Equipment Reliability Institute
ERI News - your reliability newsletter
February 2004 - volume 14

Hello, readers

ERI's instructor Dave Douthit recently enjoyed gathering failure information concerning motor truck-borne electronics, figuring out failure modes and how redesign and improved manufacture would eliminate those failures. Then he used a short course of instruction as a conduit to report his findings to the company’s test personnel. Dave’s short article “Onsite - Training or Consulting?” leads off this issue.

Then appears “Response-Optimized Vibration Testing and Screening” by John Starr and myself. The article will give you an idea of what John will teach at his "Optimizing Electronics Vibration - HALT, HASS, ALT and ESS" course, scheduled in four different dates and locations in 2004 (please check on the right column for more information).

Another installment of “Test Lab Musings”, contributed by Robert L. Renz, comes next. Editor Cris invites other readers to help us continue these series.

This issue's question is more of a request. It involves Military vs. Commercial Practice. We’re asking Dave Rahe (who runs a test lab in Texas) to answer the request.

NEW COURSE!!! NEW COURSE!!! NEW COURSE!!!
Finally, a practical, minimal-math, minimal-theory, maximum-practicality course in Modal Testing. March 23-25, near San Diego, CA. Please visit the course page to see the details.

Best wishes,
Wayne Tustin

“On site” - Training or Consulting?
by David Douthit

“On Site” or company specific courses provide a unique opportunity for the client as well as for the instructor/consultant. Rapid changes in the electronics industry + the present highly competitive climate ð need for across-the-board communication and coordination of information.

The instructor/consultant researches his client’s current products and processes + future plans. He interviews his client, searches the Web and other sources and tours his client’s facilities. Thus he can not only “meet and greet” some of his client’s key personnel but also literally “see” various processes in use.

Combining the “research” information with the “latest and greatest” information provided by the instructor/consultant can result in a training course tailored to his client’s needs. Key client personnel and the instructor/consultant meet on-site or off-site to share information, discuss issues, problems and possible solutions.

These are basic “concurrent engineering” training meetings, valuable because the client’s various departments can no longer operate separately. They are interdependent. Cooperation can speed solutions, reduce costs and improve morale. A winning combination for any client!

During these sessions specific current problems are of course discussed. More important: future issues are also covered. These issues relate to the client’s plans. Classroom discussions help direct client efforts in a cost-effective and timely manner.

The author recently served, in this manner, a well-known client (who prefers not to be named) who wished to avoid electronics reliability problems when redesigning several products. Three days of instruction were scheduled, but as instructor/consultant we shared ideas, information, experiences, and concepts. My PowerPoint slides, some of which showed the client’s present products and processes, elicited questions and arguments, as well as options for future products.

David has consulted over 30 years in the troubleshooting, repair and failure prevention of electronic circuitry and systems. He can be reached at douthit@equipment-reliability.com.

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Response-Optimized Vibration Testing and Screening
by John Starr and Wayne Tustin

Introduction
For many electronic systems, vibration testing is part of the qualification requirements. Vibration is also often used in environmental stress screening (ESS) programs. Many organizations have found optimized vibration to be the most efficient means of finding production flaws. Companies with optimized vibration screens (vibration test alone) have credited vibration with 60%-80% of flaws found. Many have concluded that the value of thermal screening is overrated¹.

Tests during development of electronic products can provide mounds of information, but little is gained without detailed analysis. With detailed analysis, substantial immediate cost savings are achieved during test programs and later from higher reliability.

Planning Needed
Effective vibration testing requires planning². Vibration testing of electronics is particularly complex. Electronic products (such as the military’s use of COTS) have many physical dimensions and material properties that cannot be tightly controlled, yet are very critical to life. Understanding the product is a critical element in all vibration testing³.

The most common method of using vibration in developing reliable electronic systems has been a combination of testing and evaluation with empirical relationships. This approach is adequate for many, but not all circuit cards. Failures due to a faulty design approach can be very costly. Empirical relationships provide guidelines, but not real "numerical definition" of life at point of failure

Need For Numbers
Quoting William Thomson (Lord Kelvin): “When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind: it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science.” This article introduces numerical knowledge of why printed circuit cards fail.

