CALCE EPSC Graduate Student Theses (2007)

Elerath, Jon (Ph.D. Mechanical Engineering)

Reliability Model and Assessment of Redundant Arrays of Inexpensive Disks (RAID) Incoporating Latent Defects and Non-Homogenous Poisson Process Events

Today’s most reliable data storage systems are made of redundant arrays of inexpensive disks (RAID). The quantification of RAID system reliability is often based on models that omit critical hard disk drive failure modes, assume all failure and restoration rates are constant (exponential distributions), and assume the RAID group follows a homogeneous Poisson process (HPP). This paper presents a comprehensive reliability model that accounts for numerous failure causes for today’s hard disk drives, allows proper representation of optional processes for repair and restoration, and does not rely on the assumption of a HPP for the RAID group. The model does not assume hard disk drives have constant rates of failure or restoration, and allows each hard disk drive "slot" in the RAID group to have its own set of distributions, closed form or user defined. Hard disk drive (HDD) failure distributions derived from field usage are presented, showing that failure distributions are commonly non-homogeneous, frequently having increasing hazard rates from time zero.

Hard disks drive failure modes and causes are presented and used to develop a model that reflects not only complete failure, but also degraded conditions due to undetected, but corrupted data (latent defects). The model can represent user defined distributions for completion of “background scrubbing" to correct (remove) corrupted data. Sequential Monte Carlo simulation is used to determine the number of double disk failures expected as a function of time. RAID group can be any size up to 25. The results are presented as mean cumulative failure distributions for the RAID group. Results estimate the number of double disk failures can be as much as 5000 times greater than that predicted over 10 years when using the mean time to data loss method or Markov models when the characteristic lives of the input distributions is the same. Model results are compared to actual field data for two HDD families and two different RAID group sizes and show good correlation. Results show the rate of occurrence of failure for the RAID group may be increasing, decreasing or constant depending on the parameters used for the four input distributions.

Choubey, Anupam (Ph.D. Mechanical Engineering)

Microsturctural Changes Under Isothermal Aging and Their Influence on Thermal Fatigue Reliability for Tin-Lead and Lead-free Solder Joints, Including Microstructural Changes Under Isothermal Aging in Mixed Solder Joints

Most electronics companies have transitioned to lead-free processes, both to comply with government legislation and to avoid issues related to mixing of tin-lead and lead-free metallurgies. However, exemptions from lead-free legislation have been granted for certain products, especially those intended for high-reliability applications. One major concern with these exempt products is that, during assembly or rework, lead-free components will have to be used due to the unavailability of tin-lead components. This will result in the mixing of tin-lead and lead-free metallurgies. The mixing of metallurgies can induce new reliability concerns. This study is focused on mixed solder joints formed by attaching lead-free components with tin-lead paste. Solder interconnect reliability is influenced by the environmental imposed load, solder material properties and the microstructure formed between the solder and the metal surfaces to which the solder is bonded. Several lead-free metallurgies are being used for component terminals, board pad plating and solder materials. These metallurgies react to form the microstructure of a solder joint. Microstructure of a solder joint continuously evolves and affects solder joint properties. A fundamental understanding on the microstructure is required to analyze the changes occurring in a solder joint with time and temperature and make predictions on solder joint reliability under thermal loading conditions. This dissertation determines key microstructural features present in SnPb, lead-free and mixed solder joints. Changes in the microstructural features were determined for SnPb, lead-free and mixed solder joints exposed to isothermal aging conditions. The effect of microstructural changes on reliability was determined by conducting thermal fatigue reliability tests for SnPb and lead-free solder joints. Whereas, for mixed solder joints, hypotheses has been determined based on microstructural analysis on their thermal fatigue performance compared to SnPb joints. This dissertation doesn't include the effect of microstructural changes on the reliability of mixed solder joints. This dissertation doesn't include the reliability tests for mixed solder joints. Two microstructural features namely, intermetallic compounds (IMC) and Pb phase were characterized for SnPb, lead-free and mixed solder joints. IMCs are formed at the solder to pad metallization interface and in the bulk solder. It was determined that reaction between Sn3.0Ag0.5Cu solder and Ni/Au component side metallization result in interfacial IMCs consisting of Ni3Sn4 IMC in the as-reflowed stage and IMCs such as (NiCu)3Sn4, (Cu,Ni)6Sn5 and (Au,Ni)Sn4 after thermal aging of 350 hours at 125ºC. With pad metallization of ImAg, ImSn and OSP, IMCs such as Cu6Sn5 are formed after reflow followed by formation of a new Cu3Sn IMC phase after thermal aging of 350 hours at 125ºC. Cu6Sn5 and Ag3Sn IMC were found distributed in bulk solder joints in the as-reflowed and aged (125ºC for 100, 350 and 1000 hrs) solder joint. This dissertation demonstrated that under thermal cycling, intergranular crack propagates between Sn grains in the bulk solder and Cu6Sn5 IMCs present at Sn grain boundaries in the bulk solder influence crack propagation. It was demonstrated that isothermal aging for 350 hrs at 125ºC causes coarsening of Cu6Sn5 IMC particles in the bulk solder which results in a 50% reduction in number of Cu6Sn5 IMC particles in the bulk solder, thus promoting the crack to propagate faster along the grain boundary. This dissertation determined that isothermal aging for 350 hrs at 125ºC would cause a 25% reduction in characteristic life for lead-free solder joints due to the changes associated with Cu6Sn5 IMC particles. In conventional SnPb solder joints Pb phase present in the bulk solder coarsens as a function of time and temperature and influences thermal fatigue reliability. Due to the presence of Pb in mixed solder joint, this dissertation determined the extent of coarsening in mixed solder joints compared with SnPb joints. It was determined that mixed solder joints are not prone to Pb phase coarsening under aging for 350 hrs at 125ºC as opposed to SnPb solder joints and therefore would have better thermal fatigue performance compared to SnPb joint under these conditions. This dissertation demonstrated that the presence of Pb in mixed solder results in a 30 to 40% lower IMC thickness compared to Pb-free and SnPb solder joints by being present at the interface as a diffusion barrier between Ni and Sn for IMC formation. Presence of Pb has been known to act as diffusion barrier for SnPb solder joints.

