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Maximising test coverage with multi-strategy test techniques
The constant march of electronics technology has meant device geometries have shrunk while
functionality has increased and this has led to circuits that are complex to test and even more
difficult to fault find. Notwithstanding the technical challenges there is also one of physical access as
daughter boards, BGA devices and very small size devices have become prevalent.
Although testing from an edge connector will prove a circuit works, there is the difficulty of
detecting and identifying the cause of a fault if it doesn’t.
No single test technique can provide the level of test and diagnostic capability necessary to provide
through-life support of a circuit. Instead, multiple
techniques are required which can be applied as
necessary and integrated into a single test program flow.
With over 40 years of experience in the test, fault
finding and repair of electronic circuits, Diagnosys has
developed an integrated diverse range of different
techniques and technology to help customers keep their
electronics working.
Multi-strategy test techniques
1) Digital vector test
This technique treats each device as a separate test. It isolates the device being tested from the
rest of the circuit by using backdriving (sometimes
called node forcing) to apply signals to the device,
and guarding to ensure no other device on the
board interferes with the outputs states being
measured.
A dynamic test routine is then used to exercise the
device and check the functionality. The test routine
will perform a power-on test that uses multiple
digital test vectors (applying input patterns and
measuring output patterns) to prove the correct functionality and operation of the entire device.
Operating with data rates up to 15 MHz (15 million test vectors a second) on the Diagnosys
PinPoint system, a comprehensive and in-depth test of a device can be achieved safely.
These digital tests not only check the dynamic functionality of the device but also test for; shorts
between pins, open circuits between the pin and the circuit, correct logic levels being sensed
and driven.
This level of fault detection and identification can only be achieved by using digital dynamic
vector tests.
2) Analog in-circuit tests
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This technique tests each passive analog component in a circuit and
uses guarding to ensure measurements are not affected by the rest of
the circuit. The value of resistors, inductors and capacitors are
measured and checked to ensure they are in tolerance. Semiconductor
junctions are tested to ensure forward voltage, reverse voltage and
respective currents are correct such that diodes, transistors and gated
devices are checked for correct operation.
Using this technique provides absolute confidence in the correct
operation of each device and will also identify the exact component that is failing. This level of
test coverage and fault identification can only be achieved using this method.
3) Active analog in-circuit tests
Devices such as operational amplifiers, voltage sources, analog
to digital and digital to analog converters require voltmeters,
waveform generators and oscilloscopes to perform
meaningful tests on their functionality. The Diagnosys PinPoint
system has in-built instrumentation to cater for these types of
test which will test the dynamic functionality of these devices.
Again it is only by using this technique that the required level
of test coverage and fault identification can be achieved.
4) Edge tests (digital and analog functional)
The fastest way to test a circuit is from the edge connector and to perform a functional test
using digital test channels and in instrumentation. These functional tests are used to determine
if a circuit is good or faulty and following a repair, to give a confidence check that the circuit is
operational again. This confidence can only be provided by using dynamic digital and analog
signals through the edge connector.
The Diagnosys PinPoint range of systems are all capable of performing edge connector testing
with digital and analog signals. An extensive analog test capability is provided by a range of inbuilt high performance modular instruments. These can be supplemented with externally
integrated instruments when required for specific applications
such as RF testing. The powerful and graphic Instrument
Strategizer software is another examples of how Diagnosys has
created easy-to-use multi-strategy capabilities for test program
development. The Instrument Strategizer allows the graphic
programming of multiple instruments in a single workbench and
provides the mechanism to easily integrate external instruments.
5) Impedance signatures - VI testing
Impedance signatures, Analog Signature Analysis (ASA) and VI
are all different names for the same technique. The most common name is VI (voltage-current)
and we will use this terminology.
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VI testing is the process of applying an alternating
voltage, usually a sine wave, onto a network and
monitoring the resultant current flow. This voltage and
current is then displayed graphically in the form of a
Lissajous figure.
The VI technique learns signatures form a known good
circuit and stores these as a test program. When a circuit
is being tested the measured signature is compared to
the one stored in the test program and a pass / fail
decision is made.
VI testing does not perform a functional test but uses the
passive impedance signature for the network. The
technique is particularly useful for networks with analog
components but can also be applied to digital components where the signature is created from
the passive input and output circuit characteristics of the device. It should be noted here that
the VI test is a power-off technique and therefore no form of functional test is performed on any
component so a component may have the correct signature but the internal logic may be
broken.
As the VI signatures are generated from the analog characteristics of a network it is possible that
some faults are masked. This can occur, for a simple example, when a small value capacitive
element of the signature is “hidden” by larger value elsewhere on the network.
For digital devices the signature is created from the analog characteristics of the input or output
circuitry. Although devices from different manufacturers will have exactly the same functionality
the VI signatures can be different. This means that the VI technique can indicate a false failure
when a signature doesn’t match the one expected. A way of overcoming this is to learn
alternative signatures for the same network thereby accommodating different manufacturers of
the same device but this is an on-going process.
Similarly if two digital pins of the same type are shorted together e.g. input – input, output –
output, then the network signature is unlikely change and the VI technique will not detect the
fault.
6) Boundary Scan test
Boundary Scan is a technique which relies on the device(s) being tested having the in-built
capability to support the test method. It basically uses serial data in and out of a device which
can be used to; statically test internal logic, check for shorts and opens between pins and
connected devices, program devices in-circuit, test connected memory etc.
It is a technique which overcomes physical access issues and provides another way to enhance
test coverage.
Boundary Scan programs can be generated automatically from the BSDL (Boundary Scan
Description Language) for each device. These files describe the capabilities and structure of the
device and are available from the device manufacturer.
Boundary Scan techniques can be used with a chain of devices or on a single device. The
Diagnosys PinPoint range of systems uses Boundary Scan at a single device level. In this way the
Boundary Scan tests are combined with the digital in-circuit test capabilities of the system to
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provide a rapid way of creating a test routine for complex devices. Testing the shorts, opens and
basic functionality of the device , the technique is another example of how multi-strategy
techniques can be applied to improve the fault detection and identification in a circuit.
Summary
Comprehensive test and accurate fault identification is essential for the
rapid repair of a circuit and for providing absolute confidence that the
circuit is operational again. To achieve this requires the application of
multiple test techniques within an integrated software environment.
The Diagnosys PinPoint systems are developed with over 40 years of
experience and are able to perform all of the necessary test techniques
to deliver unrivalled test coverage and accurate fault identification.
Powered by the renowned and easy-to-use TestVue software the
systems have a proven track-record with major Defence, Rail and
Industrial customers worldwide.
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