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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.
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.
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.
- 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.
- 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.
- 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 to test for insulation. Its operation is optimised for the testing of cables.
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.