Intermittence / No Fault Found (NFF)


IFDs:   Intermittent Fault Detectors (developed & manufactured by Universal Synaptics Corporation)

Ncompass / Ncompass-Voyager: Enhanced IFD Intermittent Fault Detectors (adds Scanning Continuity, Z-Sweep, Log-Scope, Auto-Map & more)

IFDIS:   Intermittent Fault Detection and Isolation System (multiple IFD / Ncompass modules, racked and combined with environmental stimulus and custom Interface Test Adaptation (ITA)).  Up to 16,384 simultaneous test points.

NFF:   No Fault Found / No Faults Found / No Problem Found (NPF) / No Trouble Found (NTF) / No Defect Found (NDF) / Retest OK (RTOK) / Etc.

 Electrical intermittence is a "temporary" deviation from the nominal operating condition of a circuit or device.   Common root causes of intermittence are the component interconnecting devices such as solder joints, connectors, cables / wires, crimps, splices, circuit traces, via's, wire wraps, relays, circuit breakers, etc.   There are typically several thousand of these devices in a modern avionics system.  Other than through damage, these devices tend to wear or corrode gradually, over long periods, and thereby fail intermittently long before eventually failing permanently or "hard / semi-hard".  It takes only one location becoming intermittent to render an entire system unreliable. Fixing an intermittent root cause is relatively easy.  Detecting and isolating the intermittent is the challenge.  The severity (amplitude and duration) of Intermittents generally gets worse over time.  The further up this degradation curve that you can detect and eliminate these type problems (prognostics) the better your Mean Time Before Failure (MTBF) becomes.  Inter-connection reliability is critical to the dependable performance of nearly all modern electronic systems.  Comprehensive intermittence testing specifications are therefore necessary if you want to ensure this reliable performance. 

Nanosecond intermittency can result in a loss of information.  This loss, often leads to system failures, and typically occurs in the extremes of the operating environment.   The speed and performance of today’s avionics demands that industry test-specifications for testing intermittency and reliability be performed at these increased levels also!  To test otherwise, only ASSURES that intermittent defects WILL ESCAPE detection and remain in the system to further hamper and hazard the crew and the mission.  Inadequate testing specifications for intermittency ARE a primary reason that No Fault Found (NFF) is so prevalent and endemic in the avionics and electronics industry today.  Test equipment typically employed for measuring intermittents or dis-continuities (like scanning continuity testers) or any other single-line-at-a-time methodology (Reflectometry, Digital, DVM, Etc.) are simply incapable of achieving the necessary performance levels to detect the extremely low-level intermittence causing NFF.

Intermittence occurs RANDOMLY in TIMEAMPLITUDE and DURATION, and RARELY REPEATS with the same parameters.  It might repeat only once or it might not repeat at all during a given test period.  Therefore, the hope of detection revolves around test equipments SENSITIVITY to the problem (fast testing speeds with high sensitivity to low-level impedance changes) and the PROBABILITY that you will even be testing the correct circuit at the exact microsecond that a nanosecond event might occur.

For intermittence test coverage and testing-effectiveness calculation formulas and instructions, please see the technical paper found at the following link... The Right Stuff for Aging, Intermittence and No Fault Found.




ATE:   Automatic Test Equipment

WTS:  Wire Test Sets (typically a scanning continuity, single-line at a time digital testing device of some make or model with custom UUT adaptation)

AWTS:   Automatic Wiring Testing Systems / Aircraft Wiring Test Systems  / Automotive Wiring Test Systems (programmed, scanning continuity testing systems of various makes, models and manufacturers)

EWIS:   Electrical Wire Interconnect System / Electrical Wiring & Interconnection Systems


Conventional ATE and WTS / AWTS technologies are very effective at troubleshooting EWIS hard failures (opens, shorts, miswires) and some models also effectively test for high potential (Hi-Pot) insulation breakdown and circuit breaker hold and trip levels.  However, ATE and WTS / AWTS devices are severely limited and ineffective when applied to testing for intermittent problems.  An intermittent EWIS problem will continuity-test "good" 99.9999999% of the time, that's why they get labeled as No Fault Found (NFF), they pass all continuity and functional tests, but then fail intermittently in the operating environment.  Intermittent failure events rarely synchronize with a scanning measurement window during test time.  This massive testing blind spot is a leading contributor to the extensive NFF problem.