Since the empirical guidelines do not address design details, many unexpected failures can and too often do occur. Failures can be extremely costly, depending on when in development or service they occur. Replacement of empirical equations with effective analysis leads to substantial savings.

Does Analysis Work?
Some may question the capability of mechanical analysis to aid in developing electronic systems. They should recognize that all systems (including electronic systems) are subject to numerous physical laws and mechanisms. Analysis always works. It provides valuable product information. When detailed analysis disagrees with tests, the fault is usually our lack of understanding of the physical product. Time taken to thoroughly investigate why they differ results in a greater understanding of our product.

Vibration-Caused Failures
For most components on modern electronic circuit cards, the most severe stresses result from card deformations; these are defined by mode shapes at natural resonances. Random vibration is commonly used in product testing, with various resonances excited simultaneously, much as they are in service. Whenever a failure takes place, during HALT, ESS, HASS or other test, one needs to identify the root cause of that failure⁴. When vibration is understood at root cause level, design changes can be implemented with greatest probability of success and at lowest cost.

Vibration of electronics can be quite complex. Physics of Failure (PoF) analysis translates test data into data defining exposure at point of failure level. Detailed analysis of designs can show why life expectations of identical components can vary significantly with location.

Commonly-used empirical methods do not numerically define component level vibration exposure. We do not intend to impugn the empirical methods of the past. The most common of these methods, the Steinberg equation, was developed in the1970's. Detailed analysis was not feasible due to lack of computer resources. In the late '80s, detailed analysis was feasible but very expensive. Now a very complex model of a circuit card can be run in a few hours on a high powered PC. Some analysis models can be set up in minutes, using modern technology⁵.

Figure 1 - Three Modes - Actual Circuit Card Assembly⁵

Properly conducted vibration tests can provide details of the physical responses of an electronic system. Figure 1 is a graphical representation (displacement exaggerated) of the first three mode shapes of an actual circuit card. With a sine sweep, the natural frequencies of the circuit cards can be measured. Displacement mode shapes for lower natural frequencies can be viewed with a strobe light. Step stress tests can determine fragility limits.

Substitute Hardware Endangers Products
Design of reliable systems requires further knowledge. If a component has multiple suppliers, will substitution change life capabilities? Since components are designed for electronic function (not structural capability), substitute components can differ in various structural properties. Circuit boards can similarly vary in thickness and bending modulii. Differences in natural frequencies, responses and life capabilities result. All mechanical properties affect stress and stress affects life capability (Exponentially!!!). The designer must evaluate whether proposed changes will affect reliability.

With all the expected variations in circuit and component parts, how do we interpret our test results? If one prototype unit passed one test, what can we predict for other units? Electronics systems are difficult to control mechanically, because there is little or no control on important physical parameters. Testing of all variations is not practical. Analysis allows extrapolation of test experience to cover often-reliability-critical mechanical parameter variations.

Circuit Card Complexity
Common expressions illustrate our inability to understand our test results and the difficulty of defining an effective stress screen for electronics. Expressions include:

• "each electronic product is unique"

Other expressions (following life tests or field returns):

• "cannot duplicate failures",
• “no fault identified”,
• “re-test OK”

Such expressions are common because of the statistical complexity of test control and of test items.

Three large contributors to statistical variations in test results are:

1. fatigue
2. random vibration
3. mechanical imprecision

Detailed Analysis
Circuit card components fail under vibration as a result of fatigue from cyclic stresses from inertial forces and from mode shape-caused component bending, primarily the latter. These stresses, unfortunately, cannot be quantified by measurements during a test.

Empirical formulas have attempted to define life capabilities through simple curvature approximations. These methods often fail because they can't properly cover all variations in circuit card details. Component life is affected by curvature in both directions. At best, a simple formula provides a crude approximation. Since the stress/life relationship is exponential, large life capability errors result.

A test can provide response measurements and pass/fail information, with the amount of detail available determined by the allocated funds. However, when you add PoF analysis, the available information expands dramatically.

