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Inspection of de-wetting

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    De-wetting refers to insufficient solder joints soldered, resulting in poor contact, time-to-time break. De-wetting and non-wetting both the surface are not fully coated with tin layer, the components are not fixed by tin due to components not cleaned,used too little flux or welding time is too short. The so-called "solder joint later failure ", refers to solder joint  surface looks good, does not have" lap welding "," half spot welding "," pull tip "," exposed copper "and so on welding defect.  in the workshop production, also no defective on products. But after users use for a period of time, failures caused by poor electrical conductivity occurs from time to time due to bad welding, which is one of the reasons for the high rate of early rework, which is called "de-wetting "or “false welding”which is usually caused by oxidation or impurities at the welding point and poor welding temperature. The essence is that there is an isolation layer between the solder and the pin. They're not in full contact. The naked eye generally cannot see its state. However, its electrical characteristics are not conduction or poor conduction, affecting the circuit characteristics.

The main causes of de-wetting are as follows:

A. Poor solder quality;

B. Poor reducibility or inadequate dosage of flux;

C. The surface of the weld is not cleaned in advance, and the tin plating is not firm.

D. The temperature of the iron head is too high or too low, and the surface has an oxide layer (this refers to the case of soldering iron for manual or automatic manipulator).

E. The welding time is too long or too short to master well;

F. Components loose when soldering has not yet solidified;

G. Component pin oxidation.

1. Consequence of “de-wetting”

De-wetting is mainly caused by the oxide and dirt on the surface of the metal to be welded, and its solder joint becomes the connection state with contact resistance, which leads to the abnormal or unstable status of the circuit, and the noise increases without regularity, which brings great hidden danger to the debugging, use and maintenance of the circuit. In addition, there are some false welding points in the circuit start to work for a long time in normal status , so it is not easy to find. But under the environmental conditions of temperature, humidity and vibration, the contact surface is gradually oxidized, and the contact slowly becomes incomplete. The contact resistance of the false welding point will cause local heating, and the local temperature will increase  the solder joint condition of incomplete contact further deteriorates, finally even make the solder joint fall off, the circuit completely cannot work properly. This process can sometimes last up to one or two years. Statistics show that nearly half of the failures in electronics are caused by poor welding. However, it is not easy to find the false welding point that caused the failure from an electronic device with thousands of solder joints. Therefore, de-wetting is a major hidden danger of circuit reliability and must be strictly avoided. Special attention should be paid to manual welding.

2. Detection method of “de-wetting“(take TV repair detection for  reference)

First, to see, use a magnifying glass with high magnification, carefully check the bottom of the circuit board, especially to check the high-power, high-calorie components, such as, high-power resistance,three-terminal voltage stabilizer, transistor, chip, integrated block of the power pin and field block output foot, deflection coil lead line head, and so on, most of the faults can be solved by repair welding.

Secondly, to knock, after take apart machine, use a wooden stick to gently tap each circuit board, plug-in box, checking the image while knocking, if there is reflection on image, you can roughly determine whether the failure is on power board, scan board, signal board, or plug-in box.
 

Thirdly, to shake, after determine the problem circuit board, use a wooden stick to gently shake each component, can easily find out which one loose. If the above three methods con not be effective, then we have to fine the circuit diagram of the machine, check carefully the DC potential of each channel against the circuit diagram to determine the problem, which depends on the accumulation of ordinary experience.

3. In the actual pcb welding process," de-wetting "detection is not so easy, after the surface inspection (AOI optical inspection), it is best to use ICT to further inspect the performance of electronic circuits to ensure high quality. Carry out endurance test if necessary. 

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