CALCE EPRC ADDRESSES INDUSTRY CONCERNS (January 1997 issue of CALCE News©)
Assessment of Reliability of Low-cost Flip-chip Assemblies The task is to ensure through experimentation and physics of failure (PoF) analysis, the reliability of low-cost flip-chip interconnects. This study uses PoF analysis to quantify the lessons learned from a very comprehensive test program. Failure mechanisms associated with flip-chip on organic boards (FCOB), with and without underfill, are being investigated through detailed design of experiments, using thermomechanical and vibrational loading. PoF design models will be built, based on the CALCE Multi-Domain Rayleigh-Ritz stress analysis tool (see Project C95-13) and the CALCE Energy-Partitioning damage model. The goal is to develop design models that have the ability to predict the effect of the key design variables on the dominant damage modes. Members can use this model to design robust FCOB packages proactively, and assess the acceleration transforms for accelerated tests on FCOB packages. Contact: Dr. A. Dasgupta: dasgupta@calce.umd.edu.
Accelerated Testing of PEMs for Long-term Storage Applications Significant life-cycle cost savings are possible in electronic systems through the incorporation of plastic encapsulated microcircuits. However, accelerated aging test procedures and failure mechanism models are needed to ensure that the incorporation of PEMs does not reduce reliability. In this project, we have developed accelerated test procedures and physics-of-failure models for the dominant failure mechanisms in PEMs subjected to extended periods of dormancy and are calibrating and validating those models with the results of accelerated temperature cycling and temperature-humidity tests. Contact: Dr. P. McCluskey: mcclupa@eng.umd.edu.
Experimental Degradation Analysis of Intermittently Used PEMs and Assemblies Many applications require intermittent use of electronics, a condition which is often more damaging than either storage or continuous use alone. This project is determining the failure sites, modes, and mechanisms, and the extent of degradation in assemblies that have been intermittently used for periods up to fifteen years, and examining which duty cycle causes the most damage. Contact: Dr. P. McCluskey: mcclupa@eng.umd.edu.
Criteria for Manufacturer Parts Assessment The CALCE EPRC is currently undergoing a study to effectively determine which parts are able to meet users' engineering requirements. The objective is to document a consistent qualitative and realistic process to enable users to assess the quality and performance capability of parts supplied by component manufacturers. Contact CALCE EPSC.
Assessment of the Remaining Life in Intermittently Used Assemblies A significant understanding in the reliability of electronics can be achieved by retrieving fielded assemblies and examining the extent of degradation that has occurred over time. In this project, we are developing methods for reliability growth in commercial assemblies by assessing the degradation in fielded assemblies and conducting accelerated tests to assess their remaining life. Techniques are also being developed for transferring these technologies and methodologies to our members. Contact: Dr. P. McCluskey: mcclupa@eng.umd.edu.
Suitability of PEMs for Use in Fast Jet Avionics Systems -The overwhelming cost advantages of plastic encapsulated microcircuits (PEMs) are driving many electronics manufacturers to consider their use in avionics applications which previously mandated military grade ceramic parts. This trend is increasing as advanced microcircuits are becoming available only as PEMs. This project examines the effect of low pressure and high altitude on plastic encapsulated microcircuits, focusing on outgassing, popcorning, and long term corrosion issues. Contact: Dr. P. McCluskey: mcclupa@eng.umd.edu.
Effect of Delamination and Cracking on PEM Reliability Many components are rejected or replaced solely because delamination is present at one or more package interfaces. This can significantly increase the life-cycle cost of an electronic system. However, CALCE EPRC studies have shown that delaminated PEMs can often survive extended exposure to temperature and humidity with no evidence of corrosion. This project is extending these studies to look at the effects of delamination on the susceptibility of SOICs, PQFPs, PLCCs, and PBGAs to moisture and thermal cycling related failure. Contact: Dr. P. McCluskey: mcclupa@eng.umd.edu
Uprating of Commercial Parts for Use in Harsh Environments In this project, a generic methodology is being developed for qualifying commercial components for use outside the commercial temperature range. This methodology addresses device performance and long-term reliability issues, based on investigations into the low-temperature failure of PEMs, the delamination and cracking of PEMs at low temperatures, and the use of PEMs at elevated temperatures. In addition, it addresses practical concerns about vendor selection and legal issues. Contact: Dr. P. McCluskey: mcclupa@eng.umd.edu.
Generic Solder Joint Reliability Model The goal is to present members with an integrated state-of-the-art software tool for predicting solder interconnect durability, as part of the CALCE toolkit. This model is capable of simplified viscoplastic stress analysis at a fraction of the cost of finite element analyses of comparable accuracy, and uses the CALCE energy-partitioning damage model to predict solder interconnect durability. This model has and continues to be verified through accelerated stress tests. In the future, this model will be able to handle both vibration and thermo-mechanical loading. Contact: Dr. A. Dasgupta: dasgupta@calce.umd.edu.
