Analysis of Common Faults of Printed Circuit Board

Printed Circuit Board  Faults

1. Poor Contact

Poor contact between board and slot, broken cable, poor contact between wire plug and terminal, false soldering of components and so on all belong to this category.

The solution to this kind of fault is to carefully check the suspected connector to see if there is obvious oxidation or poor contact, scrape the oxidized metal contact point, adjust the position of the contact point, and test whether the contact is good after treatment.

2. Signal Disturbed

For the digital circuit, in a certain situation, I feel sleepy the fault will appear. It is possible that the control system is affected by too much interference, and the individual component parameters or overall performance parameters of the circuit board are changed, which makes the anti-interference ability tend to the critical point, resulting in failure.

This kind of fault focuses on checking whether the equipment is well grounded, using the test pen to check whether the equipment shell is charged, or using the multimeter to measure whether the equipment shell has a high voltage to the earth, which is generally below 1V, if it is above 10V, it is necessary to doubt whether the grounding is good.

3. Poor Thermal Stability of Components

From a large number of maintenance practice, the first is the poor thermal stability of electrolytic capacitors, followed by other capacitors, triodes, diodes, ICs, resistors, etc.

(1) This kind of fault will appear or disappear with the time of machine start-up. The essence is that the fault changes with the temperature change of a fault component. For example, the failure caused by the aging of electrolytic capacitor usually occurs as soon as it is powered on, and the failure disappears after being powered on for a period of time, that is, there is a failure when the machine is cold, but there is no failure when the machine is hot. Its essence is: the capacitance of aging electrolytic capacitor changes with the temperature, when the temperature is low, the capacity is small, resulting in poor filtering, the circuit board can not work normally, and with the power on time, the temperature of the electrolytic capacitor increases, the capacity increases, the filtering conditions are met, and the fault disappears.

(2) Thermal stability fault belongs to soft fault, so it is not easy to detect and determine the fault component directly during maintenance, but the inspection scope can be reduced by artificially heating or cooling the suspected component. A hair dryer or electric gun can be used to heat the suspected element, and a cotton swab dipped in alcohol can be used to cool the suspected element. The capacitance is easy to distinguish by VI curve test.