CALCE EPSC Graduate Student Theses (2006)

Dash, Manas (Ph.D. Mechanical Engineering)

Thermo-Mechanical Durability Assessment and Microstructural Characterization Study of 95.5Pb2Sn2.5Ag High Temperature Solder

There is an increasing need in the avionics, military, oil exploration and automotive industries for high temperature solders that perform reliably in ever-higher temperature applications. In these applications, solders are often used as large area die attaches and due to the high power involved, they need to dissipate large amounts of heat that can further increase the thermal load on the devices. The mechanical, electrical and thermal behavior of the solder must be understood to ensure devices and package reliability. There is an especially urgent need for characterizing constitutive properties and thermo-mechanical durability of high temperature solders.

A partitioned constitutive model consisting of elastic, plastic and creep models was obtained for the 95.5Pb2Sn2.5Ag solder by implementing the direct local measurement technique. The validity of the assumptions used to generate these models have been demonstrated using microstructural characterization.

The thermo-mechanical durability of the 95.5Pb2Sn2.5Ag solder is investigated using thermal cycling tests and finite element modeling. A high reliability package manufacturing technique has been followed. The extensive detailed two-dimensional viscoplastic FE stress and damage analysis is conducted for five different thermal cycling tests of 95.5Pb2Sn2.5Ag solders. The energy-partitioning durability model of the solder is obtained. It is found that 95.5Pb2Sn2.5Ag solder is creep dominant at high temperatures.

The microstructure characterization study on 95.5Pb2Sn2.5Ag solder reveals that it remains primarily a single phase in the range of temperature under study with very few Ag3Sn intermetallics. Fatigue cracks due to thermal cycling have been observed.

Deng, Yuliang (Ph.D. Mechanical Engineering)

Carbon Fiber Electronic Interconnect

Carbon fiber is an emerging material in electrical and electronics industry. It has been used as contact in many applications, such as switch, potentiometer, and commutator brush. A new technique of electronics interconnect is developed, with carbon fiber as a conductive medium. This novel invention can provide interconnection between two planes in different levels of electronics packaging, from semiconductor die, substrate, packaged component to printed circuit board. For example, it can provide a separable interconnect between a land grid array (LGA) or ball grid array (BGA) IC package to a printed circuit board, as an LGA or BGA socket. The interconnect device consists of an array of contact pins. Each contact pin consists of a large number of carbon fibers which can act cooperatively to provide a high degree of reliability and predictability to the interconnect function. An optional metal coating, such as nickel, copper, aluminum, can be applied over carbon fibers to enhance its conductivity and solderability. Analytical evaluations and experimental mechanical and electrical characterizations have been conducted to conclude that this invention is a promising interconnect technique.

Jannesari Ladani, Leila (Ph.D. Mechanical Engineering)

Damage Initiation and Evolution in Voided and Unvoided Lead Free Solder Joints Under Cyclic Thermo-Mechanical Loading