Feng, Dan (M.S. Mechanical Engineering)

Optimizing Lifetime Buy Quantities To Minimize Lifecycle Costs

Mismatches between electronic part procurement lifecycles and the lifecycles of the products that they are used in cause products with long manufacturing and/or support lives to incur significant obsolescence management costs. Lifetime buy is one of the most prevalent mitigation approaches employed for electronic part obsolescence management. Making lifetime purchases of parts when they go obsolete involves managing many interacting influences and multiple concurrent buys for multiple parts. The focus of this paper is optimizing lifetime buy quantities by minimizing lifecycle cost. The factors that contribute to the lifecycle cost associated with a lifetime buy are: procurement cost, inventory cost, disposal cost, and penalty cost.

A methodology called Life of Type Evaluation (LOTE) is described; LOTE requires component and system data and uses stochastic analysis to determine the lifetime buy quantity per part that minimizes the lifecycle cost for the system. LOTE was used to determine the optimum lifetime buy quantities for a Motorola communications system with and without life extensions.

Keimasi, Mohammadreza (Ph.D. Mechanical Engineering)

Flex Cracking and Temperature-Humidity-Bias Effects on Reliability of Multilayer Ceramic Capacitors

Multilayer ceramic capacitors (MLCCs) are known to be susceptible to cracking when subjected to excessive printed circuit board (PCB) flexure, which is called “flex cracking”. The bending of the printed circuit board causes stresses to be transmitted through the solder fillets to the surface mount capacitors. These stresses are the highest at the bottom of the capacitor, where the termination bands end. In order to reduce the amount of stress that is transmitted to the brittle ceramic body of MLCCs through end terminations, a flexible termination system which incorporates a silver-loaded epoxy in end-terminations was developed by some MLCC manufacturers.

With the transition to lead-free materials in the electronics industry there is a concern that MLCCs assembled on PCBs with lead-free solder have different susceptibility to flex cracking than those assembled with eutectic tin-lead solder. In this study, the flex cracking of MLCCs assembled with lead-free solder (Sn3.0Ag0.5Cu) was compared with those assembled with eutectic tin-lead (Sn37Pb) solder and differences in the results were explained in terms of solder mechanical properties and solder solidification temperature. Tin-silver-copper lead-free solders and eutectic tin-lead solder have different mechanical properties, which affect the stresses that are transmitted to the ceramic body of the capacitor through the solder fillet. The higher solidification temperature for lead-free solder leads to increased residual compressive stresses after the reflow cool-down process for MLCCs assembled with lead-free solder compared with those assembled with tin-lead solder. In this work, the effects of dielectric material, capacitor size, solder assembly process, solder material, and end-termination type on flex cracking of MLCCs were determined for MLCCs from different manufacturers.

