ICT, In Circuit Test Tutorial

- information and tutorial about the basics of in-circuit test, ICT and in-circuit test equipment for use in printed circuit board test to quickly find electronics faults. It includes techniques such as roving probe or roving prober, guarding, back driving, etc.

This ATE, Automatic Test Equipment tutorial is split into several pages each of which address different aspects of ATE Automatic Test Equipment and their operation:

In-Circuit Test, ICT is a powerful tool for printed circuit board test. Using a bed of nails in-circuit test equipment it is possible 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. Some functionality of digital circuits can also be measured, although their complexity usually makes a full check uneconomic. In this way, using ICT, In-Circuit Test, it is possible to undertake a very comprehensive form of printed circuit board test, ensuring that the circuit has been manufactured correctly and has a very high chance of performing to its specification.

Basic concept of ICT, in-circuit test

In circuit test equipment provides a useful and efficient form of printed circuit board test 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 for ICT

Driver-sensors are the active circuits that are used for making the measurements. Normally drivers and sensors are always present in pairs in an in-circuit test system. 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.

ICT 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 cross-talk and spurious capacitance. It also reduces the wire lengths within the fixture as the shortest route between two points can be taken within reason.

ICT 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

One advantage of an ICT as a first line form of printed circuit board test 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.

An In-Circuit tester is also very easy to programme and no long diagnostic routines are required to locate any problems. While 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 as a form of printed circuit board test 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. During the ICT printed circuit board test, 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 provide a slower form of printed circuit board test 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 form of printed circuit board test 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 as a form of printed circuit board test. It remains to be seen how long this will take.

Further pages from this tutorial
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