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The Achilles Heel of
Modern Electronics
by Brent Sorensen, Universal Synaptics
Analog fault monitoring based on neural network technology can be an
alternative to conventional digital testing for finding aging-related
intermittencies.
Digital measurement technology certainly has taken the test industry
forward, mostly with giant strides. The exact opposite may be true,
however, when it is applied to testing for aging-related failure modes
such as intermittency or no fault found (NFF).
For passive circuit elements such as resistors and capacitors, aging is
seen as a continuous or analog type of degradation that results in
parameters tending to drift out of tolerance over a very long period of
time. In contrast, aging defects in electromechanical and connectivity
elements are seen more as randomly occurring, digital-like, intermittent
events. In reality, there exist two very distinct failure modes, yet
both are being tested for with one technology. In terms of reliability,
that is a major problem.
Digital abstraction devices, perfect for parametric testing, rarely
synchronize to real-time, randomly occurring, anomalous, or intermittent
age-related failure events. This shortcoming is especially acute when
large numbers of circuit nodes require testing.
For finding randomly occurring, intermittent failures, all circuits must
be tested simultaneously and continuously. By definition, most digital
instruments, whether having single or multiplexed inputs, are unsuitable
for this task.
Figure 1 compares the capability of various measuring devices to detect
intermittencies of varying lengths. For example, a 4.5-digit DMM cannot
accurately or reliably discern the presence of an intermittency unless
it lasts for at least 200 ms.
Figure 1. Single Circuit Intermittency Detection Capabilities
The Effects of
Poor Reliability
According to several studies, the reliability of certain fielded systems
has reached a critical level. The exclusive application of digital
instruments has had a particularly devastating effect on all systems
under the auspices of the Department of Defense, the FAA, and NASA that
use multilayered maintenance schemes where black-box swapping is the
preferred diagnostic choice. Even the fleet of the newest Stealth
fighters, the F-117s, has been considered for early retirement due to
the high expense required to keep the aircraft’s aging avionics systems
flight worthy.
Similarly, many automotive and other electrical and electronic
manufacturers see their commercial products failing early in their
projected life cycles. As a consequence, they are finding themselves at
risk of class-action litigation to make right the reliability
deficiencies in their market offerings.
In addition to the huge outlay a class-action suit could cost a company,
there is the accompanying problem of high warranty return rates with
extensive NFF percentages. Closely related are the profit risks some
ISO-certified manufacturers will face when quality- and
reliability-savvy component buyers refuse to buy undertested components
that would put their company in jeopardy.
Wherein Lies the Problem
At the root of most of these early-life intermittent problems, we find
degraded circuit connectivity elements such as connectors, crimps,
solder joints, relays, switches, and other intermittent-by-design
components whose published specifications suffer from the same digital
testing deficiency. As a result, their reliability ratings are much
higher than reality warrants.
For example, a connector may post super high insertion ratings based on
how many times two pins from the connector can be rubbed together before
ohmic resistance from fretting reaches some set parametric level, such
as 100,000 insertions to reach a 0.2-W level of resistance. However,
because the measurement is performed digitally, the point at which the
connector first becomes intermittent and essentially unreliable is not
known. Actual in-service reliability and product life generally are much
shorter than predicted by results of the digital test.
We find products with engineered service lives of several years failing
prematurely under warranty due to intermittent interconnections. This
occurs even after performing extensive highly accelerated life
test/highly accelerated stress screen (HALT/HASS) testing designed to
simulate their use well beyond this period.
HALT/HASS testing can miss developing intermittencies if digital
instrumentation is used. In the case of run-to-failure testing, the
product may itself average out, filter out, or retry an operation using
elaborate error correction to fix intermittencies generated by the
life-testing process.
While the design team would not condone intermittencies in products that
otherwise may be meeting useful life test goals, the testing process
itself does not report intermittencies. For that reason, intermittent
and unreliable products pass the very screening process designed to
catch them.
Despite having used HALT/HASS to produce products meeting significant
life objectives, companies have been surprised to find themselves
devastated with warranty returns and high NFF due to the unpredictable
nature of intermittency as it relates to the aging process. By
addressing this one seemingly minor interconnection problem, your
system’s life could be extended by several years.