Since some flexible- and standard-termination MLCCs are made with precious metal electrodes (silver-palladium), there is a possibility of electrochemical silver migration under bias and humidity. In this study, the effects of temperature-humidity-bias on electrical parameters of flexible-termination MLCCs were characterized and compared with standard-termination MLCCs. In addition, the effect of temperature-humidity-bias on electrical parameters of MLCCs with base metal electrodes was compared to that for precious metal electrode capacitors.

Myers, Jessica (M.S. Mechanical Engineering)

Integration of Technology Roadmapping Information into DMSMS-Driven Design Refresh Planning of the V-22 Advanced Mission Computer

Design refreshes and various reactive mitigation solutions are used to manage technology obsolescence (DMSMS) in systems. Design refreshing solely to manage obsolescence is, however, not practical for many systems, and therefore, obsolescence management refresh activities need to be coordinated with technology insertion roadmaps. This report describes the development of an information model that details technology roadmapping information for use within obsolescence-driven design refresh planning, and also describes a business case analysis for ascertaining the value of the resulting refresh plans. A case study on the V-22 Advanced Mission Computer using the MOCA (Mitigation of Obsolescence Cost Analysis) refresh planning tool is described in which optimum refresh plans (coupled with bridge and lifetime buys) with and without the inclusion of technology roadmapping constraints are determined.

Tuchband, Brian (M.S. Mechanical Engineering)

Implementation of Prognostics and Health Management for Electronic Systems

Prognostics and health management is the estimation of a product's remaining life based on current and historic conditions in terms that are useful to the maintenance decision-making process and the improvement of product design and reliability. An assessment has been undertaken to identify the state-of-practice for prognostics and health management of electronics in industry, government, and academia. Organizations involved in research, development and/or implementation of prognostics and health management have been categorized to ascertain which focus areas and applications of prognostics research are presently needed. Next, a methodology has been developed for enabling the implementation of prognostics and health management for electronic systems. This approach integrates virtual reliability assessment with a sensor selection process to identify the sensing parameters, measurement locations, and optimal sensor systems for in-situ health monitoring of electronics. The methodology has been applied to two circuit card assemblies located inside an avionics unit.

Varghese, Joseph (Ph.D. Mechanical Engineering)

Drop Testing of Portable Electronic Devices

This dissertation investigates the durability of solder interconnects of area array packages mounted on Printed Wiring Assemblies (PWAs) subjected to dynamic flexural loads. A test methodology is proposed to quantify the durability of the solder interconnect in terms of generic empirical metrics, PWA flexural strain and strain rate. It is shown that the proposed metrics (PWA strain and strain rate) can quantify the durability of the solder interconnect, irrespective of the loading orientation or the PWA boundary conditions.

An empirical rate-dependent durability model, based on mechanistic considerations, is developed to estimate the fatigue failure envelopes of the solder. 3D transient Finite Element Analysis (FEA) with rate-dependent solder material properties is used to develop transfer functions between PWA flexural strain and the solder plastic strain, for different values of PWA flexural strain rates. A strain-range fatigue damage model, based on strain-rate hardening and exhaustion of ductility principles, is used to quantify the durability and estimate the fatigue constants of the solder for high strain rates of loading. The test data also shows the existence of multiple competing failure sites (solder, copper trace, FR4, bulk intermetallic, interface between the intermetallics) and that the dominant failure site is strongly dependent on the loading conditions. Of these competing failure sites, this dissertation focuses on the solder and the interface between two intermetallic compound (IMC) layers. Interfacial fracture mechanics is used to provide a mechanistic perspective on the failure site transitions. Durability metrics, based on the mechanics of the problem is used to quantify the damage at the competing failure sites for a given loading condition and to estimate the durability of the solder interconnect. The test data shows good correlation with the model predictions.

This dissertation focuses on Sn37Pb eutectic solder interconnects. But the proposed test methodologies and mechanistic models are generic enough to be easily extended to other emerging lead free solder materials. Wherever possible, suggestions are provided for the development of test techniques or phenomenological models which can be used for engineering applications.

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