initial supposition. However, it was also found that the rate of moisture ingress into dry, delaminated packages was greater than dry, non-delaminated packages.

An accelerated test was designed to model the life of a rapid-climbing-jet. The results of the accelerated test showed a higher failure rate for components in environments with altitude cycling than without altitude cycling. Subsequent analysis of the failed components revealed an increase in delamination in PEMs subjected to altitude cycling. This delamination, as shown in the moisture experiment, provided additional pathways for moisture and environmental contaminants to migrate into the package causing subsequent corrosion of the elements. Had there have been no increase in delamination, the theoretical predictions would have held true. But with delamination, rapid altitude cycling can lead to premature failure of some PEMs. Based upon this work, recommendations were developed to minimize delamination and corrosion failures.

Topolosky, Zeke (M.S. Mechanical Engineering)

Reliability Analysis of Springs Used as Interconnects in Press-Pack Power Electronic Modules

Recent designs of power electronic interconnections in Insulated Gate Bipolar Transistors (IGBT) press-pack devices utilize a spring to establish a gate contact to the IGBT chips. The use of the mechanical contact replaces wirebonds, which are used in conventional IGBT power modules. The goal of this project is to establish design guidelines for the springs used as interconnects based upon their failure mechanisms and operating conditions.

IGBTs are being used in a wide range of power applications, which impose increasingly rigorous demands on the life cycle of the IGBT devices of upwards of 30 years of failure free operation. However, with the increased power density of IGBTs, the thermo-mechanical stresses in conventional IGBT devices can cause failure from wire bond fatigue, substrate cracking, and solder fatigue during the operational lifetime of the device.

Press-pack assembly techniques for IGBTs are being used to mitigate the failure mechanisms of conventional IGBT packages. A press-pack assembly does not use wire bonds or solder joints, rather, compressed spring contacts are used to contact the IGBT chips, eliminating conventional failure mechanisms. However, new failure mechanisms are introduced from the spring contacts such as spring fatigue, stress relaxation, wear and fretting. These are investigated, and the corresponding failure models are developed using a physics-of-failure approach.