October 2007 Issue

  October 2007
Journal of the IEST


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The October issue of the Journal of the IEST offers peer-reviewed technical papers as well as complimentary Tech Talk articles. Subjects include surface corrosion of hard disk drive media due to SO2 gas adsorption, the new ISO Committee Draft (ISO/CD) 14644-9 Cleanrooms and associated controlled environments—Part 9: Classification of surface particle cleanliness, chemical filters for the control of airborne molecular contamination, microstructural-based physics of failure models, dynamic characterization of fan filter unit operation, 217Plus reliability prediction methodology, and accelerated reliability growth testing.

The following Tech Talk articles are accessible free of charge (access Journal of the IEST).

New ISO Draft Standard Provides for Classification of Surface Particle Cleanliness
An interview with officials of ISO/TC 209 Working Group 9, “Clean Surfaces”

Clean surfaces are of vital importance for many high-tech industries and applications. The International Organization for Standardization Technical Committee (ISO/TC) 209 is taking steps to fill the need for global standards addressing clean surfaces. A draft Standard, ISO/CD 14644-9 Cleanrooms and associated controlled environments — Part 9: Classification of surface particle cleanliness, is the first in a series of standards covering different aspects of surface contamination. This article provides an overview of the key provisions in this new document.

New Reliability Prediction Methodology Incorporates Field and Test Experience
David Nicholls, ASQ CRE, Reliability Information Analysis Center (RIAC)

The 217Plus reliability prediction model, designated by the Department of Defense as the replacement for the PRISM methodology, addresses shortcomings in earlier models and provides for the incorporation of field and test data through Bayesian analysis techniques. This article highlights features of the new methodology and includes a simple example that illustrates how the improved prediction results compare to more traditional reliability prediction approaches.

Scanning Laser Vibrometer: Evaluation of an Alternative Modal Survey Tool
Sarah M. Brennan, Reem Hejal, Mike Ryan, Stepan Simonian, Chad Stimson, and David Velasco, Northrop Grumman Space Technology

This paper is the winner of the Otto Hamberg Technical Paper Award from the 23rd Aerospace Testing Seminar Proceedings.

In preparation for the challenges presented by future spacecraft systems, state-of-the-art testing technologies are explored and the results to demonstrated, proven methods are compared. A non-contact Polytec PSV-400 scanning laser vibrometer (SLV) is used to characterize the frequency response of a 2-meter reflector in a typical modal test configuration. Complementary measurements are made simultaneously using a conventional accelerometer-based system, and the results are compared to determine the effectiveness of the non-contact tool. The SLV has two distinct advantages that make it an attractive alternative for studying the structural dynamic response of lightweight, high-precision, aerospace structures—increased bandwidth (0.01 Hz–80 kHz) and resolution at the nanometer scale. These features are available without modifying the structure, and they do not require labor-intensive preparation of the test article. Typical setup time is less than 1 hour. However, the one-dimensional (1D) SLV system is limited to measurement of the out-of-plane response of a surface one discrete point at a time, and the system relies on software to render the global response. This differs fundamentally from a dedicated accelerometer system that may collect tri-axial information at hundreds of points simultaneously. Several different system configurations were tested, and the results, as well as lessons learned, will be presented.

The Journal of the IEST is pleased to reprint the Otto Hamberg Award paper and wishes to acknowledge the following papers from the 23rd Aerospace Testing Seminar Proceedings awarded the recognition of Honorable Mention:

The Third Dimension of Random Vibration Control
John Van Baren, Philip Van Baren, Vibration Research Corporation

Developments and Qualification Programme of the Videogrammetry
System for the Herschel Spacecraft Flight Model Test
Alessandro Cozzani, Matteo Appolloni, ESA/ESTEC; and Gianluca Casarosa, Advanced Operations and Engineering Services (AOES)

New Dimensions in Test Automation Methodologies and Technologies
Thomas Wissink, Lockheed Martin Space Systems Co.

Lessons Learned from Space Vehicle Test Trends
Bruce Arnheim, The Aerospace Corporation

The October 2007 issue of the Journal of the IEST also contains peer-reviewed technical papers related to the fields of contamination control; design, test, and evaluation; and product reliability. IEST members and Journal of the IEST subscribers have full access to technical papers in the current issue as well as a 20-year archive of technical papers. Others may obtain full-text abstracts of peer-reviewed technical papers and have the option to purchase individual papers (access Journal of the IEST).

