Testing and implementation of electronic modules based on flying probe tester

With the continuous enhancement of electronic module functions, the integration is getting higher and higher, the device size is small, and the density is high, which increases the difficulty of manual inspection and debugging, especially for multi-variety, variable batch production. Due to the lack of testing technology for highly integrated electronic modules, manual inspection methods have seriously restricted the rapid development and production of electronic modules. Therefore,
it is very urgent to replace manual inspection with automation and mechanization of electronic module assembly and welding post-welding inspection. Flying probe inspection is a flying probe inspection system that uses a stepper motor to control the flying probe to realize the flying probe movement test board. It is the latest solution to some major problems of electronic modules. This article will introduce the detection principle of flying probe test and how to use flying probe tester to test electronic modules.

1 Structure and principle of flying probe test system
Flying probe test is one of the methods for online inspection of electronic module assembly and welding quality. It is a system for testing electronic module solder joints in a manufacturing environment. It uses probes instead of traditional needle beds. Four probes that can move at high speed are installed on the XY mechanism. When working, the unit under test is placed in the tester manually, and the probes of the flying probe tester contact the test pads and vias to test the welding effect of the components of the unit under test. The test probe is connected to the driver (signal generator, power supply, etc.) and sensor (digital multimeter, frequency counter, etc.) through a multiplex transmission system to test the components on the unit under test. When a component is being tested, other components on the unit under test are electrically shielded by the probe to prevent reading interference. The flying probe structure is shown in Figure 1.

The flying probe tester can test the resistance, capacitance, inductance of electronic modules, common devices such as diodes, triodes, three-terminal regulators, etc. It can also test integrated circuits, short circuit and open circuit tests.
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2 Implementation of flying probe tester
In order to realize real-time monitoring of the welding quality of electronic modules after assembly and to master the welding quality of each electronic module, a flying probe tester is needed to test the electronic modules. The short circuit, open circuit and damaged components of the tested components can be eliminated through flying probe testing. The test and debugging process is usually shown in Figure 2.

In the process of achieving the goal of flying probe test, the process file of EDA design will generate a netlist file in ASCII code format, which is converted into the NOD file of the electronic module under test. The NOD file is sorted and modified to generate the source file. Modify the source file according to the parameter requirements of the electronic module, and then process the software expected to be compiled. Use the camera to set the coordinate conversion point, generate MRK and MZF files, and after the coordinate conversion point is set, check whether the setting of the test point is appropriate. The selection of test points is combined with the change of NOD files. The process of checking the test points is also the process of modifying and correcting the points under test. Finally, select the statements that need to be debugged. Each type of component corresponds to its own debugging statement. Modify the debugged statement, that is, set the isolation point, voltage and other parameters of the device under test, and run it after modification until the test requirements are met. Among them, the processing of NOD files is the most cumbersome. The following introduces how to quickly modify NOD files. The file format is as follows:

Among them, the last column is the channel number of the solder joint of the tested part, which needs to be modified manually. The modification method is:
(1) Modify CHANNEL from top to bottom, according to the name of SIGNAL. If the name is the same, change CHANNEL to natural number 1. If the name of SIGNAL is changed, change CHANNEL to natural number 2. Then search downward until there are other differences, and continue to accumulate.
(2) For parts that cannot be tested or affect the operation of the probe, the "TEST" attribute should be changed from "F" to "N" (not tested) in the *. NOD file. Such as vias that are not filled with tin after electrical installation, around programming pins, around higher components, chips with socket conversion soldering, etc. The
common processing method is to modify them one by one manually, and proofread them one by one after the modification is completed. The disadvantage is that it is inefficient and prone to errors. In order to change this situation, we must find ways to improve efficiency. Adjust the modification method:
(1) Use Notepad to open *. NOD file, copy all the contents into a WORD document;
(2) delete the first line ("* SIGNAL COMPONENTPIN XY SHEET POS TECN TEST CHANNEL");
(3) select all the data (Ctrl+A);
(4) click Table\Insert Table (to put the data into the table);
(5) select all the data in the table and copy it into an Excel spreadsheet;
(6) use the convenient operation of the Excel spreadsheet to number the tenth column, starting from 1, and increment the different signal names (refer to the first column) by 1, and save after completion;
(7) then modify the second to last column, the method is: first fill all with "Y", and then change to "F" (forced) or "N" (not tested) according to the point selection rules and error conditions, and save after completion;
(8) select all (Ctrl+A), copy (Ctrl+C);
(9) open the *. NOD file with Notepad, keep only the first line and delete the rest, paste (Ctrl+V) the contents of the clipboard from the second line, save, and the changes are complete.
The modified processing method takes advantage of the processing convenience of Office software and effectively improves the efficiency of NOD file processing.

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3 Problems and solutions of electronic modules in testing
After the flying probe program is debugged and frozen, the electronic module needs to be tested next. The flying probe test process is shown in Figure 3. During the test, some errors may occur, such as failure of short circuit test and device test parameter out of tolerance. The processing of short circuit error is more complicated, and it is necessary to solve the fault location problem of the tested module based on the principle of short circuit statement generation.

The short-circuit test generation process of the flying probe test requires the input of a value as a marginal test reference. This value indicates that a short-circuit test must be generated between all solder joints of different signals in a certain area. For example: the spacing between most solder joints on the board under test is 50 mm, and it is recommended to set the marginal parameter value to 51 mm to ensure that adjacent legs can be short-circuited. According to this principle, when the flying probe tester reports a short-circuit test error during the test, first, according to the test statement displayed on the test interface, confirm which two signals have a short circuit; secondly, appearance inspection: open the NOD file, find out the commonality between the two error signals, that is, whether the two signals are on the same component, whether they are adjacent pins of the same component, and after finding such a suspicious device, select FLYING PROBES\Autoleam, enter the channel number of the pin of the component that has an error, click GO, and the camera runs to the specified position. Use the screen to observe whether the solder joint corresponding to the center of the cross star mark and the adjacent pins are soldered to cause bridging or adjacent legs touching. If there is no bridge, select another component that meets this condition and perform the previous operation until the appearance inspection is completed; if there is a bridge, mark it with a label paper for repair. Finally, power on and check: If the appearance inspection cannot find the cause of the short circuit, it can be confirmed that the short circuit is caused by the component itself.
Take out the module, lift or suck the corresponding error pin of the suspicious component in the appearance inspection to temporarily disconnect it from the PCB pad, and use a multimeter to measure the two signals of the short circuit on the module. If there is no short circuit, locate the faulty component. If there is a short circuit, replace another suspicious component and repeat this step until the short-circuited component is located. After the above treatment, basically all short circuit problems can be solved.

4 Complementarity between flying probe and needle bed
Compared with the needle bed, flying probe is a technological innovation and is still developing. With the development of wireless communication and wireless networks, more and more electronic modules will increase wireless access capabilities. The current needle bed tester is only suitable for low-frequency bands. The probe in the radio frequency band will become a small antenna, generating a large amount of parasitic interference, affecting the reliability of the test results. The test of RF circuit is performed by subsequent functional test, which will inevitably reduce the defect coverage of electronic modules. The number of probes of flying probe tester is very small, so it is easier to take measures to reduce RF interference, realize low-frequency and RF testing of electronic modules, and improve coverage.

5 Conclusion
Flying probe is a technological innovation and is still developing. Although flying probe online testing technology faces severe technical challenges due to its own principle and method limitations, it still has incomparable advantages over other technologies in some aspects, such as the flexibility of flying probe testing and the ability to perform electrical performance testing. In addition, with the improvement of technology and the coordinated use of new technologies, it will also have tenacious vitality in the testing field.