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In modern electronic equipment, circuit boards play a key role, undertaking the task of carrying a variety of electronic components and circuits to ensure the normal operation of the equipment. In order to ensure the quality and functionality of circuit boards, a number of testing and verification tasks are required. Among these testing methods, flying probe testing and test rack testing are two common means. Although they are both designed to check the connection status and performance of circuit boards, there are some differences between the two methods.
Flying probe testing is a method of inspecting test points on a circuit board by using metal pins to contact them. Generally, the test points on the circuit board are realised through metal contacts, and flying probe testing makes use of these contacts to carry out measurements and inspections. In flying probe test, a set of needles will be mechanically moved to contact the test point with a certain speed and strength, through the measurement of resistance, voltage and other parameters to determine whether the connection and performance of the circuit board is normal. Flying probe test is suitable for single-sided or double-sided circuit boards with fewer test points, and is less suitable for densely arranged test points.
While flying probe testing makes it easy to detect shorts and opens on boards, it can also detect more complex parameters when equipped with special drivers. State-of-the-art probes are able to test both sides of a multilayer board at the same time, thus reducing the time required to test a single side separately. Flying probes in different architectures are suitable for a wide range of solutions, e.g:
Signal Integrity Testing: Using Time Domain Reflectometry (TDR) probes and specialised instrumentation, it is possible to examine a wide range of characteristics of PCB traces carrying high-speed and high-frequency signals, often capturing and analysing both time- and frequency-domain signals to identify defects in the signal path.
Phase Difference Measurement: A specially designed probe emits a high-frequency signal to measure the phase difference between a reference line and a signal line, a method that assesses crosstalk between PCB traces without the need for additional isolation testing.
High Voltage Stress Test: In view of the fact that conventional resistance tests may not be able to detect insulation defects on the PCB, e.g. the insulation resistance between two lines is high but still lower than the standard requirements, it is necessary to carry out a high voltage stress test with the aid of a high voltage generator, a suitable flying probe and a high resistance meter.
Micro-short circuit detection: For the possible existence of small short-circuit areas, the probe by applying a low voltage to measure the resistance between the two lines, and gradually increase the voltage to the test requirements, to help identify high-resistance conductive paths due to high-voltage testing caused by the carbonisation of the residual formation of high-resistance conductive paths.
Kelvin DC Measurement: This is a highly accurate DC measurement technique, mainly used for close-pitch PCB patterns such as BGAs, that relies on a combination of flying probe force contacts and sense pins to compensate for probe-induced losses through the Kelvin connection.
Flying probe testing systems vary in size, depending on the number of probes in the system. Some test equipment has up to 16 probe interfaces covering both the top and bottom sides of the PCB, and the system cost grows proportionally with the number of probes.
Flying probe testing offers several advantages over older bed of nails or ICT fixtures:
No fixturing required: Unlike bed of nails fixtures, flying probe testing does not require fixture setup. This saves the cost and time normally required to set up ICT fixtures. In fact, since manufacturers have access to Gerber data, they can set up flying probes as soon as the PCB comes off the production line. On the other hand, designing and setting up ICT fixtures takes weeks.
Short and fast programme development: Since the netlist and CAD data are the basis for generating the flying probe test programme, and since there are several open source programmes that translate this information, programme development time is short and the process takes up very little time. This also means that design changes can be easily integrated.
Process Flexibility: Unlike ICT's bed-of-nails fixtures, which are set up for any PCB, CT's bed-of-nails fixtures are specific to a single PCB and have no use for another. Simple modifications to the internal programme are required to adapt it to another board.
No Test Points Required: Since flying probes test bare boards, the probes can use component pads and no additional test points are required.
Probe contact is controlled: Flying probes can make precision connections with tighter spacing than a bed of nails. For example, high-precision flying probes can achieve test pitches of 5 µm, whereas CTs have a minimum pitch of 0.5 mm. This makes them useful for densely populated boards or to achieve greater coverage on small PCBs.
Variable test solutions and methods: Flying probe systems can offer more test solutions than ICT or bed of nails. This is possible because various types of flying test probes can be used with the help of Variable Test Solutions and Methods: Programmable Integrated Test Systems.
High measurement accuracy: The use of specific flying probes for different tests, the positioning of the probes themselves and the use of matching test instruments result in high measurement accuracy.
Fast feedback: Since flying probe test results are available on-site, sending the information to the production line helps them to quickly make appropriate corrections to the process. Similarly, PCB designers can receive rapid feedback during prototyping, allowing them to make necessary changes before going into production.
Flying probe testing also has some drawbacks. Firstly, because the contact between the needle and the board is mechanical, it is easy to damage or wear out the test points, especially for precision boards, which may have some impact on the quality of the board. In addition, flying probe testing can only detect the connection of the test points, and is relatively limited in terms of testing the electrical performance and signal integrity between test points. For more comprehensive and accurate testing needs, flying probe testing may not be the best choice.
Test fixture, also known as ICT (In-Circuit Test), is a PCB inspection method based on special test fixtures. The test frame is pre-arranged with the PCB board corresponding to a variety of test needle bed, through a one-time contact with all the test points, to achieve a rapid and comprehensive testing of the entire board.
Working Principle: Make a special test fixture, on which the test pins correspond precisely to the various test points on the PCB board. When the PCB board is inserted into the test frame, the test system applies electrical signals and collects responses through these pin beds at the same time, and quickly completes the verification of the connectivity of the circuit network, component values and functions.
Advantages:
Fast test speed: one contact can complete the detection of all test points, especially suitable for high-volume, standardised production of PCB boards, can significantly improve production efficiency.
High test coverage: Component functions can be tested in depth, such as voltage and current characteristics of analogue ICs, logic functions of digital ICs, etc., and the testing accuracy and coverage are better than flying probe testing.
Good stability: the special test frame is designed with precision, high consistency of repeated testing, and is not easily affected by factors such as probe wear and tear.
Disadvantages:
Large up-front investment: the production of dedicated test stands cost more, and the design cycle is longer, for frequent product changes or small batch production is not economical.
Poor flexibility: test racks are usually designed for specific PCBs, not applicable to large changes in the structure of the product, time-consuming and labour-intensive to adjust or replace the test fixture.
Flying probe testing with its low cost, high adaptability and fast response characteristics, suitable for diversification, small batch and R & D stage of PCB testing; while the test fixture test relies on special fixtures to achieve high-speed, high-coverage comprehensive testing, more suitable for standardisation and mass production. For different production needs and testing requirements, a reasonable choice and combination of these two test methods will provide solid technical support for the manufacturing and quality assurance of circuit boards.
