| Created: 10/19/99 |
Cluff, Kevin (Ph.D. Mechanical Engineering)
Characterizing the Humidity and Thermal Environments of Commercial Avionics for Accelerated Test Tailoring
The climatic environments, especially thermal cycling and humidity, have long been recognized as key factors in electronics reliability. Most humidity and thermal cycle accelerated tests are somewhat arbitrarily specified and rarely have demonstrated linkage to the in-service environment. This work proposes a methodology to determine realistic humidity and thermal cycle test requirements based on measured environment data.
To illustrate this approach, environment data from commercial airplanes is measured and applied to specific failure mechanisms. Temperature, relative humidity, pressure, and equipment power condition for more than 1100 flights were measured in the key avionics areas of an operating airplane. The local environment at the failure site is derived using transient assumptions. To reduce irregular field data to a from usable by acceleration transforms, counting algorithms are presented that preserve necessary attributes. The time at discrete humidity and temperature intervals is accumulated. For thermal cycle counting, a three-parameter rain flow method is introduced that preserves time-dependent parameters needed for solder fatigue analysis. Tailoring accelerated tests based on average environment parameters is compared with these damage integration approaches.
Surface-mount technology and plastic encapsulated microcircuits are increasingly used in new avionics designs. These technologies are used as example cases of developing realistic accelerated life tests. Existing and computationally simple acceleration transforms are used to estimate the environmentally induced damage.
Tests are used in component qualification. In the case of surface-mount solder join fatigue, the IPC published a definition of the commercial airplane thermal cycle environment.
Although this is not a sufficient sample to draw general conclusions about the commercial airplane environment, it provides a more detailed view of commercial airplane environments than has ever been available. This methodology will enable equipment designs to be optimized for the real environment and improve accelerated test effectiveness, thereby reducing costs while enhancing reliability.
Fowler, Andre (M.S. Mechanical Engineering - Dec. 1996)
Storage Reliability Assessment Modeling of PEMs
The knowledge base for reliability of operating electronic equipment is expansive. Conversely, studies on the effects of storage environments and intermittent operation of such equipment have been few. The goal of this research project has been to determine the "root-cause" failure mechanisms that develop in electronic equipment when exposed to such environments. These research efforts have been devoted to the evaluation of the degradation and future reliability of commercially available electronic parts and assemblies exposed to non-operating or intermittent operating conditions. Degradation analysis was performed on parts and assembles and the results utilized to gain further understanding of the danger such conditions pose to electronic equipment. The mild nature of storage conditions in general was found to be benign in it's effects on part reliability. All parts were found to be electrically and mechanically viable after storage periods of nearly 13 years. Of the parts examined, the hermetically sealed ceramic DIPs showed mechanical damage where the plastic encapsulated DIPs did not. Assemblies with intermittent load conditions showed much more damage. Severe delamination of the copper traces was found in one board, evidence of solder joint fatigue cracking was present in all boards. The area around the fly-back transformer was found to be particularly prone to damage. Plastic encapsulated DIPs utilized in the assemblies showed little damage prior to accelerated testing. Post accelerated test examination showed minor SDDV damage and Kirkendall voiding. SDDV damage was not extensive enough to cause electrical disruptions. Mechanical testing of the bond area showed no decrease in strength due to voiding.
Govind, Anand (M.S. Mechanical Engineering)
Real-Time Measurement and Characterization of Deformations in Surface Mount Packages during Infra-Red Reflow Soldering
Although several researchers have studied reflow induced package damage by post-reflow inspection techniques, few attempts have been made at real-time monitoring and measurement of package deformations during the reflow process. Knowledge of the package deformations can provide fundamental insights into the stresses seen by the package during the reflow and guide modeling, material selection, and process improvement efforts. In this study, a unique real-time, mechanical probe was designed for in-process measurement and recording of package deformations during simulated solder reflow. Experiments were conducted on plastic quad flat packages using the probe setup. Precise temperature and time resolved deformation signatures of the package due to the applied reflow profile were obtained. The results conclusively showed absorbed moisture in the package to have a profound effect on the package deformations beyond 130oC. The exact point of initiation of the delamination in terms of the package temperature and time of occurrence in the reflow were also located.
