Application of online tester in circuit board maintenance

1.1 Introduction The online tester

is an instrument for repairing PCB boards . It can complete the online/offline functional test of small and medium-sized digital integrated circuits, memory and some large-scale integrated circuit chips, and can also test, store and compare the VI curve of any node (solder point) on the PCB board (the so-called online test refers to the test of components soldered on the PCB board; while the offline test refers to the test of components separated from the PCB board). Its principle structure block diagram is shown in Figure 1.

1.2 Main Function Principles

(1) Functional test (ICFT)

uses the post-drive principle to drive the circuit, and then takes the output of the gate under test for analysis and obtains the result. The post-drive principle means that the circuit changes according to the tester's intention, forcing the level of its input end to become high or low, so that the gate under test is "isolated" from the surrounding circuits to see whether its output and input meet the specified logical relationship, thereby completing the logic function test of the circuit chip.

(2) VI curve test (VI)

Determine the fault point through comparison. The comparison process is as follows: First, test two nodes on a working PCB board to obtain the voltage (V)-current (I) relationship curve, and save the test results as a standard for future comparison operations. When a PCB board of the same type fails, the online tester can be used to perform VI test on any two nodes of the faulty board, and at the same time, the VI curve between the corresponding two nodes of the normal PCB board can be called up for comparison. If the comparison result exceeds the normal range, the two nodes should be one of the fault points.

(3) Memory test

For random read/write memory (RAM), the write/read method is used for testing, that is, first write a data to the storage unit, and then read it out to see if the two data are the same. If not, the memory is faulty. For read-only memory (ROM), first read out the contents of its storage unit and store it in the computer. Later, compare it with the contents read out from the corresponding memory on the faulty circuit board. If they are the same, it means there is no problem.

(4) Status test

The level status of each pin of the device is tested and compared with the good device to make a judgment.

(5) Custom test

The online tester also provides a digital circuit function test platform. Users can define the stimulus of the input pin of the device under test and describe the logical relationship between the output and the input. If it meets the logical relationship, the device is fine.

2 Basic principles and methods of maintenance

2.1 Basic principles

(1) Power on first, then test - This is an important principle in the maintenance process. That is, before using the tester for testing, the working power supply of the circuit chip should be checked first. If the power supply is short-circuited, the instrument may be damaged during the power-on test.
(2) Test first, then analyze - that is, test the PCB first, then analyze based on the test results, and then determine the fault location.
(3) Diagnosis first, then treatment - try to reduce the fault to the smallest scope as much as possible, and then replace the components when you are more confident.

2.2 Repair method

(1) Offline first, then online - Since offline testing has high accuracy, pluggable devices on the board should be removed for offline testing, and then other components on the board should be tested.
(2) Interface first, then component - When repairing, it is best to perform VI curve test on each interface pin on the PCB board. Because many faults are caused by interface circuits, testing the interface first can sometimes quickly find the fault.
(3) Discrete first, then integrated - test discrete components first, then test integrated chips, because discrete components have a higher failure rate.
(4) Function first, then VI - that is, it is best to perform functional test on the integrated blocks on the PCB that can be functionally tested first, and then perform VI curve test on the integrated blocks that cannot be functionally tested. Because the results of functional testing are more intuitive and reliable, they can ensure that faults are found faster. [page]

3 PCB board maintenance

3.1 Preparation before maintenance

(1) If there are no labels on the components on the PCB board, a PCB component location diagram should be drawn and the numbers should be marked on the diagram. During testing, the numbers should be used for identification.
(2) Establish a comparison library. This is an important task before using an online tester to repair a PCB board. The so-called library building means that by testing a normal PCB board, the test results such as the pin status of the components on the board, the VI curve waveform, and the data in the read-only memory are archived so that they can be called up and compared with the faulty board during maintenance.

3.2 Maintenance steps

(1) Understanding. When a PCB board needs to be repaired, the fault phenomenon should be asked and the board should be carefully observed for obvious faults such as burning, blown fuses, loose plugs, or disconnected wires.
(2) Measure the power supply. Use a multimeter to check whether the working power supply of the components on the PCB board is short-circuited.
(3) Offline test. Remove the pluggable components on the board and perform offline testing.
(4) Online test. Test other components on the board to determine the fault point. The fault location process is shown in Figure 2.
(5) Test run. After the fault point is identified and eliminated, it is also necessary to conduct an online test run to verify whether the fault has been completely eliminated. If it still does not work properly, it is necessary to re-find the fault according to the above steps.
(6) Organize the records. During the test process (including building a comparison library), you should keep records while testing, especially in suspicious areas, and record the phenomena observed during the test in detail. Finally, classify and organize the records and save them so that they can be used as a reference when encountering the same type of template or similar fault phenomenon in the future.

4 Precautions in maintenance

(1) Before testing, use a multimeter to check whether the power supply of the components on the board is short-circuited. If so, eliminate it first.
(2) When testing with power on, touch the surface of the components to check whether there are overheated components. If so, eliminate or remove the overheated components first.
(3) If there is an oscillator on the board, remove it first or short-circuit it to stop working. Prevent pulses from being generated during the test and affecting the test results.
(4) Check whether there is a battery-powered memory on the circuit board. If so, ask the relevant personnel whether the internal data is useful. If it is useful, do not test it, otherwise the data may be modified or lost.
(5) Since the characteristics of components of the same model are somewhat different, when comparing the VI curves, the corresponding node curves of each board of the same model are not exactly the same. Generally, when the difference between the two curves is large, it is considered that the place may be one of the fault points.
(6) When testing the VI curve, its voltage-current curve usually uses the power ground on the board as the reference point. However, the resistance between some components or interfaces on the board and the power ground is large or disconnected. In this case, a custom circuit node can be used as a reference point.
(7) During online functional testing, there are two possibilities when "illegal power or ground pins" appear: ① The test clip does not clamp the device under test properly, resulting in poor contact. ② The device does have other pins (except the device's own working power and ground pins) connected to the power or ground. The solution is to temporarily disconnect these pins from the power or ground and then test again.

References
[1] Han Rong. Discussion on several basic technical issues of circuit maintenance testers [J]. China Equipment Management, 1999, (11).