Flying probe in-circuit testing

Notes about the flying probe test, sometimes called the roving probe version of in-circuit testing.

Automatic Test Equipment, ATE Includes:
In-Circuit test, ICT ICT technology & techniques Flying probe Manufacturing defect analyzer, MDA ICT fixtures ICT design for test
See also: ATE basics PCB inspection techniques Functional test, FATE Developing test strategy

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.