CirVibe⁶ is an example of PoF analysis useful in developing reliable electronics. CirVibe is a software program that converts a geometric description into a mathematical model, then solves this model, extracting detailed stress cycling data for every component on the test item. The software program methods were developed based on decades of experience in applying numerical analysis to design and development of structures (what fails, how it fails, what level of model accuracy is required in each product, etc.). This experience included extensive design, development and testing of numerous electronic systems. Test programs supported by CirVibe level analysis become extremely cost effective. This highly automated program develops finite element analysis (FEA) models from simple geometric descriptions of a circuit card and its components. The finite element detail is generated internally, so the user need not have FEA expertise. Interfaces to Computer Aided Design (CAD) programs speed the development by translating CAD data to circuit card analysis models.

FEA applies laws of mechanics and determines product information that is beyond the capability of any test program. Analysis can optimize accelerometer positioning. Taking advantage of new tools to utilize current PC computer power, the detailed calculations can turn a few accelerometer measurements into:

• modal shapes for critical modes
• peak responses of critical modes
• stresses for every component for every critical mode
• fatigue damage from component cyclic stress

Adding modern FEA to the test process turns accelerometer readings into definitions of life capability of every component. Extending the analysis to include any design variations is very simple: repeat the analysis with new parameters. Design changes such as component details can be evaluated in minutes. More complex design ruggedizing changes such as layout, support conditions, stiffener additions or similar changes can be performed in a few hours. Design changes can be qualified virtually, without the time and expense of building and testing a prototype. Many options can be considered.

Product understanding gained from detailed analysis is valuable in the design of test fixtures used to attach circuit card(s) to the shaker’s vibrating table. Too often, decisions are based on results obtained with faulty fixtures in an attempt to match the geometry (but unfortunately not the dynamics) of in-service usage conditions.

ESS can be optimized earlier in the process. Since PoF analysis can provide life-use data for every component under each modal response shape and amplitude, this data can optimize the screening profile.

Detailed pretest PoF analysis identifies which components are to be driven at each natural frequency and to what level each is to be driven. It can also predict the change in damage that will result from a change in drive level over a frequency range. This product understanding is used to optimize the screen. Screens can be tailored by excitation control in frequency bands to properly excite critical parts of the test article without using excessive test article life.

Figure 2 illustrates stress screen effectiveness at a component level. It also illustrates life capability under one set of requirements.

Figure 2 - CirVibe Screen Effectiveness and Service Life Capability Plot

Conclusions – Cost Savings
Since a test cannot provide any measurements that are descriptive of point of failure, test alone can be considered a blindfolded hit-or-miss approach to gaining knowledge about a product. The industry phrase, "the ESS process is unique for each electronic product", demonstrates this fact. It's the combination of test and analysis that provides real knowledge. Tests can create real failure data. Subsequent analysis provides numerical definition of the failures experienced. By numerical definitions we mean “definitions of exposure to fatigue damage at component level”. Note that these numbers are transferable from one design configuration to another. " When you can measure what you are speaking about, and express it in numbers, you know something about it..."

When numbers are transferable from one design to another through the benefit of analysis, each test program can benefit from all past experience. Test programs become more efficient. For life testing, definitions of life capabilities relative to requirements can be more accurately defined, reducing the risk of failures. For stress screening, screen effectiveness can be defined at a component position level. Screens can be optimized much earlier in the process. The benefits of analysis include (1) cost savings of stress screen programs and (2) savings from producing a more reliable product.

References
1. "Reliability - Past and Present", G.K. Hobbs; Sound and Vibration/April 1997
2. “Prepare for Better Vibration Tests”, Tustin, Smith and Reeder, Test & Measurement World, October 1999
3. “Understanding Vibration of Electronic Systems”, Starr and Abner;
4. “Electronics Testing into the 21st Century: Success in Test Is in Capabilities, Not Specifications”; Gray and Tustin
5. "Optimizing Electronic Circuit Card HALT, ESS and HASS".
6. CirVibe Circuit Card Vibration Software; Users Manual Version 3.0

J. Starr is a registered professional engineer in the state of Pennsylvania with over 35 years of continuous and varied experience in structural capabilities in Nuclear, Chemical, and Defense industries. He works as a Consulting Engineer at CirVibe Inc. To contact John send an e-mail to jstarr@equipment-reliability.com.

Wayne Tustin, ERI's president, can be reached by e-mail or phone (805) 564-1260. Read more about Wayne at ERI's website.

