In Circuit Testing - Testing and Testability Unit-VIII Topic

ICT, In Circuit Test Tutorial

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. These can easily be checked using simple measurements or resistance, capacitance, and sometimes inductance between two points on the circuit 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. Some in-circuit tester are able to test some of the functionality of some integrated circuits, and in this way give a high degree of confidence in the build and probability of operation of the board. 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 a number of elements:

• In circuit tester:   The in circuit test system 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
• Fixture:   The in-circuit test system 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".
• Software :   Software is written for each board type that can be tested. It instructs the test system what tests to perform, between what points and details of the pass / fail criteria.

These three elements for the major parts of any in-circuit test system. The tester will be used for a variety of boards, while the fixture and software will be board or assembly specific.

Basic concept of In-Circuit Test System

In circuit test system are normally relatively expensive items of equipment. They are typically sued on high volume production lines. The fixture and programme generation costs mean that they are not viable for small runs of less than 250 to 1000 items. A cost analysis should be undertaken to ensure that the cost of generating the fixture and programme is viable.

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.

ICT advantages and disadvantages

Like any other form of test technology, in-circuit test has several advantages and disadvantages. When determining the best form of test for any given application, it is necessary to investigate the advantages and disadvantages of each system carefully.

In circuit test advantages:

• Easily detect manufacturing defects:   It is that most board faults arise from problems in manufacture - incorrect component inserted, a wrong value component, diodes, transistors or ICs inserted with incorrect orientation, short circuits and open circuits. These are very easily and quickly located using ICT as the in circuit tester checks components, continuity, etc..
• Programme generation is easy:   An In-Circuit tester is very easy to programme - files can be taken from the PCB layout to generate much of the programme required.
• Test results easy to interpret:   As the system will flag a particular node as having a short of open, or a particular component as being faulty, location of a problem in a board is normally very easy - and do not require the application of the most highly skilled test staff.

In circuit test disadvantages:

• Fixtures expensive:   As the fixtures are mechanical and require general and wiring assembly for each printed circuit board, they can be a costly item.
• Fixtures difficult to update:   As the fixture is a fixed mechanical item, with the probes or "nails" mechanically fixed, any updates to the board changing the position of the contact points can be costly to change.
• Test access becoming more difficult:   With the size of boards becoming ever smaller, access to nodes becomes increasingly difficult. In an ideal system, special contact points should be provided, but because of the constraints caused by miniaturisation, these contacts are rarely available. Some nodes may not even have accessible contact points. This makes ICT difficult, and reduces the fault coverage obtainable.
• Back-driving:   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.

Types of ICT

Although the generic term In-Circuit-Tester is widely used within the electronics manufacturing industry, there are actually several different flavours of tester that are available. The type of tester required is dependent upon the manufacturing / test process used, the volume and the boards that are used.

The main types of ICT machines that are available include:

• Standard ICT machine:   Although this is the generic test for this form of testing, testers that are referred to in this way are generally the more capable machines that can offer not only basic resistance / continuity measurements, but also capacitance and some device functionality as well.   Read more about the In-Circuit Tester
• Flying probe tester:   In view of the issues of developing and manufacturing complete bed-of-nails access fixtures - they are costly and difficult to change if any component positions or tracks are moved - another approach is to use a roving or flying probe. This has a simple fixture to hold the board and contact is made via a few probes that can move around the board and make contact as required. These are moved under software control so any board updates can be accommodated with changes to the software programme.   Read more about the Flying Probe Tester
• Manufacturing defect analyser, MDA:   This form of tester offers a basic In-circuit test of resistance, continuity and insulation. As the name implies, it is just used for the detection of manufacturing defects like short circuits across tracks and open circuit connections.   Read more about the Manufacturing Defect Analyser
• Cableform tester:   This form of tester is used to test cables. It uses the same basic functions as an MDA, although some form of high voltages may need to be applied occasionally

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.

In Circuit Testers & ICT Test Systems

There are many different in-circuit test systems available on the market.

Each ICT tester type has its own specification and the facilities that they offer are slightly different. Some are more sophisticated and provide additional capability, while others are less capable, but still offer a useful capability.

ICT test system basics

There are many elements to an in circuit tester. They utilise a number of elements to enable them to run the test programme and access the required points on the circuit.

The ICT tester can be split into a number of elements:

• Controller:   The controller is essentially the part of the In-Circuit tester that
• Switch:   The switch or switch matrix enables the measurement system elements of the system to be routed to the right area of the board under test.
• Interface:   The In circuit tester requires an interface to allow a variety of board fixtures to be placed onto the tester. This incorporates the electrical connection, often using zero insertion force, ZIF connectors. But it may also include power if the board is to be powered and possibly an air supply or vacuum if the board is to be pulled down onto the bed of nails using this type of process.
• Fixture:   The fixture or bed of nails is the interface of the tester to the board under test. The fixture has a bed of nails arranged for the particular board to enable connection to the required nodes on the circuit.