The effect of process-induced voids on the durability of Sn-Pb and Pb-free solder interconnects in electronic products is not clearly understood and researchers have reported conflicting findings. Studies have shown that depending on the size and location, voids are not always detrimental to reliability, and in fact, may sometimes even increase the durability of joints. This debate is more intensified in Pb-free solders; since voids are more common in Pb-free joints. Results of experimental studies are presented in this study to empirically explore the influence of voids on the durability of Ball Grid Array (BGA) Pb-free solder joints. In order to quantify the detailed influence of size, location, and volume fraction of voids, extensive modeling is conducted, using a continuum damage model (Energy Partitioning model), rather than the existing approaches, such as fracture mechanics, reported in the literature. The E-P approach is modified in this study by use of a successive initiation method, since depending on their location and size; voids may influence either the time to initiate cyclic fatigue damage or time to propagate fatigue damage, or both. Modeling results show competing interactions between void size and location, that results in a non-monotonic relationship between void size and durability. It also suggests that voids in general are not detrimental to reliability except when a large portion of the damage propagation path is covered with either a large void or with many small voids. In addition, this dissertation also addresses several fundamental issues in solder fatigue damage modeling. One objective is to use experimental data to identify the correct fatigue constants to be used when explicitly modeling fatigue damage propagation in Pb-free solders. Explicit modeling of damage propagation improves modeling accuracy across solder joints of vastly different architectures, since the joint geometry may have a strong influence on the ratio of initiation-life to propagation-life. Most solder fatigue models in the literature do not provide this capability since they predict failure based only on the damage accumulation rates during the first few cycles in the undamaged joint. Another objective is to incorporate into cyclic damage propagation models, the effect of material softening caused by cyclic micro-structural damage accumulation in Pb-free solder materials. In other words the model constants of the solder viscoplastic constitutive model are continuously updated with the help of experimental data, to include this cyclic softening effect as damage accumulates during the damage-propagation phase. The ability to model this damage evolution process increases the accuracy of durability predictions, and is not available in most current solder fatigue models reported in the literature. This mechanism-based microstructural damage evolution model, called the Energy Partitioning Damage Evolution (EPDE) model is developed and implemented in Finite Element Analysis of solder joints with the successive initiation technique and the results are provided here. Experimental results are used as guidance to calibrate the Energy Partitioning fatigue model constants, for use in successive initiation modeling with and without damage evolution. FEA results show 15% difference between the life predicted by averaging technique and successive initiation. This difference could significantly increase in the case of long joints such as thermal pads or die-attach, hence validating the use of successive initiation in these cases. The difference between using successive initiation with and without damage evolution is about 10%. Considering the small amount of effort that has to be made to update the constitutive properties for progressive degradation, it is recommended that softening be included whenever damage propagation needs to be explicitly modeled. However the damage evolution exponents and the corresponding E-P model constants obtained in this study, using successive initiation with damage evolution, are partially dependent on the specimen geometry. Hence, these constants may have to be re-calibrated for other geometries.

Jinka, Krishna Kumar (M.S. Mechanical Engineering)

Thermo-mechanical Analysis of Encapsulated Ball-Wedge Wire Bonds in Microelectronics, using Raleigh-Ritz Modeling

This study addresses encapsulated wire bonds in chip-on-board (CoB) multi chip modules, which provide a low cost option for dealing with the current trend towards compact microelectronic packages with increased I/O, higher reliability and lower cost. The focus is on thermomechanical stresses caused in the bond wires when the encapsulant is cooled from high curing temperatures and subsequently subjected to thermal cycling loading. The stresses generated in bond wires due to thermal expansion mismatches, in an encapsulated CoB are very complex and are driven by both global and local thermal expansion mismatches between: (i) glob-top encapsulant and the silicon die, (ii) encapsulant and the wire, and (iii) encapsulant and the substrate assembly.

A 2D stress analysis model based on the variational Raleigh-Ritz (RR) method is developed, to estimate thermomechanical stresses in the bond wire, based on elastic analysis. The study focuses on detailed parametric investigation of different encapsulated CoB configurations. The initial wire profile, before encapsulation, is first modeled with RR-2D trial functions based on cubic splines. This predicted geometry is then used for the subsequent thermomechanical stress analysis after encapsulation, based on trial functions composed of polynomials and exponential functions. The results are validated by FEA. Plastic deformation is ignored in the current analysis, as a first-order approximation. This model is therefore suitable for parametric design sensitivity studies and qualitative ranking of design options. The results show that the region above the ball bond is the predominant failure site. The RR 2-D model has a well-defined range of validity for CoB Ball-Wedge wire bond configurations with stiff encapsulants (E >= 3 GPa) and thin wires (dia <= 2 mils). Also, the trend of maximum elastic strains obtained from the RR 2-D model agrees qualitatively with thermal cycling fatigue test data obtained from the literature.