Maximizing Yield and Reliability for SMT Manufacturing Maximizing yield and reliability for SMT manufacturing of solder joints in adhesively bonded fine-pitched SMT components is being studied via a DOE and POF investigation. In applications where thermally conductive adhesives are called out, 20 mil pitch and smaller CQFPs with high pin counts are almost impossible to reflow because the adhesive cures as the assembly is preheated. At reflow the part doesn't settle into the solder paste and can't self-align through surface tension, resulting in shorts and opens. We expect to take a simultaneous two-pronged approach to this problem: (i) vary the process parameters to enable successful solder joint formation of the components requiring thermal epoxy and (ii) conduct a theoretical simulation of the solder joint formation process, to understand the interaction between the critical variables in this process. Without changing material types, adding new process steps, or modifying equipment, the practical approach involves the consideration of three primary factors: (i) the volume of epoxy deposited, (ii) stencil aperture length, and (iii) stencil thickness. A fractional factorial experiment has been designed to consider these three primary variables, while holding all other manufacturing parameters constant. Contact: Dr. Abhijit Dasgupta dasgupta@eng.umd.edu.
Conductive Filament Formation Modeling in Low-voltage/Fine line PWB's Fine line PWB's used in board level and component level packaging are often subject to failure due to Conductive Filament Formation (CFF). CALCE is currently extending its previous work in CFF to develop design guidelines for use on low-voltage printed wireboards. The study is concentrating on the effects of thermal cycling and moisture ingress from relative humidity with a variety of laminate materials in different voltage and power ranges. Contact: CALCE EPSC.
Characterization of Properties of Thin Films and Laminates Polyimides and thin films are being used extensively in the microelectronics industry. They may be found as dielectric insulators, passivation layers, inter-metal insulators, and encapsulants for integrated circuits. Many of these films exhibit an affinity for moisture and can absorb several percent by weight depending upon their environment. The absorbed moisture can degrade the reliability of the package by both inducing stresses and degrading the mechanical properties of the film. The degradation of the mechanical properties has not been well documented. CALCE is conducting research on thin films and polyimides to assess the amount of mechanical degradation attributed to moisture absorption in a controlled environment. Along with characterizing the degradation, the absorbed moisture will also be monitored insitu. The equipment used to conduct the experiments are the thin film analyzer (TFA) and the Micro Quad 8000 Moisture Absorption System. The TFA is a "custom-built" computer-controlled system developed to measure thin-film properties such as strength, strain, stiffness, creep, and dimensional stability. The relative humidity and temperature of the system can be altered by computer for various effects, including load effects. Utilizing this combination of equipment, in conjunction with the environmental scanning electron microscope (E-SEM), and the tensile substage mounted inside of the E-SEM, research scientists at CALCE are able to monitor and characterize the structural damage and degradation on different types of thin films, polyimides, and laminates due to environmental changes. Contact: Keith Rogers: dilcia@eng.umd.edu.
A Physics-of-Failure Approach to Accelerated Testing of Assemblies This project illustrates a physics-of-failure (pof) method for maximizing test-time compression cost-effectively in accelerated stress tests, using simultaneous application of different stresses. In accordance with IAB interests, the emphasis is on fatigue mechanisms prevalent in assembly-level interconnects. Three specific issues were identified, based on the interests of IAB members, for investigation in this project: (i) the synergy between vibrational stresses and temperature cycling stresses for test time compression; (ii) the effectiveness of repetitive shock (RS) random vibration stresses vs. electrodynamic (ED) random vibration stresses for test-time compression of failures; (iii) the physics-of-failure approach for deriving quantitative damage metrics, acceleration transforms, and stress margins for these stresses. Contact: Dr. A. Dasgupta: dasgupta@calce.umd.edu.
PWA Curvature-Driven Overstress Failure Models The new static solver in the CALCE software developed this past year can calculate the curvature at any point in the PWA due to the applied boundary conditions. Similarly, the new shock spectrum solver can calculate the curvature at any point in the PWA due to an applied shock pulse. Existing failure mechanism models in the software process this curvature data to ultimately predict a fatigue life for component leads and attaches. The project this year will process the curvature data to predict lead and attach failures that will occur under a single application i.e., overstress failure and not cumulative damage mechanism. For a first approximation of the dynamic loading or shock applications, the nonlinear strain-rate response of the materials will be handled with a linear simple factor of safety. Minimally, the following models will be placed in the software to predict an overstress failure due to the single application of PWA curvature or bending: PTH lead (DIP or PGA package); SMT lead and solder attach (most J and gullwing leaded packages, ceramic as well as plastic styles); LCCC solder joint attach; and PWA delamination and trace failures. Contact Dr. Don Barker: dbarker@eng.umd.edu.
