Improving Reliability in the Next Generation of US Army Platforms Through PoF Analysis, Part 1

Estimated reading time: 8 minutes

AbstractPublished studies and audits have documented that a significant number of U.S. Army systems are failing to demonstrate established reliability requirements. In order to address this issue, the Army developed a new reliability policy in December 2007 which encourages use of cost-effective reliability best practices. The intent of this policy is to improve reliability of Army systems and material, which in turn will have a significant positive impact on mission effectiveness, logistics effectiveness and life-cycle costs.

Under this policy, the Army strongly encourages the use of mechanical and electronics physics of failure (PoF) analysis for military systems. At the U.S. Army Materiel Systems Analysis Activity (AMSAA), PoF analyses are conducted to support contractors, program managers (PMs) and engineers on systems in all stages of acquisition from design, to test and evaluation (T&E) and fielded systems.

This paper discusses using the PoF approach to improve reliability of military products. Physics of failure is a science-based approach to reliability that uses modeling and simulation to eliminate failures early in the design process by addressing root-cause failure mechanisms in a CAE environment. The PoF approach involves modeling the root causes of failure such as fatigue, fracture, wear, and corrosion. CAE tools have been developed to address various loads, stresses, failure mechanisms, and failure sites. This paper focuses on understanding the cause and effect of physical processes and mechanisms that cause degradation and failure of materials and components.

A reliability assessment case study of circuit cards consisting of dense circuitry is discussed. System-level dynamics models, component finite element models and fatigue-life models were used to reveal the underlying physics of the hardware in its mission environment. Outputs of these analyses included forces acting on the system, displacements of components, accelerations, stress levels, weak points in the design and probable component life. This information may be used during the design process to make design changes early in the acquisition process when changes are easier to make and are much more cost-effective.

Design decisions and corrective actions made early in the acquisition phase leads to improved efficiency and effectiveness of the T&E process. The intent is to make fixes prior to T&E to reduce test time and cost, allow more information to be obtained from test and improve test focus. PoF analyses may be conducted for failures occurring during test to better understand the underlying physics of the problem and identify the root cause of failures which may lead to optimal fixes for problems discovered, reduced test-fix-test iterations and reduced decision risk. The same analyses and benefits mentioned above may be applied to systems which are exhibiting failures in the field.

Key WordsDesign-in reliability, failure mechanisms, fatigue, physics of failure, shock, thermal overstress, vibration.

Introduction

ManyArmy programs have taken an iterative loop approach, i.e., test-fix-test, with an emphasis on beginning the testing process early and learning from soldiers' experiences with the equipment. However, as time goes on, the cost to fix failures that were not addressed earlier in design can become very large and could adversely affect operational effectiveness and increase life-cycle costs.

The use of commercial off-the-shelf (COTS) products in military applications has been a standard practice to reduce the lifecycle costs in recent years but they may require extensive modification to withstand the harsh conditions of a soldier’s environment [1]. By designing reliability into systems early, many potential failure mechanisms and sources of failure can be eliminated with little cost [2]. Early analysis of the engineering design, combined with early low-level testing and substantial integration testing, can greatly improve the reliability of the product before designs are locked in, and well before any formal testing program.

Through PoF analysis, AMSAA provides guidance and support to the Army, DoD, and defense contractors in understanding the root–cause of failures by analyzing why, where and how failures occur and in using the obtained information to predict or prevent manifestation of potential failures in a product’s operational lifecycle. This paper briefly describes mechanical PoF and further describes the electronic PoF process in detail. An example PoF analysis of a circuit card with predominant failure drivers is discussed. In addition, the knowledge of the system’s expected life-cycle usage and environmental stress load profiles and an overview of the failure drivers in electronics and the fatigue models that work well in accounting for the failures are presented in this paper.

Physics of Failure (PoF) of Mechanical Systems

Themechanical PoF process involves the determination of the dynamics forces that act on a system during its mission usage. In early program stages these forces are obtained from a system-level dynamics model. The model reveals the forces that act on individual components. The forces are used in finite element analysis (FEA) models to determine the cyclic stresses and strains that are induced in the components of interest. Alternatively, depending upon the program phase, hardware availability and resources, the stresses and strains may be gathered directly from hardware tests. Test gathered data can be used to baseline the FEA models. FEA models may also serve as a mapping tool to predict the failure initiation locations. The two top rows of the Figure 1 below depict this general process. The lower row of the figure depicts the processes of rainflow cycle counting, damage cycles counting, and fatigue life prediction. These methods are used to determine the events within a mission cycle that are the most damaging and the actual life of a component under mission usage.