Liu, Yan (Ph.D. Mechanical Engineering)

Reduction of Skin Stretch Induced Motion Artifacts in Electrocardiogram Monitoring Using Adaptive Filtering

Cardiovascular disease (CVD) is the leading cause of death in many regions worldwide, accounting for nearly one third of global deaths in 2001. Wearable electrocardiographic cardiovascular monitoring devices have contributed to reduce CVD mortality and cost by enabling the diagnosis of conditions with infrequent symptoms, the timely detection of critical signs that can be precursor to sudden cardiac death, and the long-term assessment/monitoring of symptoms, risk factors, and the effects of therapy. However, the effectiveness of ambulatory electrocardiography to improve the treatment of CVD can be significantly impaired by motion artifacts which can cause misdiagnoses, inappropriate treatment decisions, and trigger false alarms. Skin stretch associated with patient motion is a main source of motion artifact in current ECG monitors. A promising approach to reduce motion artifact is the use of adaptive filtering that utilizes a measured reference input correlated with the motion artifact to extract noise from the ECG signal. Previous attempts to apply adaptive filtering to electrocardiography have employed either electrode deformation or acceleration, body acceleration, or skin/electrode impedance as a reference input, and were not successful at reducing motion artifacts in a consistent and reproducible manner. This has been essentially attributed to the lack of correlation between the reference input selected and the induced noise.

In this study, motion artifacts are adaptively filtered by using skin strain as the reference signal. Skin strain is measured non-invasively using a light emitting diode (LED) and an optical sensor incorporated in an ECG electrode. The optical strain sensor is calibrated on animal skin samples and finally in-vivo, in terms of sensitivity and measurement range. Skin stretch induced artifacts are extracted in-vivo using adaptive filters. The system and method are tested for different individuals and under various types of ambulatory conditions with the noise reduction performance quantified.

Qi, Haiyu (Ph.D. Mechanical Engineering)

Plastic Ball Grid Array Solder Joint Reliability Assessment Under Combined Thermal Cycling and Vibration Loading Conditions

Concurrent vibration and thermal environment is commonly encountered in the service life of electronic equipment, including those used in automotive, avionic, and military products. Though extensive research exists in literature for solder joint failures due to thermal cycling, limited research has been conducted on investigating solder joint failures due to a combination of vibration and thermal cycling.

In this study, experiments were conducted on PBGA assemblies under thermal cycling, vibration loading, and combined thermal cycling and vibration loading conditions. The results showed much earlier PBGA solder joint failure under combined loading compared with either thermal cycling or vibration loading alone. It was found that traditional linear superposition can overpredict the solder joint fatigue life since it neglects the interaction of the vibration and thermal cyclic loadings.

An incremental damage superposition approach using finite element analysis was applied to PBGA solder joint reliability assessment. This approach can model the nonlinear interactions between vibration loading and thermal cycling. It considers the temperature effect on vibration response and the effect caused by thermomechanical mean stress affects. This approach was validated through experiments and reflects the actual damage trends.

Based on the incremental damage superposition approach, a rapid solder joint fatigue life prediction simulation approach for PBGA was also developed for combined temperature cycling and vibration loading conditions. This approach included a thermomechanical stress model and a vibration stress model to analyze the interconnect stress under thermal cycling and vibration loading conditions. The mean stress during thermal cycling was obtained from the response curve. The damage due to two different loadings was then calculated using the generalized strain approach and superposed. This approach was also validated using experimental data.

Sengupta, Shirsho (M.S. Mechanical Engineering)

Effects of Solder-Dipping as A Termination Re-Finishing Technique

Solder-dipping may be used to replace tin-rich finishes with eutectic tin-lead for tin-whiskering risk mitigation purposes. Electronic parts may however, be exposed to new risks from the solder-dipping process such as those due to thermo-mechanical damage from thermal shock, finish non-uniformity, incomplete replacement of the pre-existing finish and potential poor solderability from re-finishing. This work presents an overview of solder-dipping as a re-finishing technique and identifies key process and part parameters that could result in each of these risks.