Jackson, Margaret (M.S. Mechanical Engineering)
Developing a Part Selection Plan using Benchmarking and Other Techniques
A company wide electronic parts management program offers a methodology for electric suppliers in the high reliability industry to minimize cost and risk, maximize reliability and availability, and profit from state-of-the-art procedures. Customers no longer dictate parts management policy to suppliers, forcing suppliers to assume responsibility for their own competitive product development. Parts tested, screened, and derated according to the mil-spec system are increasingly unavailable and more costly than their commercial equivalents. Suppliers, who have for years depended on the mil-spec system and on customer parts management requirements, are left to fend for themselves. Some suppliers have chosen to ignore the problem, while others scramble to work within what remains of the mil-spec system. Either way, shrinking profit margins and increasingly fierce competition from the commercial market place result. This document leads the user to competitive product development through the effective management of commercial electronic parts, and presents the rationale for each step in the process.
Jordan, Jill (M.S. Mechanical Engineering)
Effectiveness of Screening Procedures
Burn-in is an accelerated screen test, performed to precipitate defects in microelectronic parts, so that defect-related failures do not occur in the field. The data presented in this paper show that burn-in does not precipitate a significant amount of failures and the test has the high potential to cause problems that are not detected during post burn-in examinations. This has serious safety implications especially in terms of military electronics and commercial avionic where burn-in is generally recommended. Further, the results suggest that the philosophy of burn-in elimination (that is burn-in until there are few problem) is no longer appropriate.
Kelkar, Nikhil V. (M.S. Mechanical Engineering)
Phenomenological Reliability Modeling of Plastic Encapsulated Microcircuits
This thesis reviews those key studies that describe the mean time-to-failure of plastic encapsulated microcircuits as a function of steady state temperature and ambient relative humidity, using phenomenological, activation energy based reliability models. The models are then assessed for their applicability with respect to the use of an activation energy concept, influence of environmental "stresses" other than steady state temperature and relative humidity, and variabilities in failure mechanisms, modes, sites, materials, geometry, manufacturing processes, defects and failure distribution. Finally, we address the monotonic increase in activation energy that has occurred over the years and its impact on future reliability assessment and qualification procedures.
Product Realization Process Modeling: A Study of Requirements and Needs for Successful Implementation
This thesis presents a bottom up view of the requirements, industry practices, and research questions modeling of product realization. Today, process model, though relatively high-level and organizational in scope, are attracting interest among design engineers, system engineers and technical managers whose decisions affect the complex downstream task interactions that cross organizational boundaries. Unfortunately, the current state of process modelers lack the functionality to adequately capture the information that engineers require. To meet the nees of the engineering domain, process modelers need the functionality and representations provided by multiple tools. An object oriented modeler is required to document objects such as resources and attribute information required to support design activities. State Transition Diagrams must capture the multiple states (such as active, wait, in process) that are encountered when evaluation an engineering activity. Flow Diagrams are needed to capture the temporal information that is a critical discriminator between information models and models which define the product realization process.
The thesis does not prescribe, recommend, or discuss in detail, formal language specifications for modeling product realization processes. Comprehensive, enterprise-level models of the diverse human and machine task interactions necessary to build an electromechanical product are still premature. Instead, the intent of the thesis is to address a wede range of industry-relevant modeling issues which can help focus discussion on future research direction. A recent study of industrial usage, industrial requirements, and research issues is presented.
Martens, Rod (Ph.D. Mechanical Engineering)
Automated Contact Resistance Probe
As the density of separable interconnects increases, the magnitude of normal force available in a connector to make and maintain electrical contact must decrease. Unfortunately, as the normal force decreases, electrical contacts become more susceptible to such degradation mechanisms as film and pore corrosion. This work identifies the significant effects and interactions of the following electric contact design parameters: normal force, contact geometry, gold plating thickness, corrosion (mixed flowing gas exposure), and wipe at the contact interface.
To examine these parameters, phosphor bronze test coupons with two contact geometries were plated with thicknesses of cobalt gold ranging from 0 to 90 microinches. The samples were then exposed to the Battelle class II mixed flowing gas environment for 1 to 18 days. Contact resistance testing was performed using an automated contact resistance probe developed for this work which measures applies a normal force and can introduce wipe at the contact interface. The nickel plated results were examined separately from the gold plated results. The gold plated results were then divided into two cases: contact and wipe on or off of a corroded pore site due to the significant difference in contact resistance between the cases. For each of these cases, the effects and interactions of the design variables were ranked and statistical models are presented for the nickel and gold cases. Significant results include identifying the benefit of back wipe on nickel plated surfaces which has not been seen on softer contact surfaces such as gold and copper.