In addition to fretting corrosion, oxidation, loose connections, and bad
solder joints, occasionally contaminates from certain manufacturing
processes create intermittent discontinuities. For example, an otherwise
perfectly good gold-plated connector becomes intermittent because flux
and cleaning processes leave an invisible residue on some of the pins.
Also of special note, we see and hear of excessive problems with
assemblies or systems that incorporate wire-wrap connections. While not
designed to be opened or taken apart, the wrapped connections are
neither soldered nor protected from corrosive environmental elements and
are virtually impossible to visibly inspect on the contacting surfaces.
Inappropriate Testing for Reliability
The present digital-only testing model cannot certify that an item is
free from any age-related or manufacturing-induced intermittency.
Functional, component, or continuity testing can only determine that a
given test passed certain requirements at a specific moment in time.
There is no rational basis for expecting that these severely limited
results could be used to reliably predict the future performance of a
product. As an example, you can momentarily open the test leads of a
highly accurate DMM by hand then rapidly re-establish continuity, and
the meter’s digits may not even blink. Not seeing the glitch is not
equal to it didn’t happen.
Because of its mechanical nature, intermittency itself is not linear,
repeatable, or predictable. A small, inconsequential, one-shot ohmic
glitch one moment may just as easily become a significant wide-open
break or short circuit the next.
With aging, the probability of this random intermittent disconnect
happening at any level or degree may increase faster than the actual
severity of it. For these mechanical connectivity elements to function
properly, the relatively rough micromating surfaces, consisting of
hundreds or thousands of protruding hills and recessed valleys, must
physically make contact on the hills of both surfaces in sufficient
numbers to meet design goals.
As fretting wear and oxidation begin to reduce the number of available
connecting protrusions, vibrations or thermal expansion that cause the
surfaces to slip will tend to create shorter but more numerous
micro-intermittencies. As age and wear continue to take their toll,
these intermittencies will become more ohmic and longer in duration
because of the longer mechanical distances the surfaces may have to
travel before continuity is reestablished. As a result of the often
subjective demands for higher levels of accuracy in test measurements
and the difficulty of digitizing such unpredictable failure events,
intermittencies that can reset or shut down a system literally escape
detection.
This more numerous microbreak phenomenon actually may be the only good
thing about intermittency. While often functionally benign in the
beginning stages, with the right testing equipment, they become like red
flashing warning lights that a problem is developing that could easily
result in more serious operational hazards if left unfixed.
There also is the perceived problem of digital equipment creating false
failures or measurement glitches somehow associated with the digital
measurement process itself. Upon encountering a glitch or a failed test,
some ATE may recycle the test, often numerous times, effectively
allowing any real product intermittency to clear itself without logging
even a hint of the failure event to the testing record.
The likelihood of seeing an intermittent defect under all of these
digitally unique conditions usually is rather small and decreases
exponentially as the number of test points increases.
A Simple Example
To enlighten the National Transportation Safety Board’s (NTSB)
investigators about the problem of digital averaging during avionics
testing of the crash of Flight 587, we put together a rather simple
glitch test to demonstrate how today’s digital testing instruments often
do not see dangerous latent glitches that can later cause serious
problems.
In Figure 2, programmed, one-shot glitches from a pulse generator were
fed to a DMM connected to a computer to capture and record the results.
The glitches were created at random by dropping an otherwise steady 4-V
signal to ground potential 25 times for 100, 10, and 1 ms. The shorter
the duration of the applied glitch, the less likely the sampling and
averaging digital meter was able to detect it. While a few glitches were
completely missed at 100 ms, hardly any registered as defects at 10 ms
and 1 ms.
Figure 2. Glitch Testing of a 4.5-Digit DMM
The Solution
Analog testing can resolve the reliability-testing dilemma. While
continuous testing overcomes sampling and averaging problems related to
digital techniques, there also is the need to test all points
simultaneously and do it economically.
For these reasons, we developed neural network technology in a hardware
form called the Intermittent Fault Detector/NFF Analyzer. In a neural
network, all inputs converge to a common go, no-go output through
several layers of analog and eventually digital processing. In the
neural network, the incoming glitch is the active element that triggers
certain hard-time processing events that automatically yield detection,
test-point location, and a digitized trace.