A physical analysis procedure was developed and implemented to assess these risks on electronic parts. A quantitative metric was established to assess propensity for thermo-mechanical damage for solder-dipping parts. Surface mount dipped parts were prone to exposure of base-metal or interfacial intermetallics at termination knees and heels. Dipped insertion-mount parts showed regions of low finish thickness and possible deviations from eutectic tin-lead composition at portions close to the part-body.

Song, Bo (M.S. Mechanical Engineering)

Reliability Evaluation of Stacked Die BGA Assemblies under Mechanical Bending Loads

This thesis presents a reliability evaluation of stacked die ball grid array (BGA) assemblies under mechanical bending loads. During assembly and use conditions, it is not uncommon for these products to be subject to dynamic bending loads. It is therefore necessary to understand how stacked die BGA assemblies respond to dynamic loads.

The test specimens used in this investigation were four die stacked BGAs assembled on printed circuit boards (PCBs) with eutectic tin-lead solder and gold over nickel finishes, both as-reflowed and after aging. The failure envelopes of both types of specimen were quantified in terms of PCB flexural strain and strain rate. The experimental data from cyclic bending tests at three strain amplitudes with a constant strain rate have been used to determine the effect of strain amplitudes on cycles to failure. The experimental data from cyclic bending tests were combined with the data from impact tests to determine the effect of strain rate to cycles to failure. For the aged specimens, the relationship between durability and PCB flexural strain and strain rate was determined using an existing power law model. It shows that durability is strongly dependent on strain and weakly dependent on strain rate within the range of conditions studied. The failure sites associated with each test condition were identified, and failure site transition phenomena are reported and discussed.

Vichare, Nikhil (Ph.D. Mechanical Engineering)

Prognostics and Health management of Electronics by Utilizing Environmental and Usage Loads

Prognostics and health management (PHM) is a method that permits the reliability of a system to be evaluated in its actual application conditions by assessing the extent of degradation or deviation from an expected normal condition. By determining the advent of failure, procedures can be developed to mitigate, manage and maintain the system. Since, electronic systems control most systems today and their reliability is usually critical for system reliability, PHM techniques are needed for electronics.

To enable prognostics, a methodology was developed to extract load parameters required for damage assessment from irregular time-load data. As a part of the methodology an algorithm that extracts cyclic range and means, ramp rates, dwell times, and correlation between load parameters was developed. The algorithm enables significant reduction of the time-load data without compromising features that are essential for damage estimation. The load parameters are stored in bins with a-priori calculated (optimal) bin-width. The binned data is then used with Gaussian kernel functions for density estimation of the load parameter for use in damage assessment and prognostics. The method was shown to accurately extract the desired load parameters and enable condensed storage of load histories, thus improving resource efficiency of the sensor nodes.

A generic approach to prognostics that accounts for the uncertainties in measurement, model-input, and damage models was developed. The approach utilizes sensitivity analysis to identify the dominant input variables that influence the model output, and utilizes the distribution of measured load parameters and input variables in a Monte-Carlo simulation to provide a distribution of accumulated damage. Using regression analysis of the accumulated damage distributions, the remaining life is then predicted with confidence intervals. The proposed method was demonstrated using an experimental setup in which an electronic board was subjected to completely irregular temperature cycles. The board temperatures were monitored in-situ and processed using the proposed PHM technique to predict remaining life of component to board interconnects due to thermal fatigue. It was found that given the system uncertainties, the actual failures in testing were observed within the predicted failure distribution.

A failure precursor based approach was developed for remaining life prognostics by analyzing performance (resistance continuity) data in conjunction with usage loads (temperature cycles). Using the data from the PHM experiment, a model was developed to estimate the resistance based on measured temperature values. The difference between actual and estimated resistance value in time-domain were analyzed to predict the onset and progress of interconnect degradation. The method was shown to predict remaining life by trending several features including means, mean peaks, kurtosis, and 95% cumulative values of the resistance drift distributions.