Pal, Debabrata (Ph.D. Mechanical Engineering)
Application of Phase Change Materials to Passive Thermal Control of Electronic Modules
A combined experimental and computational study of melting of organic phase change materials (PCMs) in rectangular enclosures is described. Phase change materials are widely used for energy storage and thermal control applications, and the present study demonstrated their use for passive thermal management of electronics. Detailed three-dimensional heat transfer models for melting of PCMs with or without porous metallic enhancements were developed, and were validated with the current experimental data. Both, side-heated, and bottom-heated configurations were investigated, where heat inputs were from uniformly dissipating sources. Two different porous enhancements, aluminum foam, and aluminum honeycomb were used. Natural convection effect on melting, inside honeycomb was negligible, when heated from bottom. On the other hand, natural convection effects were significant for side-heated configurations, both without, and with porous aluminum foam. The effect of a non-isothermal heating, for the present study resulted in a wide variation of temperature and heat transfer coefficients, on the heated wall. With an increase in the metal volume fraction for the porous aluminum foam, the effect of resulting increase in the thermal conductivity was offset by the decrease in the natural convection, during the later stage of melting. The computational models were used for evaluation of possible PCM thermal control of electronic components, and to perform parametric studies, for two different heating conditions. In the first study, a PCM laminate was used to cool a plastic quad flat package. An increase in the thermal conductivity of the printed wiring board (PWB) resulted in better spreading of heat to the PCM, causing uniform melting, opposed to localized melting, with a lower thermal conductivity of PCM. Higher thermal conductivity flat fins in the PCM, were found to provide better thermal performance. Re- orientation of the package assembly from vertical to horizontal direction did not result in significant difference in thermal performance. For the second study, passive thermal control of a flush mounted heat source was investigated. The heat source was used on one vertical side wall of a container filled with PCM. An increase in the height/width ratio of the enclosure was found to increase the rate of melting.
Poddar, Anindya (M.S. Mechanical Engineering)
Development of a Ruggedized Laptop Computer for Mobile Applications
There is a rapidly emerging market for rugged, portable computers that are capable of withstanding harsh field environments. With rapid advances in computer technology, manufacturers of rugged laptops need to constantly upgrade their products. It therefore makes sense to ruggedize existing, commercially available products, instead of building a product from scratch. The present work aims at ruggedizing IBM Thinkpad computers for field use. The concept of a generic ruggedized case, capable of handling most future products is presented. The IBM computer is surrounded by foam and effectively `floats' in a rugged shell. The product must be capable of withstanding accidental drops, rain, dust and dirt, road vibrations and extremes in temperature. However, customer surveys show that size, more than ruggedness is the critical design factor. The reduction in size however conflicts with shock and vibration isolation. The different components in the system and their performance under field conditions is investigated. Experiments are conducted to determine the design of shock isolating mounts. The behavior of the unit at temperature extremes is characterized and the need for additional thermal management investigated. Detailed mechanical design is carried out in order to transform the conceptual idea to a real, profitable product. The ruggedized computer has far greater resistance to rain and accidental spills. The ability to withstand shock and vibration loads is far superior to that of the IBM office computer. The overall performance of the product is greatly enhanced without sacrificing any of the features in the original laptop.
Ranade, Yogendra (M.S. Mechanical Engineering)
Delamination and Cracking Effects in PEMs
Major advantages in cost, size, weight, and market lead time have given plastic encapsulated microcircuits (PEMs) more than 95% of the worldwide electronic chip packaging market - and that figure is growing. PEMs now enjoy a virtual monopoly in the commercial market, and are increasing being accepted for use in high reliability military applications. This is not to say that plastic packages have been free of problems. Early PEMs were plagued by moisture-induced failure mechanisms, such as interfacial delamination, cracking, and corrosion, and by thermally induced intermittence problems that caused devices to suffer from open circuit failures at elevated temperatures [Pecht 1993]. Substantial improvements have been made in the reliability of PEMs as a result of advances in passivation technology and quality of the plastic encapsulants. Highly accelerated reliability tests, such as temperature/humidity bias and thermal cycling tests, performed to address the thermal and moisture related concerns in PEMs, have now shown PEMs to exhibit long term reliability comparable to, and, for some applications, better than hermetic packages. Although PEMs have exceptional reliability, one of the few remaining reliability concerns arises from the inherent moisture sensitivity of the epoxy encapsulants. Given the propensity of PEMs to absorb moisture, leading to possible interfacial delamination and cracking, it is important to address the impact of this delamination on future package reliability. Delamination and cracking may destroy improvements in the reliability of PEMs, since surface breaking cracks can serve as a path for the entry of ionic contaminants into the device, causing corrosion-induced failures. The shear stress developed can lead to relative motion between the plastic and the die surface resulting in hazardous "metal shifting". Delamination may also result in damage to the passivation layer, wire bonds, or to the metallization itself. A primary objective of this study is to determine if delamination affects subsequent time to failure in samples subjected to reliability tests. For instance, delamination may facilitate the accumulation of moisture and ionic contaminants at potential corrosion sites, and cause corrosion related failures. The study also addresses the possible relationships between the amount of existing delamination and the following:
