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:
- 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..
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Fill plated through holes: If there is a 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.
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.