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How do Flying Probe Test Systems work?
Jan 26, 2018

       Flying Probe test systems were originally developed to test PCB assemblies which had previously been un-economical to test using traditional in-circuit test methods such as ICT testing due to either their low volume or design stability. The costs of ICT test fixtures and programmes were simply too prohibitive.

Flying probe or "fixtureless" test systems have subsequently proven to be extremely useful in the detection of solder bridges, open solders, presence and value verification of analogue components for prototypes, low to medium volume PCB assemblies and field returns.

       In some of the more capable machines, vectorless test capabilities enable the testing of semiconductors inQFPsSOICs, PLCC and BGA package type devices. Boundary scan and on-board programming can be performed and fixed pins (negating the need for the movement of test pins) connected to nets with high pin count can be extremely useful in reducing test time whilst increasing fault coverage.

       Additionally, many flying probe test machines come equipped with features such as on-board memory module programming providing an equivalent performance to an in-circuit test machine.

Their role in an engineers test strategy has evolved in recent times to accommodate the testing of miniaturised, more densely packed electronic circuits where traditional ICT methods are becoming impractical and expensive. Furthermore, continued improvements in test speed combined with the ability to test a greater range of componentry has resulted in flying probe testers becoming an essential component in most manufacturer's test strategies, irrespective of volumes and product mix.

However, test speed is still a big weakness of flying probe testers. It can take between 5-10 minutes to test a PCB assembly of average complexity which is often much longer than the takt time of the production cell.

     The Flying Probe test machines uses each assemblies design CAD data from which component reference designators and x,y co-ordinates are extracted to allow the system software to generate a test programme. This can take a matter of hours. Each test programme still needs to be "debugged" by a programmer, the debug time varying on board size and complexity. The best flying probe test systems have software that offer a variety of user friendly debugging tools, including graphical interfaces, PCB layout and intelligent schematics tools.

     Most flying probe systems are available with standard component package libraries enabling simple programmes to be generated without the need to 'teach' new component types to the machine.

Typically utilising 4 high speed independently moving probes, flying probe systems can access test points, vias, component pads and even fine pitch components. Other variables such as probing technique (via several probe tip styles), the speed/pressure of the probe, and minimisation of "fly height" (probe elevation in the z axis during transit) can be adjusted by the programmer to optimise the test programme and bring about further reductions in test time.

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