Due to the high labor intensity of manual inspection, eyes are prone to fatigue and the rate of missed tests is high. And with the development of electronic products toward miniaturization and digitization, printed circuit boards are also developing toward high density and high precision, and manual inspection methods are basically impossible to achieve. For higher density and precision circuit boards (0.12 ~ 0.10mm), it is completely impossible to inspect. The backwardness of testing methods has led to the current product qualification rate of domestic multilayer boards (8-12 layers) being only 50 ~ 60%.
1. Chip with program1. EPROM chips are generally not damaged. Because this chip requires ultraviolet light to erase the program, it will not damage the program during the test.
However, there is information that due to the material used to make the chip, over time (the year is old), it may be damaged even if it is not used (mainly refers to the program). Therefore, it is necessary to back up as much as possible.
2.EEPROM, SPROM, etc. and RAM chips with battery are very easy to destroy the program. Whether this kind of chip will destroy the program after scanning the VI curve using <tester>, it is not yet determined. We are still careful when encountering this situation. The author has done many tests, and the big reason may be: the leakage of the casing of the repair tools (such as testers, soldering irons, etc.).
3. Do not easily remove the chip with battery from the board.
2. Reset circuit1. When there is a large-scale integrated circuit on the circuit board to be repaired, pay attention to the reset problem.
2. It is best to reinstall the device before testing, turn it on repeatedly, try the shutdown machine, and press the reset button several times.
3. Function and parameter test1. <Tester> The device can only detect the cut-off area, amplification area and saturation area, but it cannot measure the working frequency and speed.
2. Similarly, for a TTL digital chip, it can only know the output change of high and low levels, and it is impossible to find out the speed of its rising and falling edges.
4. Crystal Oscillator1.Usually you can only use the oscilloscope (the crystal needs to be powered up) or the frequency meter to test, the multimeter can't measure, otherwise you can only use the substitution method.
2. Common failures of crystal oscillators are: a. Internal leakage, b. Internal open circuit c. Deteriorated frequency offset d. Leakage of externally connected capacitors. Here the leakage phenomenon can be measured using the VI curve of <tester>.
3. Two kinds of judgment methods can be used during the whole board test: a. The relevant chips around the crystal oscillator failed during the test. B. No other fault points were found except the crystal oscillator.
4. There are two common types of crystal oscillators: a. Two pins, b. Four pins, of which the second pin is powered, please be careful not to short circuit arbitrarily.
5. Distribution of fault phenomena
a. Incomplete statistics of circuit board faults:
1) chip damage 30%,
2) discrete components 30% damage,
3) The wiring (copper wire on the PCB board) is broken by 30%,
4) the program is broken or lost by 10% (there is an upward trend).