In addition to this in-circuit testers utilise a number of techniques to enable them to be able to access components and measure their values in the presence of other paths within the circuit.

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.

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.

Flying probe in-circuit testing

The flying probe tester is a form of automated test equipment that has been in use since around 1986 when the first testers were introduced. Flying probe testers provide many advantages over other forms of automated test equipment for particular applications. As a result, flying probe testers are now in widespread use in a variety of areas of the electronics manufacturing industry.

Initially, flying probe testers were introduced to cover the prototype and very small quantity production areas. Now the use of this type of automated test equipment has expanded, and although not used as the main test in high volume production, they are nevertheless used in many areas.

Flying probe test basics

The concept of a flying probe tester is that rather than having a comprehensive fixture for a given PCB assembly that can access all the required nodes via a "bed of nails", the system uses a generic board holder, and one or more probes moves across the board accessing individual nodes under software control.

The flying probe tester is therefore able to cut down on the number of test fixtures required and it is also much easier to introduce changes, especially to features such as component or pad positions because it is just a matter of changing the software.

The flying probe tester can be considered as a form of in-circuit tester, ICT. Early flying probe testers were only able to offer relatively basic capabilities and were more akin to Manufacturing Defect Analysers, MDA capable of testing for shorts and opens as well as basic tests on components such as diodes and transistor junctions. Advancements made in the technology mean that flying probe testers now include facilities such as on-board memory module programming and boundary scan testing. With these capabilities, they are able to offer the equivalent performance of an advanced in-circuit tester.

One advantage of the flying probe tester is that as the flying probe assembly itself is a precision mechanical item the probes can be placed very accurately. This enables them to be placed on small pads or component solder connections with high levels of accuracy. Some manufacturers state that their systems can probe pins on IC types including PLCCs, SOICs, PGAs, SSOPs, QFPs, etc as the probe placement accuracy is sufficiently high.

Advantages and disadvantages of flying probe test

Like any system, the flying probe tester has its advantages and disadvantages. This means that it is ideally suited to use in some applications, but not in others. Essentially as it is a form of in-circuit test, it is normally compared to other full in-circuit testers.

    Advantages of a flying probe test system:

• No special fixture required:   In view of the fact that the probes move according under software control to make contact with the required nodes, the "bed of nails" fixture required for ICT is not required. SA simple generic mechanism to hold the board in place is needed.
• Changes can be made easily:   As the probes move under software control, any changes to pad positions or components can be made by purely changing the software. It is not necessary to make any mechanical changes to a fixture as in the case of a "bed of nails" fixture.
• Test development time reduced:   Software for a flying probe tester can be developed relatively quickly from the PCB design files. The big saving is that no mechanical fixture is required and its manufacturing time is not needed. Accordingly development of the test programme for the flying probe tester simply requires the PCB files, and ultimately a first off board or boards on which to test the programme.

The advantages of the flying probe test system mean that it is ideally suited to many applications. However the disadvantages also need to be considered as well.

    Disadvantages of a flying probe test system:

• Speed of operation is slow:   When compared to other forms of automated test equipment such as an ICT, the flying probe tester is much slower because the probes have to physical move to each position in turn. For an ICT system all the connections are in place in the fixture
• It may not always possible to make complicated tests:   Using early flying probe testers it was not possible to test components beyond passive components or diodes. To achieve higher levels of fault detection technologies such as boundary scan and the use of on-board memory enable more complicated tests to be undertaken. It is necessary to check the performance of the individual flying probe tester to ensure it can meet the requirements.

Balancing the advantages and disadvantages of the flying probe test system, it is ideally suited to prototype applications and also areas where small volume production is undertaken. In view of the test times taken, it is not suitable for volume production applications in view of the test times unless it is used only for sample testing.

Flying probe testers are now widely used throughout the electronics manufacturing industry. They provide a much cheaper and more flexible form of in-circuit test. While these flying probe testers have their limitations, their advantages outweigh these in small volume and prototype applications where their flexibility, low development costs and short development times mean they are ideally suited for these areas.

MDA, Manufacturing Defect Analyzer

The Manufacturing Defect Analyser, MDA is a basic form of In-Circuit Tester.

As the name implies, the MDA is aimed at only providing a straightforward test of the board to reveal manufacturing defects.

As the majority of manufacturing defects are simple connectivity issues, the MDA is restricted to making measurements of continuity. This significantly reduces its cost making it more viable in many areas of test.

