REF: Equipment Reliability Institute
ERI News - your reliability newsletter
November, 2001 - volume 5


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Environmental Test
Understanding Vibration of Electronic Systems
by John Starr and Ed Abner

Vibration Exposure
All electronic systems experience vibration as they get transported on land, by air or rocket sometime during their life cycle. In addition, vibration is very efficient for environmental stress screening (ESS) of electronics because of the large number of cycles of stress that can be applied in a short time. When setting up a vibration testing program for electronic systems, through in-house testing or the services of an environmental test lab, the most important part of obtaining a successful test is understanding the product being vibrated. This applies for accelerated life testing and ESS. For life testing, what are the life capability expectations of each of the component parts of the system being tested? For ESS, what areas of the system are adequately screened and what areas are questionable?

In order to develop and test electronic systems with confidence, someone in the development chain and preferably all (design, development, production, test..), should understand vibration capabilities. This requires understanding at the component level. Understanding vibration means being able to list the few components that might be at risk of failure during a vibration test, and list the many components that would survive if tested at that level for 100 years.

Analysis of Vibration of Electronics
Vibration has always been very complex, often oversimplified by applications of empirical formulas to predict capabilities. Empirical methods used in the 1970's often served to define a conservative limit for design of electronic card systems. These methods fail to account for the complex configurations of electronic systems; and as a result, often result in expensive failures.

Life capability of electronics is usually dominated by modal response of circuit cards, with failure resulting from high cyclic bending occurring in a few critical components. Often these failures are a result of local effects - stresses caused by structural discontinuities such as supports, card cutouts, or adjacent components. Fatigue failure of any structural system has wide variations in life capabilities. The life capability of electronics is further complicated by the expected variations in geometric features and material properties of electronic parts.

The empirical formula approach served well during a time when all companies involved in electronics could not afford the computer power and expertise necessary to evaluate electronics at the point of failure. No equation is capable of covering configuration variations and modal response combinations for modern electronics. But today, desktop computer power and computer software programs make the capability to evaluate vibration life available to all.

Product Understanding
The CirVibe software package is an example of a PC Windows tool purpose-built for vibration life analysis of electronics. CirVibe methods compensate for the complexity of a non-precise mechanical composition and configuration. When interfaced with Electronic CAD packages (i.e. Mentor Graphics, Cadence Allegro,) that supply geometric detail and coupled with CirVibe's Life Usage / Expectations method, experienced users can create fully detailed models in a few minutes. The detailed analysis performed by the computer may take several hours, but provides detailed information on the life capabilities at component level and modes dominating failure of each. With this level of understanding, all parts of the development process considering vibration become more efficient. This is especially true for testing. Testing programs, which tend to be very expensive, become far more efficient because they are now understood with extensive detail. Some think of vibration ESS as black magic, mystically causing flaws to turn into failures. But flaws become failures by application of stress cycles. Understand the system at stress level and you understand the process. Figure 1 shows the second vibration mode of a circuit card. The local curvature in the circuit card for critical response modes define the vibration life of the card.



Figure 1: Animation of 2nd mode of vibration of an actual circuit card

The industry has a name for failures for which no cause can be determined. They are called "Cannot Duplicate Failures" (CND's). Other terms to describe this include "no fault identified" or "re-test OK". These are terms that could have been applied by a designer not supporting the test with analysis or using the wrong analysis tool for evaluating the electronic system. ESS is called a process (trial and error), ESS supported by proper analysis is an informed process.

Virtual Qualification
Ed Abner at CPI Satcom is involved in design, development and test (HALT/HASS/ESS/QUAL) of satellite communications equipment and has been using CirVibe in his development processes. His responsibilities include ruggedization of equipment and upscreening of COTS to meet the Mil Specs, Bellcore specs, European/Asian/Russian specs, IEC/EMC/RFI specs and others. Many of these specs are more severe than the Mil-Specs. The demands on his schedule to support products through the development cycle require Ed to understand the product with minimum time investment. Incorporating CirVibe analysis early and throughout the development process has been one of the keys to meeting tough development schedules with confidence in the capability of the product. An example product which included CirVibe vibration analysis was the Outdoor LPA Hub Mount TWT Low Power Amplifier, Figure 2. This product was fully qualified with virtual testing using CirVibe.

Figure 2: Outdoor LPA Hub Mount TWT Low Power Amplifier

Defining a vibration test plan for electronic systems with knowledge of the expectations of the outcome of the test program create the most efficient and cost effective interaction with the test facility. It also allows the quickest recovery from unexpected results, allowing virtual evaluation of many alternate solutions to problems that occur. Virtual evaluations can most often be performed in a small fraction of the time required to create design revisions and test.

J. Starr is a registered professional engineer in the state of Pennsylvania with 33 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@CirVibe.com

To contact Ed Abner:
CPI SATCOM, 811 Hansen way, M/S S-600 Palo Alto, CA - 94303
Phone (650) 846-3844 and FAX (650) 846-3849
mailto:ed.abner@satcom.cpii.com