Surface Corrosion of HDD Media and Subsidiary Particle Formation Due to SO2 Gas Adsorption
Norikazu Namiki, Kyoritsu Gokin Co., Ltd.; Akihiro Komtasu and Keiji Watanabe, Fujitsu Ltd.; and Naoki Kagi, National Institute of Public Health

The storage capacity of hard disk drives (HDDs) for personal computers has increased more than 10,000 times in the past decade. Meanwhile, the gap between the disk and the magnetic head (flying height) has decreased from sub-micrometers to a few nanometers. The lower flying height leads to more sensitive disk-to-surface contamination linked to fatal failures. Many studies have reported that disk surface contamination is related to the adsorption of volatile organic compounds (VOCs) and the corrosion of the magnetic layer of the disk. However, surface contamination derived from sulfur dioxide (SO2) gas in the surrounding area has been discussed insufficiently. In this work, to ascertain the mechanism of disk surface contamination and the subsidiary particle generation, HDD disks were intentionally exposed to SO2 gas, followed by an evaluation of disk operation performance during endurance testing. After the series of endurance tests, a large quantity of sulfate and nickel, which is a main component of the disk substrate layer, was detected on the SO2-contaminated disk surface, as well as a small quantity of cobalt, which is a main component of the magnetic layer. From these findings, the mechanism of surface contamination of HDD disks in the coexistence of SO2 and water was inferred. Nickel is supplied from the substrate layer to the top layer through holes and cracks in the films of disks because of the corrosion triggered by adsorbed water. High temperature and humidity cause the desorption of SO2 from SO2-contaminated disk surfaces to react with nickel. Eventually, the products of nickel sulfate are precipitated on the surface to be released in the form of particles by contact with the head.

Comparison of Chemical Filters for the Control of Airborne Molecular Contamination
Chris Muller, Purafil, Inc.

Airborne molecular contamination (AMC) continues to grow as a major contamination control issue for microelectronics manufacturers. Because of this growth, chemical filter manufacturers are offering a large number of AMC control options from which the facility, process, and contamination control engineers must determine the best solution for their specific AMC control application(s). This paper presents information on the various types of chemical filters available, filter strengths and weaknesses, and the results of independent performance evaluations for a number of different chemical filter types. Several AMC control strategies employing chemical filtration are discussed. Other topics include an introduction to gaseous contaminants and a definition of AMC; classifications of AMC; contaminants common to cleanroom applications, their sources, and effect; implementing AMC control—a three-step methodology for optimum control of AMC; gas-phase air filtration principles—adsorption, absorption, chemisorption, and factors affecting each; dry-scrubbing air filtration media—plain and chemically impregnated adsorbents; chemical filtration equipment designs—thin-bed and deep-bed systems; and AMC monitoring—passive and real-time reactivity monitors.

Microstructural-Based Physics of Failure Models to Predict Fatigue Reliability
Robert G. Tryon, Animesh Dey, and Ganapathi Krishnan, VEXTEC Corporation

The fatigue life of materials is strongly influenced by their microstructure. The scatter in fatigue life is caused by the scatter in grain size, grain orientation, and stresses induced in each grain of the material due to external loading. A novel microstructural fatigue crack growth model that accounts for real-life variability in the material is presented in this study. Ti-6Al-4V, a common aerospace titanium alloy used in numerous turbine engines, is studied to predict its fatigue life and scatter at load ratios of 0.1 and -1.0. The predictions are compared with experimental data. Results indicate that the microstructural material models accurately predict the behavior of Ti-6Al-4V.

An Innovative Method for Dynamic Characterization of Fan Filter Unit Operation
Tengfang (Tim) Xu, Lawrence Berkeley National Laboratory, University of California

Fan filter units (FFUs) are widely used to deliver re-circulated air while providing filtration control of particle concentration in controlled environments such as cleanrooms, minienvironments, and operating rooms in hospitals. The objective of this study is to document an innovative method for characterizing operation and control of an individual FFU within its operable conditions. Built on a draft laboratory method previously published, this study presents an updated method including a testing procedure to characterize dynamic operation of FFUs, i.e., steady-state operation conditions determined by varied control schemes, airflow rates, and pressure differential across the units. The parameters for dynamic characterization include total electric power demand, total pressure efficiency, airflow rate, pressure differential across FFUs, and airflow uniformity.

Accelerated Reliability Growth Testing and Data Analysis Method
Milena Krasich, Bose Corporation

Traditionally, a reliability growth test was performed at the levels of various operational and environmental stresses, often at a level equal to that expected in use. Other than formal failure modes and effects analysis, reliability growth tests were often the only practical means for identification and mitigation of failure modes of a newly designed product.

With the present high reliability requirements and long product useful life, the length of reliability growth tests may become cost and schedule prohibitive; therefore, accelerated testing is taking the place of prior testing at the use levels. This practice, however, does not address the dilemma of possibly unrealistically skewed test results highly dependent on the sequence of individually applied stresses as, unfortunately, it is often difficult to impossible to apply all of the environmental and operational stresses simultaneously.

This study shows how data analysis applied to an accelerated life test based on reliability growth methodology may produce a viable solution to the calculation problems. The stresses applied in this test are an accelerated application of most of the stresses expected to take place during product use. Each of the tests represents a lifetime exposure to an individual stress. If those stresses are applied individually and in sequence, they are considered to be equivalent to being applied in parallel with one another, as the duration of each stress is calculated to represent life of the product.

Time to failure in each test is re-calculated to represent time to failure in real life. Failure occurrences are then sorted in their increasing order and analyzed using one of the reliability growth test analytical methods.

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