MDA basics

The concept for the MDA is based around the concept that the design of a board has been previously proven, and parts are reliable and very few defective components will be delivered. Therefore it should only be manufacturing defects that will impact the performance of a board or assembly. As most of the defects consist if solder splashes and poor or open joints, then the majority of failures will be detected by testing for a relatively simple spectrum of failure types.

While Manufacturing Defect Analysers are primarily focussed on the detection of the very basic faults, even the most basic testers these days will also detect missing components, although the exact functionality for any given MDA will only be revealed in the datasheet / specification. Often the tester will be able to detect the presence of resistors, capacitors and transistors. The detection of integrated circuits can also be achieved using the protection diodes to indicate whether the component is correctly placed.

The tester makes connection to the board under test using a bed of nails fixture, and this means that a different fixture is generally required for each board. It needs to make contact with specific points on the board, where often there may be a test point or land area for the probe.

Like other forms of In-Circuit Tester, an MDA will use the printed circuit board CAD data for the generation of the fixture design and the test programme. This often allows up to around 80% of the test programme to be generated automatically.

MDA Advantages / Disadvantages

Like any other technology the manufacturing defect analyser has its advantages and disadvantages. These need to be considered when choosing which type of tester and test technology should be used.

SUMMARY OF ADVANTAGES AND DISADVANTAGES OF A MANUFACTURING DEFECT ANALYZER, MDA
ADVANTAGES	DISADVANTAGES
Machine is much less costly than a full ICT Can detect open and short circuits which form the major number of defects Can detect some values dependent upon the MDA capability	Limited component diagnostics Still requires bed of nails fixture and its associated costs Component access can be an issue with current board density levels

An MDA machine is very much simpler than a full ICT. This makes it an attractive proposition for many situations, particularly within smaller companies where the capital expenditure investment of a full ICT machine may not be viable.

In many other situations a Manufacturing Defect Analyser may be a viable option is where the fault spectrum does not warrant a full In-Circuit test. This decision can only be made in the light of an analysis of existing fault spectra for a given manufacturing line.

ICT, In Circuit Test Fixtures / Bed-of-Nails & Probes

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 term bed of nails is a rather graphic description of what many fixtures look like, having a large number of test points or probes proud of a board that holds them in place.

Although the concept of the in-circuit test fixture or bed of nails is broadly the same whatever manufacturer is used, there are a number of variations on the basic theme.

In circuit test fixture basics

The In-circuit test fixture is required to interface the main tester with the particular board under test. It will have a main connector that interfaces to the tester and wires that are taken from the connector to individual pins / probes / or "nails" that make contact with the required nodes on the board under test.

The probes are held in place by what may be termed a base-board. This is precision drilled to ensure that the probes are held in exactly the right place for the fixture to make contact with the required nodes on the board.

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 are about three main methods for pulling the board onto the probes:

• Vacuum:   This form of fixture uses a vacuum to pull the board down onto the pins. It has the advantage that as the vacuum exists over the whole area of the board, the board is evenly pulled down onto the pins, but it does require any holes in the board to be sealed before the in-circuit test stage of the production process.
• Pneumatic:   This form of fixture uses a compressed air source, present in most manufacturing areas to be used.
• Mechanical:   This uses a simple lever or other mechanical arrangement to pull the board down onto the pins.

Wireless In-circuit test fixtures

Another form of ICT fixture is known as a wireless fixture. This does not mean that it uses wireless / radio communications, but instead the fixture does not use traditional wires but it uses a printed circuit board. This provides a number of advantages:

• Reduces complexity of fixture:   Most ICT fixtures require many wires to run between the individual probes and the fixture connector that interfaces to the main ICT system connector. There can often be several hundred wires, making the fixture very complicated and difficult to work upon.
• Reduces spurious resistance   The wires within an ICT fixture need to be long enough to allow the fixture to be opened to enable proper access. The length of the wires can introduce a significant amount of resistance that can reduce the overall measurement accuracy of the system. Using a wireless ICT fixture enables the track lengths to be shortened and resistance level decreased.
• Improves reliability:   the large number of wires in an ICT fixture introduces a means of failure. Wires can easily break and become disconnected. The use of a wireless fixture, using PCB technology significantly improves the level of reliability.
• Reduces fixture cost:   Using modern software, it is possible to reduce the cost of the fixture production by using a PCB. Using automatic routing of the tracks in the PCB layout software means that PCB design is automated to a large degree. This means that the complex wiring is removed from the fixture production process.

Test pins / probes for in-circuit test fixtures

There is a great variety of different types of pin or test probe that can be used for in-circuit test fixtures.

The in -circuit test probes or test pins or probes are spring loaded and comprise a barrel with its internal spring, and the plunger. The test probes fit into a socket that enables them to be replaced when they become worn of damaged.

