Flying probe testing is the best solution to some major electrical testing issues today. It replaces traditional test beds with probes, using multiple motor-driven electrical probes that can quickly move to contact the pins of devices and perform electrical measurements.
During the production process of PCB boards, it is inevitable to encounter electrical defects such as short circuits, open circuits, and leakage due to external factors. As PCB circuit boards continuously evolve towards higher density, finer pitch, and multilayer designs, failing to timely screen out defective boards will inevitably lead to greater cost waste. Therefore, in addition to improving process control, enhancing testing technology can also provide PCB manufacturers with solutions to reduce scrap rates and improve product yield.
The methods of electrical testing include: Dedicated, Universal Grid, Flying Probe, E-Beam, conductive cloth (glue), capacitive, and brush testing (ATG-SCANMAN). Among them, three commonly used devices are dedicated testing machines (automatic universal PCB testing machines), high-quality universal testing machines, and flying probe testing machines.
What are the differences between flying probe testing and test fixtures? What are their respective advantages?
Flying probe testing:Flying probe testing utilizes 4 to 8 probes to conduct high-voltage insulation and low resistance continuity testing on the PCB, testing for open and short circuits without the need for specialized test fixtures. The PCB is simply loaded onto the flying probe tester, and the testing program is run. The advantages of flying probe testing lie in its convenient testing methods and operation processes, saving testing costs, eliminating the time required to create test fixtures, and improving shipping efficiency, making it suitable for small batch production of PCBs.
In contrast, test fixtures are specialized testing jigs designed for mass-produced PCBs, which have higher production costs but better testing efficiency and no charge for reorders.
In terms of the applicability of testing technology, flying probe testing is currently suitable for small batch and sample electrical testing equipment. However, when applied to medium to large-scale production, the slow testing speed and high equipment costs will significantly increase testing costs. Universal and dedicated types, regardless of the level of PCB, can achieve economies of scale in testing costs as long as the production volume reaches a certain quantity, accounting for only 2-4% of the selling price. This is why universal and dedicated types are currently the main types of testing machines for mass production.
The working principle of flying probe testing
The flying probe tester is an improvement over traditional bed-of-nails online automatic high-voltage dedicated PCB testing machines, using probes to replace the test bed.
During operation, the unit under test (UUT) is conveyed into the tester via a belt or other UUT transfer system, and the probes of the tester contact the test pads and vias to test individual components on the UUT. The test probes are connected to drivers and sensors through a multiplexing system to test the components on the UUT. While one component is being tested, other components on the UUT are electrically shielded by the probe to prevent reading interference.
The flying probe tester can check for short circuits, open circuits, and component values. A camera is also used in flying probe testing to help locate missing components, inspecting the shapes of components with clear orientations, such as polarized capacitors.
With the probe positioning accuracy and repeatability reaching a range of 5-15 micrometers, the flying probe tester can precisely detect the UUT. Flying probe testing addresses many existing issues seen in PCB assembly: such as testing development cycles that may last 4-6 weeks; the inability to economically test small batch production; and the inability to quickly test prototype assemblies.
Flying probe testing is a method for checking the electrical functionality of PCB boards (open/short circuit testing). The flying probe tester is a system for testing PCB circuit boards in a manufacturing environment. Instead of using all the traditional bed-of-nails interfaces found in conventional online testing machines, flying probe testing uses four to eight independently controlled probes that move to the components under test. The unit under test (UUT) is conveyed into the tester via a belt or other UUT transfer system. Then, the fixed probes of the tester contact the test pads and vias to test individual components on the UUT. The test probes are connected to drivers (signal generators, power supplies, etc.) and sensors (digital multimeters, frequency counters, etc.) through a multiplexing system to test the components on the UUT. While one component is being tested, other components on the UUT are electrically shielded by the probe to prevent reading interference.
