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By: Louis
Y. Ungar, Editor, The BestTest Newsletter
The Flying Probe Tester
Intuitively, a robotic arm accessing circuit nodes and feeding the information to a central computer seems like a good choice for automatic testing. But when we talk about a “Flying Probe Tester” we have to remember that we are referring to the manner in which we access the circuit nodes, rather than provide information about what we test and how we make measurements. Theoretically, flying probe access, like bed-of-nails fixtures, can be used by any ATE type – Connectivity, Manufacturing Defects Analyzer (MDA), In-Circuit (ICT), or Functional Board Test (FBT). Practically, however, the latter two provide some complications because accurate measurements have to be made and an appropriate amount of time is required to make them. Some of the measurements, as well as stimuli may also require synchronization to come along – all of which would further burden the probe time. For these reasons, when one refers to a Flying Probe Tester, we can usually assume it to be a Connectivity or MDA tester.
Historical Perspective
The flying probe tester found its niche during the 1990s. While it was still slow compared to grid testers, it did not require a test fixture. Also, flying probe could probe small targets on close centers.
Flying probe access can be made to contact pads as small as 6 mills with accuracies down to 75 micrometers. Test fixtures typically do not access pads smaller than 25 mills.
The major advantage of going fixtureless was the cost of fixtures, but there were also other advantages. Prototype testing could be performed without waiting for a fixture to be constructed. Changes in the circuit (and therefore in the fixture) required much less time. Also, the flying probe tester enabled direct inputs from computer aided design (CAD) data, allowing changes to the test program to go into effect much faster.
Initially, flying probe was slow – about 10 minutes to test both sides of a fairly large and complex inner layer using 16-head flying probes. As such, they were primarily used to confirm automatic optical inspection (AOI) detection. Initial results confirmed that opens and shorts were indeed escaping AOI, but the flying probe introduced some false errors. Upon closer examination, it was determined that AOI results were subjective and was greatly influenced by the tolerances set by operators. When probing speeds were improved to 30-60 sec typically, flying probe became more widely used.
(History of Inner-Layer Testing, by Brian Marcinak and Gary Stoffer, Surface Mount Technology, June
2006).
Flying Probe Capabilities
In their early development, flying probe testers provided only simple connectivity or MDA capabilities. Thus only shorts, opens, and discrete components could be tested. Now, they can provide power on capabilities used by in-circuit testers (ICTs). As ICTs these days tend to be equipped with boundary scan, flying probe test can be combined with boundary scan to provide a greater set of test information for fault detection and diagnoses.
Though not common, flying probe and functional test is also possible. When available on a functional tester, the user can have the best of all worlds. When functional test detects a failure on a circuit board output, the flying probe can be invoked to provide diagnostic capabilities. With boundary scan also available, the tester can be provided with additional circuit state information, thus assisting fault isolation and diagnoses.
Flying Probe Limitations
Flying probe is not a complete answer. It’s slow test time is the major concern.
Some improvements are being made to speed up probing. To optimize Z-plane motion as well as X-Y movement, flying probe software calculates the highest component level along the path of motion and raises the probe just above this level rather than to a generic PCB area fly height.
Another concern is the possibility of damage caused by the probing. Several techniques, such as Digitaltest’s “Soft Landing” technology, provides a level of protection. As a buyer of test systems you should make sure you get some assurances on this point. You should especially be careful of systemic damage that can affect a large portion of your production volume.
Even without causing permanent damage, probing can create false test results due to flexing and probing pressures masking opens. As a design for testability effort, circuit board layout should take into account flying probe physics and ensure the validity of the tests.
Conclusion
Flying probe methods are certainly catching on as a way to access hard-to-reach places. They have improved in speed and probing accuracy. They found their way to ICT applications and have made a bridge to boundary scan tools. A new bridge to FBT is also very likely. The limitations, however, still exist and as a result one should consider flying probe as a useful tool rather than a comprehensive test solution.
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