It’s the Analysis That Counts
Evaluation Engineering, October 2002
by John Starr, CirVibe, and Wayne Tustin, Equipment Reliability Institute
Many manufacturers have found optimized vibration
test to be the most efficient means of finding production flaws,
crediting it with discovering 60% to 80% of the problems. In fact,
many companies have concluded that the value of thermal screening is
Tests during the
development of electronic products can provide mounds of
information, but little is gained without detailed analysis. With
detailed analysis, substantial immediate cost savings can be
realized during production test and later from higher
products, such as commercial off-the-shelf (COTS) products used by
the military, have many physical dimensions and material properties
that cannot be tightly controlled, yet are very critical to life. As
a result, understanding the product is a critical element in
The most common use
of vibration test in developing reliable electronic products has
been a combination of testing and evaluation with empirical
relationships. This approach is adequate for most circuit cards.
But, empirical relationships provide guidelines, not a real
numerical definition of life at the point of
Since the empirical
guidelines do not address design details, many unexpected failures
on printed-circuit cards can and too often do occur. Failures can be
extremely costly, depending on when in development or service they
occur. Replacing empirical equations with effective analysis leads
to substantial savings.
You may question
the capability of mechanical analysis to aid in developing
electronic systems. But remember that all systems, including
electronic systems, are subject to numerous physical laws and
mechanisms. Analysis always works. When detailed analysis disagrees
with tests, the fault usually is our lack of understanding of the
For most components
on modern electronic circuit cards, the most severe stresses result
from card deformations defined by mode shapes at natural resonances.
Random vibration commonly is used in product testing, with various
resonances excited simultaneously, much as they are in
Whenever a failure
takes place during highly accelerated life testing (HALT),
environmental stress screening (ESS), highly accelerated stress
screening (HASS), or other testing, you need to identify the root
cause of that failure.3 When vibration is understood at
the root cause level, design changes can be implemented with the
greatest probability of success and at the lowest cost.
electronics can be quite complex. Physics of failure (PoF) analysis
translates test data into data defining exposure at the
point-of-failure level. Detailed analysis of designs can show why
life expectations of identical components can vary significantly
Commonly used empirical methods do not numerically
define component-level vibration exposure. Properly conducted
vibration tests provide details of the physical responses of circuit
cards. 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.
The design of
reliable products requires further knowledge. If a component has
multiple suppliers, will substitution change life capabilities?
Since components are designed for electronic function, substitute
components can differ in various structural properties. Circuit
boards can vary similarly in thickness and bending
in natural frequencies, responses, and life capabilities. 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?
products 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 critical mechanical parameter
illustrate our inability to understand our test results and the
difficulty of defining an effective stress screen for electronics,
such as “each electronic product is unique.” Other expressions,
following life tests or field returns, are “cannot duplicate
failures,” “no fault identified,” and “re-test OK.” Such phrases are
common because of the statistical complexity of test control and
contributors to statistical variations in test results are the
components fail under vibration as a result of fatigue from cyclic
stresses, i.e., inertial forces and mode-shape-caused component
bending. Unfortunately, these stresses cannot be quantified by
measurements during a test.
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 provides
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
software program is an example of PoF analysis useful in developing
reliable electronics.4 It converts a geometric
description into a mathematical model, then solves this model,
extracting detailed stress cycling data for every component on the
software-program methods were based on decades of experience in
applying numerical analysis to design and development of structures.
This experience included design, development, and test of numerous
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 you don’t need FEA
expertise. Interfaces to computer-aided-design (CAD) programs speed
the development by translating CAD data to circuit-card analysis
FEA applies laws of
mechanics and determines product information beyond the capability
of any test program. Analysis can optimize accelerometer
positioning. Taking advantage of new tools to use current PC power,
the detailed calculations can turn a few accelerometer measurements
- Modal shapes for
- 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,
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 also can
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
PoF analysis identifies which components are to be driven at each
natural frequency and to what level each is to be driven. It also
predicts 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 stress 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
Figure 1 shows stress-screen effectiveness at
a component level. It also illustrates life capability under one set
Since a test cannot
provide any measurements descriptive of point of failure, test alone
can be a 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 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 the component level.
These numbers are transferable from one design configuration to
When numbers can be
transferred from one design to another through analysis, each test
program benefits from all past experience. Test programs become more
efficient. For life testing, definitions of life capabilities
relative to requirements are 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 cost savings of stress-screen programs and savings from
producing a more reliable product.
1. Hobbs, G.K.,
“Reliability—Past and Present,” Sound and Vibration, April
2. Starr and Abner,
“Understanding Vibration of Electronic Systems,” www.vibrationandshock.com/news/news5/nl5.htm.
3. Gray and Tustin,
“Electronics Testing into the 21st Century: Success in Test Is in
Capabilities, Not Specifications,” www.equipment-reliability.com/articles/art2.htm.
4. CirVibe Circuit
Card Vibration Software, Users Manual Version 3.0, http://www.cirvibe.com/.
is a consulting engineer at CirVibe. He has an M.S. in
engineering mechanics from Michigan State University and extensive
experience in structural analysis and test development. CirVibe,
P.O. Box 47394, Plymouth, MN 55447, e-mail: firstname.lastname@example.org
Tustin is president of Equipment Reliability Institute and a
graduate of the University of Washington with a B.S.E.E. He has
broad experience in shock and vibration testing and teaching of
these test methods. Equipment Reliability Institute, 1520 Santa Rosa
Ave., Santa Barbara, CA 93109, 805-564-1260
All contents © 2002 Nelson Publishing