Design for Automated Optical Inspection / Test

- key design guidelines for Functional Automatic Test, FATE as part of a design for test, or testability strategy.

Functional test can take a variety of forms. It may be undertaken on a specific dedicated tester, alternatively another way in which functional test can be undertaken is assemble test instruments can run them under computer control using technologies such as GPIB, VXI, PXI, PXIExpress or the new LXI technology. Whatever methodology is used it is necessary to apply design for test guidelines or rules when the circuit design is undertaken.

The design for functional test guidelines apply in particular to the tests undertaken on a PCB or other sub-assembly. Those undertaken on the completed unit should not require particular test access to the internal workings of the PCBs, although in many cases a special test adaptor may be required for running the test programme and controlling the unit under test.

Most of the design for test, DFT, guidelines given below apply to sub-assembly test, although some are applicable for the main unit test as well.


Design for functional test guidelines

In order to maximise the coverage and capability of a functional test syste, FATE, it is necessary to ensure that the board is sufficiently testable for the FATE 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 FATE system performance:

  1. Provide accessible location holes where PCB fixtures are used:   many FATE systems that test PCB assemblies will use fixtures that mate with the bourds under test. Where these fixtures are used it is often necessary to have location holes (these may also be used for AOI and ICT systems). To provide accurate location of the board to the fixture the following guidelines may be useful:

    • 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. Do not connect control pins directly to a rail or ground:   Use pull-up resistors or pull down resistors to connect key control lines such as et, reset, clear, etc. In this way the FATE system can override these states if it needs as part of its test. This is only likely to be applicable to functional test systems where complete control of the PCB is required.

  3. Provide facility to inhibit clock signals:   For test systems that may run much slower than the real time system, it may be necessary for the FATE system to take control of the clock. Where this may be applicable, provide a mechanism within the circuit to allow the FATE system to control the clock.

  4. Enable digital feedback loops to be turned off:   For digital FATE systems it may be necessary to break a digital feedback loop. For systems and PCBs where this may be a problem, a method of inhibiting part of the loop for fault finding, etc may be required.

  5. Consider test connector:   In some instances where additional lines are required for testing, it may be worth adding a test connector to provide the additional access. This may be a low cost connector, or even a series of test pads to which the FATE system can interface. It may also be possible to provide this access via the main connector, although size and cost constraints for the main connector may limit the required access.

  6. Consider buffered connector for RF connections:   If any RF measurements are to be made of signals that are not normally accessed, it is worth considering a buffer amplifier and coax connector to enable the RF to be sampled accurately without affecting the existing circuitry.

  7. Consider the use of test points:   For some test situations, test points may be required for diagnostic testing. These test points should be inserted at critical points within the circuit. If possible, these test points should have a small series resistor to prevent damage should the test point be shorted to ground, etc with a test probe.


Summary

There is a large variety of functional test systems, with a variety of design for test requirements, and this means that each design for test guideline should be reviewed against what is actually needed. By using the design for functional test guidelines, each point can be considered and those that are applicable can be selected and implemented.

By Ian Poole


Read more automatic test, ATE, tutorials . . . . .

ATE basics PCB inspection ICT, In-Circuit Boundary scan
Flying probe Functional, FATE Test Strategy  

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