PWA Failure Mechanism Model Handbook: Thermal Fatigue and PWA Failure Mechanism Model Handbook: Vibration Fatigue These two projects are related but deal with two different classes of failure mechanisms. As the physics-of-failure approach to electronic equipment life assessment gains acceptance, there is increasing demand for a one-source location for information about the various failure models. Information about each model is available in the open literature, but archival journal publications are not necessarily easily located nor readable by the practicing engineer. In addition, relevant information about each model is more likely an evolving story, as independent investigators critique their colleagues' ideas. A practicing engineer does not have the time to get up to speed on any one model, let alone the entire field. Thus, there is a need for a summarized handbook or resource document that not only has the models, but explains their strong and weak points and when they can or cannot be used. The document is envisioned to be in electronic format i.e. in HTML format. This is not to preclude publication as a conventional document, but to take advantage of context-sensitive searches and jumps. The key to success will be to condense the information on each model down to a concise summary and list references for the interested individual to search out more details. Caution will be exercised in summarizing the models so that ranges of applicability and implied assumptions are not ignored. Critical comments include the expected accuracy, any validation results, and critical parameters and variables that dramatically influence the results. Contact Dr. D. Barker: dbarker@eng.umd.edu.
Passive Cooling for High-heat Fluxes Using Thermosyphons Thermosyphons are thermal management devices that utilize natural circulation of a working fluid for heat transport. During the past year, a single-phase thermosyphon was developed at CALCE EPRC to dissipate 8 W/cm2 in an entirely passive mode. The current project will investigate the geometric optimization of passively cooled thermosyphons. Since the overall size of the thermosyphon is strongly dependent on the heat rejection region, parametric computational studies will be performed to optimize the fin structure at this end. As part of the current effort, the performance of a two-chamber thermosyphon, in which the heating and cooling sections are separated, will also be investigated. Such a thermosyphon will be evaluated both for single-phase and two-phase operation. Contact: CALCE EPSC.
Health Management of Electronic Systems The goal of this project is to identify and develop methods for on board monitoring of electronic systems. These early detection fuses are calibrated from accelerated testing of the system so that they fail at a designated time before the system fails. This early detection fuse failure notifies technicians in the field the device is near the end of its serviceable life thus needing to be replaced. Contact: CALCE EPSC.
Rapid Processing of BGA Underfills by Variable-frequency Microwave Cure of Liquid Encapsulants Current methods of chip-on-board and surface-mount attach technologies involve slow production throughput due, in part, to time-consuming thermal epoxy cure techniques. Application of a variable frequency microwave (VFM) energy source to this area represents a rapid processing method that will enable significant improvement over the conventional approach. We will characterize the mechanical performance of BGA packages that feature microwave-cured underfill encapsulant materials. Performance will also be compared to the performance of conventional thermal cure packages. The testing will include a battery of environmental tests to determine stress-induced damages, and functional integrity of the logic chip will be verified as well. Contact: CALCE EPSC.
Performance and Reliability Assessment of Thermoelectric Cooling Thermoelectric cooling is an active thermal management technique based on the Peltier effect, which describes the generation of a temperature difference across a material in response to an applied potential difference. Thermoelectric coolers (TEC) are solid-state semiconductor devices that operate from DC power sources and have no moving parts. This project will focus on the assessment of performance and reliability of TEC for the thermal control of conventional and high temperature (up to 200 Celcius) electronics. Contact: CALCE EPSC.
Evaluation of Temperature-Humidity Interaction Effects Using Mechanical Accelerators This study will explore the feasibility of detecting damage induced by combined temperature and moisture exposure through application of subsequent accelerated vibration stresses. Experiments will be conducted on surface-mount circuit card assemblies with daisy-chained components to facilitate real-time electrical monitoring of failures. This technique has the potential to significantly cut down the time and cost required for temperature-moisture accelerated stress tests of electronic assemblies. Contact: Dr. A. Dasgupta: dasgupta@calce.umd.edu.
Distributed Filament Contacts It is well known that metallic contacts can suffer electric failure as the result of contamination and long-term environmental corrosion. As a result, a new composite material that can be fabricated into distributed filament contacts (DFCs) was invented by Xerox as a more robust alternative to metal contacts for certain applications. DFCs are carbon-fiber filled polymeric composites that are processed into contacts rich in highly oriented, conductive carbon fiber. While these contacts have somewhat higher contact resistances than metals, the multiple redundancy of many thousands of fibers coupled with excellent corrosion resistance and thermal properties indicate that DFCs will have exceptional stability and reliability.