Wu, Meiling (Ph.D. Mechanical Engineering)

Ball Grid Array (BGA) packages are a relatively new package type and have rapidly become the package style of choice. Much high density, high I/O count semiconductor devices are now only offered in this package style. Designers are naturally concerned about the robustness of BGA packages in a vibration environment when their experience base is with products using more traditional compliant gull or J leaded surface mount packages. Because designers simply do not have the experience, tools are needed to assess the vibration fatigue life of BGA packages during early design stages and not have to wait for product qualification testing, or field returns, to determine if a problem exists.

This dissertation emphasizes a rapid assessment methodology to determine fatigue life of BGA components. If time and money were not an issue, clearly one would use a general-purpose finite element program to determine the dynamic response of the printed wiring board in the vibration environment. Once the response of the board was determined, one would determine the location and value of the critical stress in the component of interest. Knowing the critical stress, one would estimate the fatigue life from a damage model. The time required building the FEA model, conducting the analysis, and post-process the results would take at least a few days to weeks. This is too time-consuming, except in the most critical applications. It is not a process that can be used in everyday design and what-if simulations. The rapid assessment approach proposed in this research focuses on a physics of failure type approach to damage analysis and involves global and local modeling to determine the critical stress in the component of interest. A fatigue damage model then estimates the life. Once implemented in software, i.e. the new version of CALCEPWA, the entire fatigue life assessment is anticipated to be executed by an average engineer in real time and take only minutes to generate accurate results.

Zhan, Sheng (Ph.D. Mechanical Engineering)

Surface Insulation Resistance Degradation and Electrochemical Migration on Printed Circuit Boards

Widespread adoption of lead-free materials and processing for printed circuit board (PCB) assembly has raised reliability concerns regarding surface insulation resistance (SIR) degradation and electrochemical migration (ECM). As PCB conductor spacings decrease, electronic products become more susceptible to these failures mechanisms, especially in the presence of surface contamination and flux residues which might remain after no-clean processing. Moreover, the probability of failure due to SIR degradation and ECM is affected by the interaction between physical factors (such as temperature, relative humidity, electric field) and chemical factors (such as solder alloy, substrate material, no-clean processing).

Current industry standards for assessing SIR reliability are designed to serve as short-term qualification tests, typically lasting 72 to 168 hours, and do not provide a prediction of reliability in long-term applications. The risk of electrochemical migration with lead-free assemblies has not been adequately investigated. Furthermore, the mechanism of electrochemical migration is not completely understood. For example, the role of path formation has not been discussed in previous studies. Another issue is that there are very few studies on development of rapid assessment methodologies for characterizing materials such as solder flux with respect to their potential for promoting ECM.

In this dissertation, the following research accomplishments are described: 1). Long-term temp-humidity-bias (THB) testing over 8,000 hours assessing the reliability of printed circuit boards processed with a variety of lead-free solder pastes, solder pad finishes, and substrates. 2). Identification of silver migration from Sn3.5Ag and Sn3.0Ag0.5Cu lead-free solder, which is a completely new finding compared with previous research. 3). Established the role of path formation as a step in the ECM process, and provided clarification of the sequence of individual steps in the mechanism of ECM: path formation, electrodeposition, ion transport, electrodeposition, and filament formation. 4). Developed appropriate accelerated testing conditions for assessing the no-clean processed PCBsĄŻ susceptibility to ECM: a). Conductor spacings in test structures should be reduced in order to reflect the trend of higher density electronics and the effect of path formation, independent of electric field, on the time-to-failure. b). THB testing temperatures should be modified according to the material present on the PCB, since testing at 85oC can cause the evaporation of weak organic acids (WOAs) in the flux residues, leading one to underestimate the risk of ECM. 5). Correlated temp-humidity-bias testing with ion chromatography analysis and potentiostat measurement to develop an efficient and effective assessment methodology to characterize the effect of no-clean processing on ECM.

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