In contrast, the IFD / IFDIS / Ncompass does not scan, sample, or average..., rather it monitors all of the circuits, individually, all of the time, in parallel, sensing for random, low-level dis-continuities occurring down to *50 nanoseconds on any line ( .00000005 seconds ).  The result is; low-level intermittence which causes No Fault Found (NFF), is not missed.   Comparisons of intermittence test coverage and detection probability between IFDIS &  ATE / WTS / AWTS type equipment is calculable and typically equates to several billion and even trillions of times more effective in favor of  the IFDIS for avionics and EWIS reliability testing (the total calculation depends on the number of circuits which should be simultaneously tested for intermittence).  The IFDIS can test up to 16,384 wires or circuits individually, in parallel, without losing a nanosecond of test coverage on any line.  For the intermittent / NFF problem, IFDIS advantages over ATE / WTS / AWTS are like comparing the Hubble Space Telescope to field binoculars... you obviously won't see, what you cannot see.

In addition to its patented all lines, all the time, intermittence testing technology, the  IFDIS & Ncompass also performs ohmic continuity and AC Impedance tests similar to ATE / WTS / AWTS technology.  The IFDIS / Ncompass employs safe, low-energy stimulus, it can be quickly adapted and broadly applied in-situ with reduced concern for damaging electronic circuits.  To be clear, the IFD / IFDIS / Ncompass does not perform high voltage Hi-Pot or high current circuit breaker tests.  If you want or need to test for Intermittence / NFF you should use an IFD / IFDIS / Ncompass to be effective.  If you want to test wiring insulation degradation (Hi-Pot) or Circuit Breakers, then you should use some other ATE / WTS / AWTS technology, if you want to test all of these wiring / circuit issues... then you will need to use both.  There is no comprehensive way around it, each technology performs distinct and important functions that the others do not... making them all important and in most cases complimentary technologies.

Research shows that intermittent / NFF and all the quasi-intermittent defects are the most prevalent failure modes and typically account for 50% to 80% of all LRU / WRA depot and EWIS repairs, especially in the avionics boxes.  Some mix of opens, shorts, miswires & other component failures account for the rest the defects.   

Intermittent fault detection specifications measured using Hewlett-Packard 8111A Pulse/Function Generator

For intermittence test coverage and testing-effectiveness calculation formulas and instructions, please see the technical paper found at the following link... The Right Stuff for Aging, Intermittence and No Fault Found.

No, not very well, WTS / AWTS scanning continuity technologies serially test only one circuit or wire at a time checking for continuity or nominal operation.  WTS / AWTS devices verify proper function and continuity at that one very brief moment, but this does not test the reliability of the circuit to continue performing correctly into the future.  While WTS AWTS devices might indeed be physically connected (adapted) to all of the circuits or wires in the Unit Under Test (UUT) at the same time, they are still just serially scanning and  testing them only one circuit at a time.  In other words, if you have a momentary (microsecond) intermittent fault occurring on circuit #75 but the scanning continuity test set is currently testing circuit #25 at the exact time that the random intermittent event occurs on circuit #75, a WTS / AWTS device misses the fault completely.  Imagine trying to sync up to a random nanosecond or even microsecond intermittent if you have 1,000-3,000 or even 10,000 test points as found in typical modern avionics chassis boxes. These intermittence-testing dead zones with scanning test equipment are a leading contributor to the massive No Fault Found (NFF) problem.  They simply just don't see developing problems, so over time you get a cumulative or compounding effect and low reliability.

