In Circuit Test (ICT)
a form of test and associated test equipment that is able to
quickly detect and locate electronics circuit board faults
In-Circuit Test (ICT) is a powerful tool in the testing of manufactured
boards. Using a bed of nails in-circuit test equipment is able gain access to
the circuit nodes on a board and measure the performance of the components
regardless of the other components connected to them. Parameters such as
resistance, capacitance and so forth are all measured along with the operation
of analogue components such as operational amplifiers. The functionality of
digital circuits can also be measured, although their complexity usually makes a
full check uneconomic.
Basic Concept
In circuit test equipment operates by measuring each component in turn to check
that it is in place and of the correct value. As most faults on a board arise
out of the manufacturing process and usually consist of short circuits, open
circuits or wrong components, this form of testing catches most of the problems
on a board. Even when ICs fail, one of the major reasons is static damage, and
this normally manifests itself in the areas of the IC close to the connections
to the outside world, and these failures can be detected relatively easily using
in-circuit test techniques. Naturally an in-circuit test does not give a test of
the functionality of a board, but if it has been designed correctly, and then
assembled correctly, it should work.
In circuit test equipment consists of two main parts. The first is the tester
itself. This consists of a matrix of drivers and sensors that are used to set up
and perform the measurements. There may be 1000 or more of these driver sensor
points. These are normally taken to a large connector conveniently located on
the system.
This connector interfaces with the second part of the tester the fixture.
In view of the variety of boards this will be designed specifically for a
particular board, and acts as an interface between the board and the in circuit
tester. It takes the connections for the driver sensor points and routes them
directly to the relevant points on the board using a "bed of nails".
Driver-Sensors
Driver-sensors are the active circuits that are used for making the
measurements. Normally drivers and sensors are always present in pairs. As the
name suggests the drivers supply a voltage or current to enable a node in the
circuit to be driven to a particular state. They normally have a reasonably high
capability to enable the node to be driven to the required state despite the
condition of the surrounding circuitry. Typically they may need to force the
output of a digital IC to a given state despite the natural output state of the
device. To achieve this the output impedance of the driver must be very low.
Sensors are used to make the measurements. Like most other measuring devices
these need to have a high impedance so that they do not disturb the circuit
being measured.
Guarding
The key to the success of in-circuit testing is a technique known as guarding.
It is very easy to measure the value of a component when it is not in a circuit.
For example a resistor value can be measured by simply placing an ohmmeter
across it. However when the component is in a circuit, the situation is somewhat
different. Here it is most likely that there are other paths around the
component that will alter the value that is measured.
To overcome this problem and gain a far more accurate indication of the value
of the component a technique known as guarding is used. Here the nodes around
the component under test are earthed and in this way any leakage paths are
removed and more accurate measurements made.
Fixtures and Connections
In order to carry out the test it is necessary to gain access to each node on
the board. The most common way of achieving this is to generate a "bed of nails"
fixture. The board is held in place accurately by the fixture and pulled onto
spring loaded pins that make contact with connections on the board. The board
may either be pulled down under the action of a vacuum or it may be achieved
mechanically.
At one time when board component densities were much lower it was often
possible to place special ATE pads onto the board to enable good connection to
be made. Nowadays with very much more compact boards this is not possible.
Instead connections are made onto the component pads. This is obviously more
difficult because of the solder and the component connection itself, but can
still be achieved to a high degree of reliability. Typically each spring exerts
a force of between 100 and 200g to ensure that good contact is made. This
obviously means that the total force required for all the pins on a board can be
very significant. Sometimes supports for the board are required to ensure that
it does not flex too much as this may result in cracking some delicate surface
mount components.
Typically pins are placed on a 0.1 inch matrix. Many new surface mount IC
packages require a much finer pitch, and to achieve this an adapter is often
used.
There is a great variety of different types of pin that can be used. The
major design changes are within the head or tip that contacts the board under
test. Each type of head has a particular application for which it is best
suited. Concave tips may be used to connect onto terminal posts, flat tips or
those with a spherical radius may be used to connect onto card edge fingers,
whilst those with a sharp point may be used to connect onto component pads.
These sharp tips will penetrate any oxide layer, giving a high level of
reliability on soldered areas.
The wiring in the fixtures is generally not neatly loomed together. Whilst
this may not be as aesthetically pleasing, it reduces the levels of crosstalk
and spurious capacitance. It also reduces the wire lengths within the fixture as
the shortest route between two points can be taken within reason.
Programme generation
One of the advantages of the in-circuit tester is that programme generation can
be made much simpler than that of a functional tester. It is possible for much
of the programme to be generated automatically from a knowledge of the circuit.