Test Lab Musings (part 3)
by Robert L. Renz

A few years ago, the US Army evaluated the tool boxes used by helicopter mechanics, and found that they could replace a 100 Lb tool box with a set of about six tools for daily inspections and routine service. Why not do the same thing to your test lab? Keep what is needed available, and put the rest away for special situations.

When you are setting up for a test, try to do it all at once. Its easy to go to lunch, but then try to figure out where you were before you left - be particularly careful when you are bolting down a head expander, or a fixture. Finger tight just doesn't do it in a vibe test.

If I'm using a new fixture, I like to install it on the shaker, torque the attachment bolts, do a sine sweep (5 - 2000 Hz), then re-torque the bolts again after the washers are seated. Its also a good way to check out a fixture for resonances.

When I'm running a test, I stop and re-torque the fixture bolts whenever I start to see my oscilloscope trace getting dirty. Odds are, something is loosening up. I also like to re-torque whenever the test stops, for example, when changing the UUT.

If I use petro wax to install a small accelerometer, remember to tape the lead down within about 2-3" of the accelerometer.

Robert L. Renz of General Dynamics - Advanced Information Systems at Bloomington, Minnesota.

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Questions our readers have asked...

This section of our newsletter was created for you, reader! Feel free to send questions or suggestions to the webmaster. One of our specialists will respond.

Q: Panasonic tests their new "toughbooks" to this standard. See this link. I know nothing about MIL standards so would be interested
if someone here could contrast and compare (briefly of course :-)
this standard from a typical HALT test geared towards this
type of product. (Steven Nahas, Intel Corporation, Beaverton, OR)

A: Steven,
There are a number of differences between a HALT stress and the Mil Std 810 tests to which Panasonic subjected their notebooks.

A HALT chamber applies vibration and thermal stresses, additional stresses such as voltage and frequency margining, power cycling and others as appropriate can additionally be applied.

The 810 document contains numerous environmental stresses, outside of the stresses that are applied during a HALT test. There are 24 test methods called out in the 810 document. The stresses that have some correlation/comparison to HALT stresses are the temperature tests – High Temperature (501.4), Low Temperature (502.4), Temperature Shock (503.4), Vibration (514.5), and Shock (516.5). The temperature stresses are the most comparable stress of the previous list. The differences during the temperature tests are the 810 breaks both high and low temperature exposures into what are believed to be field / life cycle environments of material during storage (Procedure I) and operation (Procedure II). These temperature levels, 4 generally (2 Op and 2 NonOp), are chosen prior to the test based on application environments. The HALT process utilizes an exploratory temperature step stress approach to incrementally identify what the product is thermally capable of surviving, for both operation and non-operation. The dwell times during HALT are shorter at a specific temperature. The airflow rates (LFM) are higher and the thermal rates of change are faster (30-60ºC/minute vs. 3-10ºC/minute) in a HALT chamber vs. typical environmental chamber where Mil Std 810 are generally performed.

The Temperature Shock test is normally non operational and the product is shuttled between two thermal zones (hot and cold). The temperature recovery rate, after a transfer, in this environment is dependent on the mass of the product. The rates can be similar to those during HALT (typically 30-70ºC per minute). HALT chambers typically would have a faster product recovery rate.
The Vibration is created using an ElectroDynamic system (ED) vs. a repetitive shock system (HALT). The ED is a single axis stress with a frequency range from 5Hz to 2000 Hz. HALT is multi-axis from 10Hz (or less) to 5000 to 10,000Hz (depending on system). The Mil Std 810 has defined 25 different vibration profiles which attempt to mimic transportation and application environments to varying degrees. The vibration profiles chosen were a Minimum Integrity test (non operational) and a US Highway Truck (operational) level. The Minimum Integrity test has aggressive levels and should demonstrate that the product is fairly robust. I would like to see the product operational during the exposure with the understanding that the HDD is inoperable (as it is anyway during the Truck test). The low frequency excitation may be greater, and typically is, with the ED. Keep in mind however that the test is single axis and the cumulative stress of multiple axes is minimized in comparison to HALT.
The Drop test stresses the product in a realistic fashion. The impact type test stresses the chassis and support frames in ways that the HALT test cannot. However the HALT repetitive shock will generally produce similar failure mechanisms within the product as those occurring as a result of dropping the product.