In contrast, the IFD / IFDIS / Ncompass monitor each and every circuit individually (all-lines-all-the-time) in a SIMULTANEOUS and CONTINUOUS manner.  By monitoring every single circuit at the same time in parallel, those random, intermittent events are not missed.  The IFD / IFDIS / Ncompass were specifically designed from the ground up to overcome all of the conventional ATE and WTS AWTS technologies intermittence testing limitations. The IFD / IFDIS / Ncompass was conceived to detect,  identify and isolate the low-level intermittence causing NFF through the use of a patented analog, hardware neural network, which allows it super sensitive circuitry to monitor all of the test lines, all the time, whereas scanning continuity test sets are designed to only check one wire or circuit at a time.

See Brochure: It Doesn't Take A Genius...

Rarely, and only the very worst of them!  Be aware that some "intermittence" testing claims should really more properly be termed "semi-hard failures".  If the intermittence occurs so frequently and is so present in a test as to be there every time a scanning, cycling, continuity tester plods through all of the test points one at a time individually (this can take several minutes in large systems)... then this type of malfunction is really more properly termed a "semi-hard, or even hard" type of failure.  Most testing devices could detect these frequent intermittent failures.  On the other hand, if your equipment is not detecting nanosecond and microsecond intermittence, then you are missing an entire "class" of intermittents (the really tough and elusive ones).  This intermittence is the type causing high NFF, low MTBF and poor reliability.  Regardless of the stimulus applied to the UUT, the probabilities for typical AWTS technologies to detect the low-level intermittence causing NFF are so infinitesimal it simply is not worth your time and money and would not result in any real reliability gains.  Scanning Continuity & Environmental Chamber technologies have been employed for decades and have had no significant affect on the NFF problem.  Intermittence occurs randomly in time, place, amplitude and duration.  The very nature of the failure mode suggests that the ability to detect and further isolate the intermittence root cause is a function of TEST-COVERAGE and that is based on detection SENSITIVITY and PROBABILITY rather than conventional methods concentrating on ohmic measurement accuracy.

With IFDIS you can expect to see significant improvements in reliability and Mean Time Between Failure (MTBF) for the IFDIS units tested.  IFDIS not only catches the primary intermittence that was likely bad enough to drive the box in for repair, it will also detect developing intermittent problems which can be fixed at the same time and for very little cost.  Fixing a problem at the depot before it causes a system malfunction, obviously keeps the devices working longer in the field. Therefore, IFDIS is a true prognostics testing technology and the demonstrated MTBF improvements on IFDIS tested devices thus far are actively substantiating it!

See Brochure: It Doesn't Take A Genius...

You can’t detect an intermittent event until it occurs, and then you might have limited opportunities to catch it on the specific circuit when it does. Trying to measure fractions of a milliohm, scanning one circuit at a time, is completely ineffective for intermittent failure modes with or without environmental stimulus.


TDR:  Time Domain Reflectometry  (High Frequency Test Signals for Distance to Fault Tests)

SWR:  Standing Wave Reflectometry (Low Frequency Test Signals for Distance to Fault Tests)

SSTDR:   Spread Spectrum Time Domain Reflectometry (Static & Live Circuit Test Signals for Distance to Fault Tests)

PASD:   Pulsed Arrested Spark Discharge (Damaged Wiring Insulation / shorts & Distance to Fault Tests)

There are several variations of Reflectometry technologies.  The most prevalent are TDRSWR, & SSTDR.   These technologies are primarily for determining “distance to faults” and can be very accurate at telling the approximate physical distance of a hard-failure down a single length of wire.  Because they use high, low and spread spectrum “frequencies” for test stimulus, they may detect some AC impedance issues which is a step-up from just DC resistance measurements you get with a typical continuity test. 

