PCB Tech - Application of Online Tester in Circuit Board Maintenance

Introduce a new maintenance tool-online tester, and focus on the functional characteristics of the tester and the methods and steps to use it to repair circuit boards.

1 The functional characteristics of the online tester

1.1 Introduction

The online tester is an instrument for repairing PCB boards. It can complete on/offline functional testing of small and medium-sized digital integrated circuits, memories and some large-scale integrated circuit chips, and can also test, store and compare VI curves on any node (solder) on the PCB (the so-called Online testing refers to the testing of components soldered on the PCB board; while off-line testing refers to the testing of components separated from the PCB board).

(1) Functional test (ICFT)

Use the post-drive principle to drive the circuit, and then take the output of the gate under test for analysis, and get the result. The principle of post-driving is to make the circuit change according to the intention of the tester, and force the level of its input end to be higher or lower, so that the door under test is "isolated" from the surrounding circuit to see if its output and input meet the specified requirements Logical relationship to complete the logic function test of the circuit chip.

(2) VI curve test (V-I)

The fault point is judged by the comparison form. 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 in future comparison operations. When the same type of PCB board fails, you can use the online tester to perform VI test on any two nodes of the failed board, and at the same time call out the VI curve between the corresponding two nodes of the normal PCB board for comparison, if the comparison result exceeds If the normal range is exceeded, then the two nodes should be one of the fault points.

(3) Memory test

For random read/write memory (RAM), use the write/read method to test, that is, write a piece of data to the memory cell first, and then read it out, to see if the two data are the same, if they are different, the memory appears Fault. For read-only memory (ROM), first read out the content of its storage unit and store it in the computer. In the future, compare it with the content read from the corresponding memory on the faulty circuit board, and the same indicates that there is no problem.

(4) Status test

It is judged by testing the level state of each pin of the device and comparing it with a good device.

(5) Custom test

The online tester also provides a digital circuit functional test platform. Users can define the excitation of the input pins of the device under test and describe the logical relationship between the output and the input. If the logical relationship is met, there is no problem with the device.

2.1 Basic principles

(1) Power supply first and test later-this is an important principle in the maintenance process. That is, before using the tester for testing, you should check the working power supply of the circuit chip. If the power supply is short-circuited, the instrument may be damaged during power-on testing.

(2) Test first and then analyze-it is to test the PCB board first, and then analyze according to the test results, and then determine the location of the fault.

(3) Diagnose first and then deal with it-try to reduce the fault to the smallest extent, and then replace the components with greater certainty. ?

2.2 Repair method

(1) Offline first and then online--Because of the high accuracy of offline testing, the pluggable components on the board should be removed for offline testing, and then other components on the board should be tested.

(2) Interface first, then components-when repairing, it is best to perform VI curve test on each interface pin on the PCB. Because many faults are caused by the interface circuit, testing the interface first can sometimes quickly detect the fault.

(3) Discrete first and then integrate-test the discrete components first, and then test the integrated chip, because the failure rate of discrete components is higher.

(4) Function first and then VI-that is, it is better to perform function test on integrated blocks that can be tested on the PCB, and then perform VI curve test on integrated blocks that cannot be tested. Because the result of the function test is more intuitive and reliable, it can ensure that the fault is found faster.

3.1 Preparations before maintenance

(1) If there is no label on the components on the PCB, you should draw a diagram of the location of the PCB components and mark the number on the diagram. When testing, use this label for identification.

(2) Establish a comparison library. This is an important task before using the online tester to repair the PCB board. The so-called library building refers to the test results of the component pin status, VI curve waveform, read-only memory data and other test results on the normal PCB board through the test, so that it can be called out for comparison and analysis with the faulty board during maintenance.

3.2 Repair steps

(1) Understand. When a PCB board needs to be repaired, you should ask about the failure phenomenon, and carefully observe whether there are obvious failures such as burnt, blown fuse, loose plug-in or disconnected connection on the board.

(2) Test the power supply. Use a multimeter to check whether there is a short circuit in the working power supply of the components on the PCB.

(3) Offline testing. Remove the pluggable components on the board and perform offline testing.

(4) Online test. Test other components on the board to determine the point of failure. The fault location process is shown in Figure 2.

(5) Trial operation. After the fault point is determined and eliminated, it must be tested online to verify whether the fault is completely eliminated. If it still does not work, you must follow the above steps to find the fault again.

(6) Organize records. During the testing process (including building a comparison database), you should make a record while testing, especially where there are suspicious places, and record the phenomena observed during the test in detail. Finally, sort and save the records so that they can be used as a reference when encountering the same type of templates or similar failure phenomena in the future.

(1) Before the test, use a multimeter to check whether the working power supply of the components on the board is short-circuited. If there is, remove it first.

(2) During the power-on test, touch the surface of the components with your hands to check whether there are overheated components. If there are, remove or remove the overheated components first.

(3) If there is an oscillator on the board, first remove it or short-circuit it to stop working. Prevent the generation of pulses during the test and affect the test results.

(4) Check whether there is battery-powered memory on the circuit board. If so, ask the relevant personnel whether their internal data is useful. If it is useful, it cannot be tested, otherwise its data may be modified or data may be lost.

(5) Since the characteristics of components of the same model are somewhat different, in the VI curve comparison test, the corresponding node curve of each board of the same model is also different. Generally, when the two curves differ greatly, it is considered that this place may be one of the fault points.

(6) During the VI curve test, the voltage-current curve usually takes 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 too large or disconnected. At this time, you can customize a circuit node as a reference point.

(7) In the maintenance of the PCB circuit board, there are two possibilities when there is "illegal power supply or ground pin" in the online function test: 1. The tested device is not clamped properly by the test clamp, resulting in poor contact. 2. The device does have other pins (except the working power supply and ground pin of the device itself) connected to the power or ground. The solution is to temporarily disconnect these pins from the power or ground, and then test.