Maintenance Method and Testing Techniques of Printed Circuit Board

As control systems are costly, in case of system failure, maintenance is the general option in considerations of economic benefits and cost saving. For maintenance of printed circuit board, serviceman must have solid theoretical foundation, strict and serious working attitude and conduct careful observation. However, in case of following cases, it is required to replace with new circuit board: The printed circuit board has reached the end of service life; printed circuit board are severely damaged and can’t be repaired; through many times of repeated maintenance, constant failure indicates that the printed circuit board is not suitable for the machine tool.

Maintenance Methods and Techniques of Printed Circuit Board: 

1. Observation method

   When you get one printed circuit board to be repaired, firstly observe its outlook carefully. If it has been burnt, make sure to check if the power supply circuit is normal before powering on it to avoid secondary damages. Observation method is one of static examination methods, and shall be implemented through the following steps. 

Firstly, observe if the printed circuit board has any artificial damages from the following aspects: 

①　Check if it has been broken, resulting in deformation of board corner or deformation and damages of chip.

②　Check if the chip socket is damaged due to forced operation as there is no special tool.

③　Observe the chip on the circuit board. If with socket, firstly check whether operator has placed the chip in wrong position or direction during maintenance. If the operator fails to correct the mistake timely, the chip may be burnt when the printed circuit board is powered on, causing unnecessary loss.

Where there is short-circuited terminal on the circuit board, check for misplug. 

Secondly, observe if any element on the printed circuit board is burned-out：

For example, if the resistor, capacitor and diode are blackened or get burned. Under normal condition, normal use of resistor will be not affected even if it is burnt because its resistance and performance remain unchanged. In this case, multimeter is required for auxiliary measurement. However, if the capacitor and diode are burnt, their performance will be changed and and couldn’t work well, and thus the whole circuit will be affected. Then it is required to replace them with new elements. 

Thirdly, check the integrated circuit, such as 74 series, CUP, coprocessor, AD and other chips, for bulge, crack, burns and blackening. If any, it is almost certain that chips are burnt out and must be replaced. 

Fourthly, observe the circuit board wiring for peeling, burns and open circuit, and if the plating through hole is separated from the bonding pad.

Finally: Check whether the fuse (including fuse and thermistor ) is burnt out. Sometimes the fuse is too thin to see, so you shall take advantage of auxiliary tool ( like multimeter) to judge if the fuse is damaged. 

The four cases said above are mostly caused by overcurrent of circuit but specific causes for overcurrent shall be determined based on actual conditions. Even so, the general idea for finding causes is as follows; first step is to carefully analyze the schematic diagram of circuit board; in the second step, based on the circuit where the burnt-out elements belong, find its superior circuit, infer step by step; the third step is to analyze parts prone to failure based on accumulated experience during work and find fault cause.

 2. Static measurement method 

The testability of printed circuit board can be explained simply as: When testing the performance of some element of printed circuit board, testing engineer shall apply the simplest testing method when possible to determine whether such element satisfies expected functional needs. Testability can be further defined as: 

1. Degree of simplicity of methods used to test the compliance (technical specification) of products.

2. How quickly the testing procedures are prepared?

3. Degree of generalization to find product fault

4. Degree of simplicity of methods to access to the test point.

In the past, if one product has failed in one test point, then operator will infer the problem lies in this test point. If any product defect can’t be found during production and testing, this defect will be simply identified and diagnosed through function and system tests.

In contrast, people today try their best to find defects in advance as early as possible. In this way, we can reduce the cost, but more importantly, some manufacturing defects can’t be found through function test due to the complexity of products. For example, this problem exists with some pre-installed software or programmed software. (such as flash memory of ISPs: programmable device inside the In-system programmable devices). Programming of such elements must be designed in the research & development phase and testing system must be equipped with such programming. 

Testing friendly circuit design certainly would cost money. However, testing circuit with fault will get you spend more money. Because testing itself may be costly and testing costs will increase with increase in testing level; from online testing to function testing and system testing, the cost will get higher and higher. If you skip one of the tests, the cost may even be greater. Generally, the testing costs will increase by ten times each time the testing level is improved. By testing friendly circuit design, we can find fault early and thus the testing costs can be made up quickly. 

Absolutely, to achieve better testability, mechanical and electrical design specification must be taken into consideration and there is a price to pay. However, this consideration can generate series of benefits from the aspect of the whole process, so it is an important premise for successful production. 

Step 2: Measure the door circuit with oscilloscope and check if it confroms with logical relation. If the output is illogical, deal with it in two cases. The first case is high level is measured while the circuit outputs low level. From this point, it can be directly judged that the chip is damaged; in another case when the circuit outputs high level but low level output is measured, chip damages can’t be inferred directly. It is also required to disconnect chip from rear circuit and carry out measurement again to observe if the logic is reasonable and then judge if the chip is damaged.  

Step 3: Measure the crystal oscillator inside digital circuit with oscilloscope to see if it has output. If not, it is required to remove the chip connected to the crystal oscillator when possible and carry out measurement. If there is still no output, the crystal oscillator will be considered as damaged preliminary; if any, it is required to reinstall and test the chip one by one to find fault cause. 

Step 4: Digital circuit with bus structure generally includes digit, address and control bus. Measure such three bus, compare with the schematic diagram and observe whether the signal is normal and find fault. 

Online measurement method is mainly used to compare with normal and faulty printed circuit board to troubleshoot fault so as to complete maintenance of printed circuit board.