Figure 1. Flow for predicting failures in military systems.Click here to see full-size image.

Physics of Failure (PoF) of Electronic Systems

Military electronic systems perform in harsh and rugged environments in the field and the electronic components within these systems are often subjected to various complex forces such as vibration, shock and thermal cycling loads. Typically these electronic components are attached to the circuit cards with the use of a solder or a lead wire and the life of these solder joints or lead wires is critical for the survivability of electronic systems.

The extreme mechanical loads the electronic packages see may induce cracks and failures in component interconnects (i.e., solder joints and leads). Figure 2 shows the bending of the electronic components due to random vibration. Many electronic failures are mechanical in nature. However, some of these failures are due to poor design practices and some of the failures are due to poor manufacturing practices. Extensive military testing experience over a period of many years has shown that about 80% of the electromechanical failures are due to some type of thermal condition about 20% of the failures are due to some type of vibration and shock [3].

Figure 2. Image depicting the bending of electronic components due to random vibration.

AMSAA conducts PoF analysis of electronics to assist the engineers in designing the electronics to survive in extreme conditions. The electronic PoF process involves the determination of the effects of operational factors/forces that act on a circuit card assembly (CCA) during its mission usage. Temperature, shock, and vibration all play a large role in how long a CCA will survive in a particular environment. An electronics PoF analysis reveals the mechanisms that cause electronic components to fail and identifies the one(s) that may cause components to fail first. Failure mechanisms include vibration induced fatigue of component to board interconnects, mechanical shock induced failure of component to board interconnects, thermal fatigue of solder interconnects, thermal fatigue of plated-through holes, and thermal fatigue of die-to-package interfaces. Once the most damaging failure mechanism is known, corrective action can be taken to remedy the potential problem and ultimately, extend the life of the system.

Electronic PoF analysis typically consists of a thermal analysis (system-level and/or board-level), CCA shock response analysis, CCA modal and random and/or harmonic vibration response analysis, and CCA thermal and vibration fatigue life assessments. The system-level thermal analysis determines the temperature distribution of the internal air, the chassis walls and the components. The CCA thermal analysis determines the steady-state temperatures of all components and the power dissipated by each component when the system is energized. The CCA shock response determines the board displacement and strain due to a given shock pulse. The natural frequencies of a CCA are determined by the FEA modal analysis. The CCA vibration response analysis determines the displacement and curvature of a CCA due to vibration loading.

Figure 3. An overview of the electronic PoF analysis process.

Based on output from the vibration analysis and transportation/usage profile(s), the CCA vibration fatigue life assessment estimates the fatigue life of all component solder joints and leads on each CCA. The CCA thermal-fatigue life assessment determines each component’s solder-joint life based on the number, magnitude, and duration of the thermal cycles (i.e., change in temperature due to powering on / powering down the equipment, change in temperature due to daily outside ambient temperature changes, etc.). The combined CCA thermal and vibration fatigue assessment predicts the fatigue life of all component solder joints and leads based on the cumulative damage due to vibration loading and thermal cycling. See Figure 3 for an overview of the electronic PoF analysis process.

Reprinted with permission from Enabling Sustainable Systems, Proceedings for the MFPT: The Applied Systems Health Management Conference 2011, Society for Machinery Failure Prevention Technology, 2011, pp. 507-523.

References

[1] M Looney, H Erdogmus, J Dean, P Oberndof, C A Sledge, S Allison and G Allan, COTS Process Issues in Military Applications.

[2] M W Deckert, A Science-Based Approach to Ultra-High Reliability.

[3] D S Steinberg, Vibration Analysis for Electronic Equipment, Third Edition, John Wiley & Sons, Inc. New York: 2000.

About the Authors

Geetha V. Chary and Gary S. Drake, Army Materiel Systems Analysis Activity, APG, MD. Contact: amsaa.reltools@us.army.mil

Ed Habtour, U.S. Army Research Laboratory Vehicle Technology Directorate, APG, MD. Contact: ed.m.habtour.civ@mail.mil