Humidity, water resistance and dust stresses are not typically performed during HALT. The altitude failures are generally thermal related and likely found with high temperatures (not all failures however).

The Mil Std 810 test is a qualification test and isn't intended to push a product to explore its capabilities (at what level will it fail - both soft and hard).
The objective is to demonstrate that it will pass the test as defined. It may be note worthy that the Mil Std 810 document has become less prescriptive in the latest revision, allowing the test engineer to interpret the product's life cycle and mission rather than defining predetermined test levels.

Professional Testing performs HALT and Mil Std 810 test daily on a wide variety of products (including ruggedized notebooks and tablets). I hope I answered some of your questions, but if you have others please contact me or just reply.

Regards,
Dave Rahe

David Rahe has over twenty years of experience in the environmental test industry, and is currently the Chief Operating Officer at Professional Testing Inc., an independent test laboratory in Austin, TX. He is the Co-Chair of the IEEE Accelerated Stress Testing and Reliability (ASTR) organization, serves as the Technical Chair for ASTR and author of the HALT Standard. Prior to joining PTI he held positions of Director of Operations, and Managing Engineer at QualMark Corp. He has performed numerous HALT and HASS processes on a wide variety of product types over the last 12 years.

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I need to apologize concerning my new text, lengthily titled “A minimal-mathematics introduction to the fundamentals of
Random Vibration and Shock Testing, HALT, ESS and HASS, also Measurements, Analysis and Calibration, with applications in the fields of aeronautical, automotive, seismic and shipboard design and production.”

We copyrighted it in 2003. A year ago that seemed credible. The biggest slowdown has been photographs that worked fine as PowerPoint slides in my lectures. Printers told us that printing in color on paper required 300 dpi (dots per inch), much greater resolution than what serves nicely on computer screens and video projectors.

So we’ve had to get originals for re-screening. Or (in cases where the originals no longer exist) to substitute somewhat different photographs. Picture changes generally require changes in the text.

All this takes time.

Oh, yes. The book has grown. It now has 33 chapters, up from 31. It will include a CD-ROM, which carries a number of video clips and animations that illustrate vibratory and shock motion. One of my favorites shows the highly repetitive motions inside an automobile engine. Another shows cantilever beams responding to random vibration.

Our webmaster will be showing the book on our websites soon. Then we will be able to inform you regarding the production process. Also we intend to offer a sample of chapter 1, which would give the readers a little more information of what's in this book. Please feel free to contact us if you have further questions. And don't forget to check on ERI or Vibration and Shock for the latest news.

Book cover
Click on the image to enlarge

A little story about SAVIAC

SAVIAC (The Shock & Vibration Information and Analysis Center) was started after WWII to gather and benefit from military and naval equipment “lessons learned” during that war. It was initially called the Centralizing Activity; and included visits to military and naval bases to gather information. To facilitate communication between experts, Shock and Vibration Symposia were established by the Shock & Vibration Information Center, SVIC, as part of the Naval Research Laboratory, Washington, DC. Early on, these met monthly, then annually. That’s why, after 56 years, number 74 met last Fall at San Diego. SVIC was “disestablished” in 1986 and reestablished in 1990 as a contractor-operated Information Analysis Center.

Number 75 will meet at the Cavalier Hotel at Virginia Beach, VA October 18-22, 2004. For information contact Joel Leifer at (301) 596-0100 or visit their website.

If you cannot attend courses in the US, ERI offers two overseas courses. ERI Specialists Deepak Jariwala who will teach at Singapore, and Markus Dumelin who will teach at Switzerland. Dates and locations are shown below. Click on the links for more detailed information.

July 13-15, 2004,
at Singapore

October 5-7, 2004,
at Zug, Switzerland

ERI - Equipment Reliability Institute
1520 Santa Rosa Ave.
Santa Barbara - CA - 93109
Tel: (805) 564-1260
Our fax number:
(805) 966-7875

Wayne Tustin tustin@equipment-
reliability.com

Webmaster webmaster@equipment
- reliability.com

Websites
http://www.equipment-
reliability.com

http://vibrationand
shock.com

Copyright © 2000-2004 Equipment Reliability Institute. All rights reserved.

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