However, just like all other one-line-at-a-time, serial, scanning testing technologies, the actual malfunction (open, short or impedance) must be very constant in nature for any problems to be seen.  Since the intermittence that causes a No Fault Found (NFF) tests good 99.9999999% of the time (hence the NFF label) these devices will also miss the bulk of intermittent / NFF problems. Unfortunately, these technologies are to often "sold" as intermittent fault detectors, but their single line or circuit at a time implementation dramatically limits their capabilities and their single circuit intermittence testing speeds are extremely slow by comparison, usually down in the low milliseconds at best.  If you are not testing in the nanoseconds detection range you are missing an entire class of intermittent / NFF problems.

To efficiently and comprehensively detect, isolate and resolve intermittence / NFF you have to have phenomenal test coverage, be super sensitive on each line, individually, to catch even the slightest degradation and since intermittence by it’s definition occurs randomly in time, place, amplitude and duration you also must be sensing every potential failing line, at the same time, in parallel.  This is exactly how the patented IFD / IFDIS / Ncompass analog, hardware neural network technology works… it does not miss intermittent events.  The IFD / IFDIS / Ncompass also perform both standard DC ohmic continuity and AC impedance tests in addition to our patented, unique intermittence testing.

Does TDR or SWR Work for Intermittent Faults?  Not according to this Sandia Labs test report.    See page-11 of this Sandia Labs test results matrix...

If you know that you have a problem (hard failure) on a particular wire in a cable or circuit and you need to know the exact distance down that wire the hard problem exists, then by all means you should use one of these technologies... they work pretty good.  In contrast, if you are having problems duplicating the malfunction, or if the system is acting unreliably or intermittently and you need to isolate the source of the intermittence, then you should use the IFD / IFDIS / Ncompass Intermittent Fault Detectors.  There are limited practical applications for TDR / SWR / SSTDR for testing the avionic LRU / WRA boxes, these reflection technologies are used primarily on aircraft wiring and other wire / cable systems.

CASS / eCASS / RTCASS and VDATS test stations are comprised of several different individual testing technologies, all integrated into an expandable, reconfigurable test stand which may then be controlled by a central computer and operating system.  These are ideally the best and most capable pieces of individual Automatic Test Equipment (ATE) for each type of parametric measurement or functional test requirement at that particular location.

The type of instruments typically found in a CASS / eCASS / RTCASS or VDATS stations are all very good and necessary technologies and are all very effective for testing and validating the specific electronic functions for which they were designed.   However, this type of Automatic Test Equipment (ATE) is geared toward the testing of hard failures or for parameters that are constant in nature.  None of these devices perform adequately when an intermittent / NFF problem is at the root of the malfunction. Therefore, the IFD / IFDIS is completely complimentary to any CASS or VDATS test station.  The IFD / IFDIS / Ncompass were designed specifically to detect intermittence and No Fault Found (NFF) type malfunctions and works extremely well for testing the LRU / WRA chassis and backplane components.

Prior to the development of the IFD’s, Intermittent Fault Detection technology, testing all lines, at the same time, simultaneously and continuously, was not really possible to any efficient degree using conventional test technology.  The only way to comprehensively detect the low-level intermittence causing No Fault Found (NFF) is to test all of the lines, at the same time, simultaneously and continuously.  The sensing technology also has to be super-sensitive on each of the individual lines to pick up the developing intermittent problems.  Having significant experience and interest in the science of Artificial Intelligence (AI), our company’s founders felt the most analogous solution to this testing dilemma would be to mimic the parallel distributed processing operation found in the operation of the human brain and sensors of the body’s nervous system. The resulting IFD and its current derivatives, the Ncompass and IFDIS all share a common intermittence testing architecture that, in its basic form, comprises a Hardware Neural Network.

A1: Those familiar with neural networks know they are most often associated with software functions.  The Ncompass / IFD / IFDIS do employ software neural networks to analyze impedance patterns and perform impedance fingerprinting and other useful pattern recognition processes.  However, at the heart of each Ncompass / IFD / IFDIS is also a patented hardware neural network that is used to monitor and sense for intermittence in electrical circuits.