Basic In-Circuit Test Probe or Contact

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:   These in-circuit test probes are often used for connecting on to terminal posts.
• Spherical radius convex tips:   These may be used when mating with an edge connection on a printed circuit board.
• Cone tips:   This format for an in circuit test probe is often used for mating with a PCB via hole, or directly onto a PCB track..
• Single pointed tips:   These are often used for mating with solder joints as the tip is able to penetrate the oxide film on the solder to make good contact.
• Multi-pointed tips:   These may be used when a connection is required through a larger area of solder - the multiple serrations mean that several points of contact can be made through the oxide layer on the solder. They may also be used to mate with the connection to a conventional component, i.e. not surface mount. The probe serrations will connect to the solder and wire that protrudes through the board.

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 In Circuit Test Programme Generation & Programming

A software programme is required for each board to be tested by an in-circuit tester.

This programme defines the basics tests that are to be made, and the circuit conditions for the whole board.

One of the advantages of the in-circuit tester is that programme generation can be made much simpler than that of a functional tester.

In circuit test programme basics

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.

Design for In-Circuit Test, ICT

In circuit testing is still a valuable tool in today's electronics manufacturing environment. While many thought ICT would be phased out many years ago because the smaller components and more compact circuit boards have been proved wrong. However to be able to used ICT satisfactorily, it is necessary design for in-circuit test right from the earliest concept of the board. In this way sufficient access can be gained to provide a high test coverage for the printed circuit board or assembly.

By adopting design for in-circuit test guidelines, it is often possible to provide a sufficiently high level of access to test most of the components on the board.

Design guidelines for In Circuit Testing, ICT

In order to maximise the coverage and capability of an In Circuit Test, ICT system, it is necessary to ensure that the board is sufficiently testable for the ICT system to provide a useful test. Guidelines can be adopted to help ensure that the circuit can be tested satisfactorily.

The ideas mentioned below are some ideas that can be implemented to improve the ICT performance:

1. Provide accessible location holes:   In order that the connections can be made to the board, it is necessary to have accurate position or location holes that can be used to accurately locate the PCB onto the test fixture. In this way the PCB can make accurate location onto any probes or connections required. Requirements for the tooling holes may include:
• Three preferred but a minimum of two, on opposite diagonal corners
• Tooling or location holes should not be plated to ensure their accuracy
• Tooling or location holes should not be obscured and they should be free from components etc in the vicinity of the hole to enable any locating spigots on the test fixture to mate with the hole.
• Location accuracy of the tooling or location holes should typically be within 0.05 mm, i.e. 0.002 inches, although with techniques changing all the time, check the requirements for the actual tester.
2. Connect resets and other key lines via a resistor:   One key design for ICT parameter is to ensure that any key reset or other lines that might be taken to ground or the supply rail, are taken there via a resistor. In this way if the In Circuit Tester needs to control these points on the chip to undertake a performance check, then it is able to have control.
3. Provide a probe-able pad for each circuit node:   When using in-circuit testing, it is necessary to get access to each node in the circuit to enable sufficient test coverage to be achieved. Probe-able test pads are ideally dedicated test pads, but with circuits becoming much smaller this is not always possible. Often ICT manufacturers claim fixtures can probe solder joints. Check this out as this technique can lead to lower reliability testing.
4. Probe-able test pads should all be on one side of the board:   When possible test pads for the test probes should all be on the same side (underside) of the PCB. This means that single sided fixtures can be employed. These are cheaper, simpler and faster to use than double sided fixtures.
5. Test pad finish:   Any test pads should have a good conductive finish. This will include solder, but often the gold plating if used elsewhere in the board can be used, although it adds cost.
6. Test pad size should be sufficient for the fixtures:   The test pad size will need to balance available space on the PCB with the size required for the test probes. They should be sufficient to allow the probe to make contact, and should have space around them to take up any tolerances in the fixture, PCB, etc, so that the probes do not cause shorts.
7. Test pad density must be considered:   The density of the test pads should not be so great that it becomes impossible to manufacture the fixture. It is necessary to check this with the fixture manufacturer as figures vary according to the type of fixture that will be used.
8. Fill plated through holes:   If there is likelihood that vacuum fixtures will be used, it is necessary to fill the plated through holes as part of the manufacturing process. Methods for filling other holes will also be required.

Summary

In order to be able to perform a sufficiently useful in-circuit test, it is necessary to ensure that the tester has a sufficiently testable board. As access to the nodes in PCBs is more difficult these days, it is necessary to ensure that testing can be accommodated. This can only be achieved by applying design for in circuit test rules from the beginning of the design, and especially during the PCB layout stages. If this is achieved, then in-circuit testing will be possible.