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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
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