Rothman, Timothy (M.S. Mechanical Engineering)
Acceleration Tranform Model for Leadless Solder Joint
The paper illustrates a methodology for using physics-of-failure models to extract acceleration transform information from limited test data under accelerated stresses. Test time compression is achieved by appropriately accelerating the stress levels in order to obtain accurate information on reliability. The critical variables are identified and their influence on the stress magnitude is quantified using physics-of-failure models. Careful experimental measurements are made to determine magnitude of the stress levels. The total amount of testing time is minimized by tailoring the critical variables in each sample such that multiple stress levels can be achieved in the samples under a single loading. This type of parametric-accelerated test eliminates the need for repeating the test at multiple load leve ls. All sources of error due to experimental variables and assumptions or simplifications in the analytical model are closely examined and discussed. A more detailed physics-of-failure model, the CALCE Energy Partitioning Model, is used to quantify the experimental results and determine the validity of the acceleration transform model. Such techniques are essential for cost effective and testing of highly reliable modules under accelerated stresses. The techniques are general enough to be applied to screening, qualification, and reliability enhancements of exiting products. Analytical predictive models for acceleration transforms will obviously result in significant savings of cost and time while providing the investigator with information other than just pass-fail.
Shukla, Anand (M.S. Mechanical Engineering)
Sorption Characterization of Laminates Used in Electronic Packaging Substrates
Since the 1930s, printed circuit boards (PCBs) have formed the physical backbone of electronic systems. They are generally comprised of layers of laminates, which are composites consisting of woven glass fabrics embedded in a resin matrix, and copper planes. Water has long been known to be a driver for failure modes and mechanisms in PCB laminates. It can increase dielectric constants, plasticize the resin, reduce glass-transition temperatures, cause swelling, facilitate ionic contamination, induce interfacial degradation, and promote conductive filament formation. Thus, the moisture content of laminates must be characterized for various environmental conditions.
Hygrothermal tests have been executed to characterize the sorption properties of laminates. The first set of experiments dealt with modeling the equilibrium sorption levels of each laminate in various environmental conditions. The second experiment involved evaluating the diffusion coefficient of each laminate. The laminates were then calibrated for their moisture content in terms of electrical capacitance for monitoring purposes.
Verma, Vani (M.S. Mechanical Engineering)
Mechanical and Thermal Analysis of a Unit Cell of a Microbridge Uncooled Focal Plane Array Infrared Detector
Ferroelectric uncooled focal plane array (UFPA) infrared temperature detectors are being used increasingly nowadays, for military as well as commercial purposes. a new microbridge structure unit cell of a focal plane array has been designed and its performance under operating conditions evaluated. The behavior of the unit cell under dynamic, thermal and thermo-mechanical loads, has been analyzed. the present research focuses on identifying the failure sites and modes of a single unit cell of the UFPA and developing analytical models to predict the performance of the unit cell. Stresses and deflections due to temperature gradients have been evaluated. The response of the unit cell to high shock loads has also been calculated.
The parametric models for assessing the UFPA microbridge unit cell have been appended to the CADMP-II software. the results of this analysis allow the designer to choose geometry and materials for the UFPA which reduce the risk of failure by the identified mechanisms. This makes reliability assessment part of the design process.
Yu, Enchao (Ph.D. Mechanical Engineering)
Passive Cooling of Vented and Closed Enclosures by Natural Convection, Conduction and Radiation
Computational and experimental investigations on combined conduction, natural convection and radiation heat transfer in discretely heated and vented three-dimensional enclosures are carried out. Both vertical and horizontal orientations of the heated surfaces are studied. Influence of computational boundary conditions is evaluated, especially the applicability of approximate boundary conditions prescribed on the enclosure apertures. Radiation is found to play a significant role in the overall heat transfer. Effects of power levels, venting configurations and substrate thermal conductivity on the heat transfer characteristics are studied. Temperature measurements and flow visualizations reveal flow and temperature fields in horizontally and vertically heated enclosures for different aspect ratios, opening area ratios, and venting configurations. These are useful in understanding the complex flow physics, as well as validating the numerical models. Effects of vents on multiple enclosure walls and influence of orthotropic thermal conductivity of substrates, both important in applications, are examined computationally. Enhancement of natural convection using pin-fin heat sinks is also examined using a porous media model and experimental techniques. Computational predictions are in reasonably good agreement with experimental results.