The impact of neural analog technology is found in Figure 3, a screen
capture of test results where an expanded glitch test, described in
Figure 2, was performed on one of our testers, the Model IFD-2000. Shown
as columns that grow with each detected event, the first six test
points, color-coded to track results in the scoreboard on the left and
the scope traces below, gathered testing data for the applied glitches
that ranged in six steps from 100 ms down to 1 µs.
Figure 3. Glitch Test
Because all 25 glitches of each pulse duration were software generated,
the scope traces on the bottom of the screen overlap and demonstrate
perfect repeatability, something not seen with random intermittency.
There also were no false failures reported.
For the user, the IFD-2000 is a simple go/no-go device with pass/fail
criteria displayed at the top of the screen. The scope traces at the
bottom are simply an indication of the severity of any detected
intermittency. They only are presented as a diagnostic indicator and to
provide a familiar scope-like presentation of what’s going on in the
item being evaluated.
Some test engineers have questioned the need for analog neural
technology when dozens of equipment manufacturers offer instruments with
digital sampling speeds in the gigahertz region. The main problems with
this approach are the size and complexity of the instrument that must be
configured to simultaneously sample hundreds or thousands of test points
and the enormous amounts of digital data that will be acquired at these
high speeds. At some time, because an instrument’s storage capability is
finite, testing must stop and the data be processed.
With analog neural network technology, the footprint is small, allowing
portable applications that analyze for pass/fail in continuous real
time, and the only data generated or saved is the failure report. The
rest of the information you don’t need.
Perhaps the best example of what this technology brings to the
aging-related testing field is illustrated in Figure 4. The more testing
information you can gather over a given period of time, the more
probable the detection of any anomalous glitches that might be likely to
occur during actual operation.
Figure 4. Comparison Between Continuity Testing and Direct Reliability
Testing
Regardless of the
number of lines under test, the glitch detectability using neural analog
technology is 1 event/320 ns.
A popular but mostly futile therapy for NFF or intermittency problems is
to run the tests again and again, hoping that something might show up.
As can be seen in Figure 4, on a single wire, a 320-ns glitch resolution
provides 3.3 million equivalent tests for random intermittency in the
same time period that a typical continuity tester using digital
technology will perform one test.
Summary
NASA-STD-8729.1 states: “The attribute of reliability, by definition,
lies in the probabilistic realm while most performance attributes or
parameters such as temperature, speed, thrust, voltage, or material
strength contain more deterministic characteristics. Within the accuracy
of the measuring devices, one can directly measure performance
attributes in the deterministic realm to verify compliance with
(reliability) requirements. No such measuring device exists for
probabilistic parameters like reliability. It is usually estimated
through comparison with similar components or systems through inference,
analysis and the use of statistics.”
This view, now superceded by the capability to directly test for
reliability with neural analog technology, needs to be updated,
especially when intermittency in the connectivity elements is so
directly linked to system safety and reliability.
Accurate reliability testing can be seen as increasing return on
investment if the costs and the benefits are correctly calculated.
Analog intermittency testing costs about 10% of traditional ATE. On
older avionics systems where NFF repair rates may run as high as 50% to
90%, the costs of ATE are somehow justified, even for test equipment
that is literally only fixing half of the present problems.
Spending a little extra to fix the other half then is indeed a bargain.
In most repair or manufacturing situations, the savings from more
efficient in-house diagnostics will offset the reliability testing
costs, and a company will garner immeasurable goodwill because its now
puts out a better, more reliable product and has increased its
protection from forced legal remedies.
About the Author
Brent Sorensen is president and founder of Universal Synaptics.
Previously, he spent 29 years in various U.S. Air Force capacities
involved with testing and researching the CND/NFF problem. Mr. Sorensen
has written and published a number of papers and worked with a variety
of high-level government and commercial organizations regarding the
effects of the NFF/aging phenomenon. Universal Synaptics, 1801 W. 21st
St., Ogden, UT 84401, 801-731-8508, e-mail:
Brent.Sorensen@usynaptics.com
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