Flying probe

In the testing of printed circuit boards, a flying probe test or fixtureless in-circuit test (FICT) system may be used for testing low to mid volume production, prototypes, and boards that present accessibility problems. A traditional "bed of nails" tester for testing a PCB requires a custom fixture to hold the PCBA and the Pogo pins which make contact with the PCBA. In contrast, FICT uses two or more flying probes, which may be moved based on software instruction.[1] The flying probes are electro-mechanically controlled to access components on printed circuit assemblies (PCAs). The probes are moved around the board under test using an automatically operated two-axis system, and one or more test probes contact components of the board or test points on the printed circuit board.[2]

Flying probe testing is commonly used for test of analog components, analog signature analysis, and short/open circuits. They can be classified as in-circuit test (ICT) systems or as Manufacturing Defects Analyzers (MDAs). They provide an alternative to the bed-of-nails technique for contacting the components on printed circuit boards. The precision movement can probe points on integrated circuit packages without expensive fixturing or programming required.

The main advantage of flying probe testing is the substantial cost of a bed-of-nails fixture, costing on the order of US $20,000,[3] is not required. The flying probes also allow easy modification of the test fixture when the PCBA design changes. FICT may be used on both bare or assembled PCB's.[4] However, since the tester makes measurements serially, instead of making many measurements at once, the test cycle may become much longer than for a bed-of-nails fixture. A test cycle that may take 30 seconds on such a system, may take an hour with flying probes. Test coverage may not be as comprehensive as a bed of nails tester (assuming similar net access for each), because fewer points are tested at one time.[5] However, net access for traditional bed of nails testing is proving more challenging as board designs become more complex and compact. This often tilts the balance in favour of Flying Probe testing since these can use targets as small as 80um or 3.2mils for net access.

Increasingly Flying Probe systems are being enhanced with multiple test techniques to achieve a very comprehensive "one stop" test strategy for circuit boards. Options such as laser test (used initially for board planarity correction, but now used for such tests as BGA planarity, no-fit component verification and component alignment testing) and automated optical inspection are now common. Flying probe systems can also be combined with bed of nails access on key nets (such as power supply nets) to add powered tests such as Boundary Scan, device programming and even full functional test capability.

Uses

Benefits of fixtureless in-circuit test
  • Automatic optical inspection for presence of components, correct polarity, and letters or numbers on ICs.
  • Value measurements on resistors, capacitors, Zener diodes and inductors.
  • IC open circuit checker finds lifted legs and dry joints on ICs.
  • Can test fine pitch printed circuit boards down to 0.3 mm with a repeatable accuracy of probe placement of ±0.05mm.
  • Test program is rapidly prepared from printed circuit board CAD data.
  • All major CAD platforms support FICT.

References
  1. Stephen F. Scheiber (2001). Building a Successful Board-test Strategy. Newnes. pp. 303–. ISBN 978-0-7506-7280-1.
  2. R. S. Khandpur, Printed Circuit Boards:Design, Fabrication, Assembly and Testing, Tata-McGraw Hill, ISBN 0070588147, 2005, page 572
  3. "ICT Performs Comprehensive Testing". NexLogic. NexLogic Technologies Inc. Retrieved 30 September 2019.
  4. Keith Brindley (22 October 2013). Automatic Test Equipment. Elsevier. pp. 12–. ISBN 978-1-4831-0115-6.
  5. Alec Cohen, Prototype to Product: A Practical Guide for Getting to Market, O'Reilly, 2015, ISBN 1449362281, pp. 83, 84
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