1. Charge/Discharge Time Method
The charge/discharge time (also known as net value) for each network is fixed. If there are networks with equal values, they may be short-circuited, and only the networks with equal values need to be measured for short circuits. The testing steps are: first board: full open circuit test → full short circuit test → network value learning; for subsequent boards: full open circuit test → network value test, and then use resistance testing for suspected short circuits. The advantages of this testing method are accurate results and high reliability; the disadvantages are that the first board takes a long testing time, with many retests, resulting in low testing efficiency. The most representative machine is MANIA’s SPEEDY machine.
2. Inductance Measurement Method
The principle of the inductance measurement method is to use one or several large networks (generally ground networks) as antennas and apply signals to them, while other networks will sense a certain inductance. The tester measures the inductance of each network, compares the inductance values of each network, and if the inductance values are the same, there may be a short circuit, which will then be tested for short circuits. This testing method is only applicable to boards with ground layers, and the reliability of testing double-sided boards (without ground networks) is not high; when there are multiple large-scale networks, since more than one probe is used to apply signals, the number of probes available for testing is reduced, leading to low testing efficiency. The advantages are high testing reliability and low retest frequency. The most representative machines are ATG’s A2 and A3 models, which are equipped with 8 and 16 probes to improve testing efficiency.
3. Capacitance Measurement Method
This method is similar to the charge/discharge time method. According to the relationship between conductive patterns and capacitance, if a reference plane is set, and the distance from the conductive pattern to it is L, and the area of the conductive pattern is A, then C=εA/L. If an open circuit occurs, the area of the conductive pattern decreases, and the corresponding capacitance decreases, indicating an open circuit; if two parts of the conductive pattern are connected together, the capacitance response increases, indicating a short circuit. In open circuit testing, the capacitance values at the endpoints of the same network should be equal; if they are not equal, an open circuit exists, and the capacitance values of each network are recorded as a comparison for short circuit testing. The advantages of this method are high testing efficiency, while the shortcomings are that it is completely reliant on capacitance, which is affected by many factors, leading to lower testing reliability compared to resistance methods, particularly due to measurement errors caused by associated capacitance and secondary capacitance, making it less reliable for networks with fewer endpoints (e.g., single-point networks). Currently, flying probe testing machines from HIOKI and NIDEC READ companies use this testing method.
4. Phase Difference Method
This method involves adding a sine wave signal to the ground layer or electrical layer and obtaining the phase lag angle from the line layer, thus acquiring capacitance or inductance values. The testing steps are to first test the open circuit for the first board, then measure the phase difference values of other networks, and finally test for short circuits; for subsequent boards, first test the open circuit, then measure the network phase difference values, and for suspected short circuits, verify using resistance testing. The advantages of this method are high testing efficiency and reliability; the disadvantage is that it is only suitable for testing boards with four or more layers, while testing double-sided boards can only use resistance testing. Currently, companies like MicroCraft use this testing method.
5. Adaptive Testing Method
The adaptive testing method allows each testing application process to select an appropriate testing process based on specific conditions and testing specifications after completing each test. For example, if the network value (charging time or capacitance, etc.) of a network is smaller than the device’s testing error, the device will automatically adopt resistance testing and electric field testing. This testing method is the fastest and yields the best testing results. However, as of now, no testing machines using this method have been encountered.
The flying probe tester is an improvement over traditional bed-of-nails online testers, allowing probes to replace the test bed, with 4 heads each equipped with 8 testing probes that can move at high speeds on an X-Y mechanism, with a minimum testing gap of 0.2mm. During operation, the unit under test (UUT) is conveyed into the tester via a belt or other UUT transfer system, and the probes of the tester contact the test pads and vias to test individual components on the UUT. The test probes are connected to drivers (signal generators, power supplies, etc.) and sensors (digital multimeters, frequency counters, etc.) through a multiplexing system to test the components on the UUT. While one component is being tested, other components on the UUT are electrically shielded by the probe to prevent reading interference.
The flying probe tester can check for short circuits, open circuits, and component values. A camera is also used in flying probe testing to help locate missing components, inspecting the shapes of components with clear orientations, such as polarized capacitors. With the probe positioning accuracy and repeatability reaching a range of 5-15 micrometers, the flying probe tester can precisely detect the UUT.
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