When manufacturing PCBs, it is necessary to check the electrical circuit of a completed PCB against the Netlist file that the design engineer of the project had defined earlier when creating the PCB layout. Using the Netlist file from the design stages ensures weeding out discrepancies occurring during production, and even those that may have crept in while laying out the PCB or while generating the Gerber file. The Flying Probe Test is one such method.
Manufacturers use the Flying Probe Test as an electrical test. The procedure uses electro-mechanical computer-controlled test probes moving sequentially across the bare PCB, testing at various predefined test points. Earlier, PCB manufacturers conducted this type of testing using the “bed of nails” fixture, popularly known as ICT or in circuit testing. However, this was an expensive and time-consuming method, as a specific fixture was necessary for each PCB, incorporating precisely placed testing and tooling pins.
Fig 1: Bed of Nails In-Circuit Tester (ICT)
Flying Probe Test
Although one can consider the flying probe test as an upgraded bed of nails fixture, there are several differences. As the name suggests, flying probes replace the testing pins, with typically four headers moving along the X- and Y-axis at high speeds.
Fig 2: Flying Probe Test Setup
A conveyor mechanism transports the PCB under test and stations it under the flying probe tester. The probes then descend and connect with test pads and vias on the PCB to find out any defects according to a predetermined test program.
The testing staff uses the Netlist or CAD data from the designer to generate an applicable file. They run this file through a test program specific to the testing machine to further generate corresponding test program formats for the PCB.
With the completion of the test program, the tester must decide on the testing item that they want to handle first, for instance, inadvertent shorts between two traces. The testing machine will pick up the data of reference points conforming to the PCB under test from the CAD data. One set of probes will connect to the origin of the two traces, while another set of probes will connect to their end points. Checking electrical connectivity between the four traces will reveal the continuity along the length of the two traces and the presence of any shorts between them. Flying probe tests help in quickly inspecting PCB fabrication issues.
Capabilities of Flying Probe Test
Although flying probe tests can easily detect shorts and opens, equipping them with special drivers makes them capable of testing more complicated parameters as well. Complex probes can probe and test both sides of a multi-layered board simultaneously, reducing the time taken to test single sides individually. Flying probes with different architectures are available for diverse solutions such as:
Signal Integrity Testing: Using TDR or time domain reflectometry probes along with special instruments, it is possible to test a wide variety of characteristics of PCB traces meant to carry high-speed and high-frequency signals. The setup typically captures and measures signals in the time and frequency domain for characterizing imperfections in the signal path.
Phase Difference Measurement: Using specially designed probes to send high-frequency signals between reference trace and the signal trace, it is possible to measure the phase difference between the two. With this test, it is not necessary to set up separate isolation tests for measuring cross-talk between traces on the PCB.
High Voltage Stress Test: PCBs can have isolation defects that may remain undetected by regular resistance tests. The insulation resistance between two traces on the PCB may be high enough to pass a regular resistance test, but it may still remain below the requirements in the specifications. A high voltage stress test is necessary to detect this, and requires the use of high-voltage generators, suitable flying probes, and high-resistance meters.
Micro-Short Detection: Presence of tiny slivers can cause micro-shorts on PCBs. Sometimes these may burn out during the high voltage stress test creating a high-resistance conductive path due to the carbonized residue the burn-out generates on the PCB surface. Micro-short detection probes apply a low voltage to check the resistance between two traces on the PCB, gradually increasing the voltage to that applicable for the test.
Kelvin DC Measurements: This is a very accurate DC measurement technique, often required for BGA and similar close-pitched PCB patterns. It requires a force and a sense pin in the flying probe. A Kelvin connection compensates for losses in the test probes.
Flying probe test systems come in different sizes—major variables being the number of headers the system is using. For instance, a tester can have as many as 16 headers, with eight for the top and eight for the bottom of the PCB. Of course, the cost of the system increases proportionately with the number of headers it is using.
Benefits of Flying Probe Test
Flying probe tests offer several benefits over the older bed of nails or ICT fixtures:
No fixtures required: unlike the bed of nails fixture, a flying probe test does not require a fixture to be set up. This saves on the cost and time typically necessary for setting up the ICT fixture. In fact, as the Gerber data is available to the manufacturer, they can set up the flying probes as soon as the PCBs emerge from the manufacturing line. On the other hand, designing and setting up an ICT fixture requires several weeks.
Program development is short and fast: As the Netlist and CAD data are the basis for generating the program for a flying probe test, and several open source programs are available to translate the information, program development time is short with the process taking very small amount of time. That also means it is easy to integrate design changes.
Process is flexible: Unlike the bed of nails fixture for ICT, which is specific to a single PCB and is not useful for another, the flying probe setup is suitable for any PCB. It requires only a simple modification of the internal program to make it suitable for another board.
Test points not necessary: As the flying probe is testing a bare board, the probes can use component pads and do not need extra test points.
Probe contact is controllable: Flying probes can make precision connections at a tighter pitch that the bed of nails can. For instance, high-accuracy flying probes can achieve test pitch of 0.1 mm, compared to the minimum pitch of 0.5 mm for the ICT. This makes them useful for densely populated boards, or allows for a greater coverage on a small PCB.
Variable test solutions and approaches: The flying probe systems can provide far more test solutions than the ICT or bed of nails can. This is possible as various types of flying test probes are available, aided by the programmable integrated test system.
High measurement precision: By using specific flying probes for different tests, intrinsic positioning of probes, and employing matching test instruments, the measurements can be highly precise.
Rapid feedback: As flying probe test results are available on the spot, sending the information to the production line can help them make suitable corrections to their processes quickly. Similarly, PCB designers can receive rapid feedback during prototyping period enabling them to make necessary changes before committing to production runs.
Conclusion
Although the flying probe test system has several advantages, the biggest disadvantage is its time per test. Setting up the system can be relatively simple, and the operator must load and unload the PCB. However, that means the operator must keep watch—10 seconds to load a PCB, 2 minutes of testing, another 10 seconds for unloading the PCB. Therefore, the decision whether to use a flying probe test depends on the volume of manufacturing.