This can be provided very easily from the printed circuit files. The information
about the nodes along with the circuit value information can be combined to give
a programme that can then be altered manually to provide
Multiplexing
Today's printed circuit boards can be very complicated. On larger boards the
node count can easily rise over a thousand and may reach several thousand on
some. To have dedicated pins on the tester for each node can be very costly as
each one requires its own driver sensor. To reduce this manufacturers introduce
a system known as multiplexing. Here a particular node may be placed through a
switching matrix so that it can address more than one node. The number of nodes
that are addressed by each tester primary node is known as the multiplex ratio.
Whilst it may appear to be an excellent idea to reduce costs, it reduces the
flexibility of the tester. Only one of the multiplexed nodes can be accessed at
any time. This can cause restrictions in the programming and also in the fixture
itself. Considerable thought has to be given to the fixture construction to
ensure that two pins on the same multiplex are not required at the same time. It
may also cause problems if the pins are allocated automatically by software that
generates the test programme and fixture wiring diagram.
When buying a machine it is worth checking whether multiplexing is used and
what the ratio is. With this information a judgement can be made of the cost
saving against the reduction in flexibility.
Fault coverage
With access to all the nodes on the board, manufacturers generally quote that it
is possible to find around 98% of faults using in circuit test. This is very
much an ideal figure because there are always practical reasons why this may not
be achieved. One of the major reasons that it is not always possible to gain
complete coverage of the board. Low value capacitors are a particular problem as
the spurious capacitance of the test system itself means that low values of
capacitance cannot be measured accurately if at all. A similar problem exists
for inductors but at least it is possible if a component is in place by the fact
that it exhibits a low resistance.
Further problems are caused when it is not possible to gain access to all the
nodes on the board. This may result from the fact that the tester has
insufficient capacity, or it may result from the fact that a point to which the
tester needs access is shielded by a large component, or anyone of a number of
reasons. When this occurs it is often possible to gain a level of confidence
that the circuit has been correctly assembled by what may be termed "implied
testing" where a larger section of circuit containing several components is
tested as an entity. However the confidence will be less and location of faults
may be more difficult.
Pros and Cons of ICT
The advantage of an ICT is that most board faults arise from problems in
manufacture. These might arise from the incorrect component inserted, a wrong
value resistor, a diode in the wrong way. These are very easily and quickly
located using ICT.
The testers are also very easy to programme and no long diagnostic routines
are required to locate any problems. Whilst the fixtures can be reasonably
expensive the production of these as well can be automated to a large degree.
However against this any changes to the board layout as a result of up-issuing
the board can result in changes to the fixture that may be difficult to
implement.
Another advantage of ICT is that the test results are easily interpreted.
This enables them to be used by a variety of people. As a result their running
costs are less than some other systems that might need highly skilled diagnostic
technicians and as a result this makes them attractive for use on the shop floor
to locate most of the problems.
There are some other limitations. The first is that they obviously cannot
provide a full functional check of the specification of the board. As the board
is not being exercised in its operational mode, its operational parameters
cannot be checked.
Another problem that is becoming more difficult to overcome is that access to
the nodes is becoming more difficult. Many years ago it was possible to place
special pads onto the boards to enable the fixture pins to connect to the board
easily. Now boards are so compact that there is no possibility of being able to
apply special pads to each node. Also the size of component connections is
becoming much smaller and this means that probing these points is far more
difficult. However it is still possible to achieve a good coverage on many
boards.
One problem that concerned people, especially some years ago was that of back
driving. When performing a test some nodes have to be held at a certain level.
This meant forcing the output of possibly a digital integrated circuit to an
alternative state purely by applying a voltage to over-ride the output level.
This naturally put a strain on the output circuitry of the chip. It is generally
assumed that this can be done for a very short period of time sufficient to
undertake the test without any long-term damage to the chip. However with the
geometries in ICs shrinking, this is likely to become more problematical.
Roving probe
To reduce the fixture costs, provide additional flexibility and enable board
changes to be accommodated by updates to a software programme, a type of in
circuit tester known as a roving probe or roving prober may be used. Instead of
having a bed of nails fixture a simple fixture to hold the board is used and
probes that move under software control are used to probe the relevant points on
the board. These systems normally have a number of probes, some that can access
both sides of the board.
These systems are slower than the systems that use a bed of nails fixture
because there is a delay between measurements as the probe moves to the next
position and this will naturally reduce the throughput. However the system is
cheaper for the maintenance and introduction of new boards because of the
reduced fixturing costs and reduced cost of changes.
Summary
In circuit test has many advantages and is an ideal system in many respects.
However as a result of the rapidly shrinking component sizes and the resultant
difficulties in gaining access to all the nodes on boards testing using ICT has
been steadily becoming more difficult. Accordingly many people have been
predicting the imminent demise of ICT. It remains to be seen how long this will
take.
|
