Basic knowledge of circuit board

1、 Chip with program

1. EPROM chip should not be damaged. Because this chip needs ultraviolet light to erase the program, it will not damage the program in the test. However, there is information: due to the material used to make the chip, as time goes on, even if it is not used, it may be damaged (mainly referring to the program). So try to back it up as much as possible.

2. EEPROM, sprom and RAM chip with battery are easy to break the program. Whether this kind of chip will destroy the program after using the tester to scan the VI curve has not been determined. Nevertheless, colleagues should be careful when they encounter this kind of situation. The author has done many tests, and the possible reason is that the shell leakage of maintenance tools (such as tester, electric iron, etc.).

3. Do not remove the chip with battery from the circuit board easily.

2、 Reset circuit

1. When there are large scale integrated circuits on the circuit board to be repaired, attention should be paid to reset.

2. Before the test, it's better to install it back on the equipment and turn it on and off repeatedly. And press the reset key several times.

3、 Function and parameter test

1. < tester > the detection of the device can only reflect the cut-off region, amplification region and saturation region. But we can't measure the working frequency and speed.

2. Similarly, for TTL digital chip, only high and low level output changes can be known. But we can't find out the speed of its rising and falling edge.

4、 Crystal oscillator

1. Usually, it can only be tested by oscilloscope (crystal oscillator needs to be powered up) or frequency meter, but it can not be measured by multimeter, otherwise, it can only be replaced.

2. The common faults of crystal oscillator are: A. internal leakage, B. internal open circuit, C. deteriorated frequency offset, D. leakage of peripheral connected capacitor. The leakage phenomenon here can be detected by VI curve of the tester.

3. Two judgment methods can be used in the whole board test: A. relevant chips near the crystal oscillator fail to pass the test. b. Except crystal oscillator, no other fault point was found.

4. There are two kinds of crystal oscillator: A. two feet. b. Four pins, of which the second pin is added with power supply, pay attention not to short circuit at will.

5、 Distribution of fault phenomena

1. Incomplete statistics of circuit board fault location: 1) chip damage 30%, 2) discrete component damage 30%, 3) wiring (PCB copper wire) fracture 30%, 4) program damage or loss 10% (there is an upward trend).

2. It can be seen from the above that when the circuit board to be repaired has problems with wiring and program, there is no good board, so we are not familiar with its wiring and can not find the original program. It is unlikely that the board will be repaired.

Compatible design

Electromagnetic compatibility refers to the ability of electronic equipment to work coordinately and effectively in various electromagnetic environments. The purpose is to make the electronic equipment not only suppress all kinds of external interference, so that the electronic equipment can work normally in the specific electromagnetic environment, but also reduce the electromagnetic interference of the electronic equipment itself to other electronic equipment.

1. Choosing a reasonable wire width, because the impact interference caused by transient current on PCB printed lines is mainly caused by the inductance of printed wires, the inductance of printed wires should be reduced as much as possible.

2. If the layout allows, it is better to use the well shaped network wiring structure. The specific method is that one side of the printed circuit board is laid horizontally, the other side is laid longitudinally, and then the cross holes are connected with metallized holes.

3. In order to suppress the crosstalk between the wires of PCB circuit board, the long-distance equal routing should be avoided as far as possible, the distance between the wires should be widened as far as possible, and the signal wire, ground wire and power wire should not be crossed as far as possible. The crosstalk can be effectively suppressed by setting a grounded printed wire between some signal lines which are very sensitive to interference.

Form

The circuit board is mainly composed of pads, vias, mounting holes, wires, components, connectors, fillers, electrical boundaries, etc. the main functions of each component are as follows:

Pad: a metal hole used to weld pins of components.

Vias: there are metal vias and nonmetal vias. Metal vias are used to connect component pins between layers.

Mounting hole: used to fix the circuit board.

Wire: copper film of electrical network used to connect component pins.

Connector: used for connecting components between circuit boards.

Filling: used for copper coating of ground wire network, which can effectively reduce the impedance.

Electrical boundary: used to determine the size of the circuit board, all components on the circuit board can not exceed the boundary.