The best way to describe the theory of how our patented Hardware Neural Network works as applied to testing for intermittence in electrical systems, is to compare how the human brain and our body’s neurons and sensors work.  The human brain functions using what is called Parallel Distributed Processing.  Rather than serially scanning through our body analyzing one sensor at a time to make sure we are operating ok, the brain is “listening” to all of our body’s sensors, all at the same time in parallel, listening / sensing for any change from the nominal or steady state.  For instance, when you’re walking down the side of a road and a pebble gets inside your shoe, the nerves (sensors) in your foot immediately signal your brain which is monitoring all our body’s circuits in parallel and immediately registers the intensity and physical address of the discomfort or change from the steady state (the malfunction). That is basically how the Intermittent Fault Detector works...  we employ a neural sensor to every line or circuit in the UUT simultaneously.  Any particular IFD sensor which detects intermittence (change from the nominal or steady state) is reported & processed through the hardware neural network which determines the sensor address and intermittence amplitude and duration, then feeds this information to the test operator via the controlling computer... no missed events.

A2: Probably the best way to explain and demonstrate the unique, comprehensive testing capability of the Ncompass-IFD / IFDIS in conventional testing terminology is to first separate the Ncompass-IFD technologies into its primary testing functions.

Ncompass Mode

Continuity:  The Ncompass’s Continuity test employs traditional, switched, one-line or circuit at-a-time, ohmic continuity tests which verify the circuit connectivity looking for opens, shorts, miss-wires or any continuity or ohmic differences from a good or gold UUT… all pretty common stuff… similar to the rest of the multitude of continuity testers available in the marketplace.

Log-Scope: The Ncompass’s Log-Scope function uses this same continuity tester as a single-circuit (selectable) ohmic measurement logging meter, which then charts the ohmic readings over a selectable test period looking for any changes or ohmic drift which may be occurring in the circuitry.

IFD Mode

The patented Intermittent Fault Detection (IFD) portion of the Ncompass-IFD employs a proprietary hardware neural network which includes the following test functions in its testing suite;

Analyze: the Analyze function performs an AC impedance test on all connected circuits, 16 circuits at a time.  Any difference(s) from a good or gold unit are flagged and reported.

Z-Sweep:  the Z-Sweep function performs a sweeping AC impedance test on all connected circuits, 16 circuits at a time.  Any differences from a good or gold unit are flagged and reported.

Intermittence:  the Intermittence testing function is the primary purpose of the Ncompass-IFD.  This is what makes the IFD technology a unique and revolutionary piece of test equipment. The IFD's Intermittence testing performs oscilloscope-type monitoring of every single connected circuit individually, yet it performs this testing on all of them simultaneously in parallel (all lines, all the time), up to 16,384 simultaneous test points.

To duplicate just the basic portable IFD’s intermittence testing function using conventional means, one would need at least 256 latching analog oscilloscope channels (each portable IFD module tests 256 test points simultaneously) all racked into a mounting structure and all connected to a central controlling computer. You would also need a DC power source on each oscilloscope-connected circuit to provide the test-stimulus.  In contrast, the 256 channel IFD module provides all of this and fits neatly into a small suitcase-size package for flight-line use.  Multiple IFD modules (up to 64) or blades can be stacked in a rackmount configuration (1,280 test points per 7U rack-space) up to 16,384 total test points.

Each individual circuit is then monitored, all of the time, sensing for any low level impedance changes or circuit “noise” which may be intermittently occurring down to 50 nanoseconds.  These "tremors" are the first tell-tale signs of an impending intermittent operation (prognostics). The amplitude and duration of each detected impedance change event is captured, reported and logged to the controlling computer, which then re-sets all of the circuit sensors (oscilloscopes) which continue to monitor and sense for the next random impedance change.  At the end of a given test period, all of the un-stable, intermittent circuit events (intermittence) are pareto-ranked, reported, saved and printed.  Custom graphics for each UUT depict the malfunctioning circuit’s physical location for an easy, surgical repair.  Use of the Ncompass-IFD / IFDIS provides the most comprehensive test coverage (several orders of magnitude greater) of any wiring/circuit tester on the market today.