What does false welding mean? Cause analysis and detection method of welding defects

Date：2022-05-10 17:18:24Views：1831

What is false soldering? It is generally caused by oxidation or impurities at the welding point, poor welding temperature and improper method. In essence, there is an isolation layer between the solder and the pin. They are not in full contact. The state is generally not visible to the naked eye. However, its electrical characteristics have no conduction or poor conduction, which affects the circuit characteristics.

Causes of false soldering

1. One is the unstable state caused by improper production technology in the production process;

2. The other is caused by the aging and peeling of the solder joints at the welding feet of some parts with serious heating after long-term use.

Detection method of false welding

1. Visual inspection method

Generally, first look for heating components, such as power tubes, high-current diodes, high-power resistors, integrated circuits, etc. these components are prone to false soldering due to heating, which can be seen directly in serious cases, and can be viewed with a magnifying glass in slight cases. Generally, the newly welded pins are very smooth. When the edge is affected, it will become rough and dull due to continuous extrusion and stretching, and gray circles will appear around the solder joint. When viewed with a high-power magnifying glass, you can see the small crack group in the shape of crack, and in serious cases, it will form a ring crack, that is, desoldering. Therefore, even if there is no desoldering in the place with circular black circle, it will be a hidden danger in the future. Large area repair welding of integrated circuits and heating element pins is one of the solutions.

2. Current detection method

Check whether the current setting complies with the process regulations and whether the current setting does not increase accordingly when the product load changes, resulting in poor welding due to insufficient current during welding.

3. Shaking method

It is to shake the low-voltage components one by one with your hand or camera to feel whether the components are loose. This is mainly to shake the larger components. In addition, before using this method, the fault range should be compressed to determine the approximate range of the fault, otherwise it will face many components. It is unrealistic to shake one by one.

4. Vibration method

When the phenomenon of false soldering is encountered, it can be confirmed by knocking. Gently tap the circuit board with a screwdriver to determine the position of the false soldering point. However, when the percussion method is adopted, the personal safety and the safety of the equipment shall be ensured to avoid expanding the fault range.

5. Repair welding method

Repair welding is a maintenance method when the fault can not be found after careful inspection, that is, welding the components within the fault range one by one. In this way, although no real fault point is found, the purpose of maintenance can be achieved.

Solution to faulty welding

1) Judge the approximate fault range according to the fault phenomenon.

2) Appearance observation, focusing on larger components and components with large calorific value.

3) Observe with a magnifying glass.

4) Pull the circuit board.

5) Shake the suspicious component by hand and observe whether the pin solder joint is loose

What is false welding? In the welding process of electronic components, it seems that the solder joint is successfully welded on the surface, but it is not welded in fact. Sometimes, the lead can be pulled out of the solder joint with one hand. This phenomenon is called false welding.

Causes of false welding

1. Poor solder quality;

2. Poor reducibility or insufficient amount of flux;

3. The surface of the welded part is not cleaned in advance and the tinning is not firm;

4. The temperature of the soldering iron head is too high or too low, and there is an oxide layer on the surface;

5. The welding time is too long or too short, which is not well mastered;

6. When the solder has not solidified during welding, the welding elements are loose;

7. Oxidation of component pins.

What are the hazards of false welding?

False soldering is mainly caused by oxides and dirt on the surface of the metal to be welded. Its solder joints become the connection state with contact resistance, resulting in abnormal operation of the circuit, unstable phenomena of good and bad, increased noise and no regularity, which brings major hidden dangers to the debugging, use and maintenance of the circuit. In addition, there are also some false solder joints that remain in good contact for a long time after the circuit starts to work, so it is not easy to find. However, under the selection of environmental conditions such as temperature, humidity and vibration, the contact surface is gradually oxidized, and the contact slowly becomes incomplete. The contact resistance of the false solder joint will cause local heating, and the local temperature rise will further worsen the incomplete contact solder joint, and finally even make the solder joint fall off, and the circuit can not work normally. This process can sometimes take as long as one or two years.

According to statistics, nearly half of the electronic machine product failures are caused by poor welding. However, it is not easy to find the faulty solder joint from an electronic device with thousands of solder joints. Therefore, false soldering is a hidden danger of circuit reliability, which must be strictly avoided. Pay special attention to manual welding.

The existence of false welding greatly reduces the reliability of printed boards and overall products, causes unnecessary maintenance to the production process, increases production costs, reduces production efficiency, causes great quality and safety hazards to the products that have left the factory, and increases after-sales maintenance costs.

Inspection method of false welding

1. Look, use a magnifying glass with high magnification to carefully check the bottom of the circuit board, especially the components with high power and high calorific value, such as high-power resistance, three terminal voltage stabilization, triode, chip, power pin of the integrated block and output pin of the field block, lead out wire pile head of the deflection coil, etc. most faults can be solved by repair welding.

2. Knock. After startup, you can gently knock each circuit board and plug-in box with a wooden stick. While knocking, you can look at the image. If there is a reflection, you can roughly determine whether it is the power board, scanning board or signal board, or the failure of plug-in box.

3. Shake. After confirming that there is a problem with the circuit board, you can shake each component gently with a wooden stick after starting the machine, and you can quickly find out which component is loose. If the above three methods are not effective, we can only find the circuit diagram of the machine, spend some time and carefully check the DC potential of each channel according to the circuit diagram to judge the problem, which depends on the accumulation of ordinary experience.

Solution of false welding

1. Distribute the thimble properly and evenly to make the amount of printing tin uniform. At the same time, confirm whether the printing scraper is deformed and worn, and replace the defective scraper.

2. Point by point calibration method is adopted to calibrate the mounting position of the element so that it is installed in the middle of the copper foil.

3. If the terminal is deformed, it needs to be shaped and then pasted. For the components with dense IC deformation that cannot be actually installed, it is generally installed by soldering iron hand. For incoming oxidized components, contact IQC and ask the supplier for improvement.

4. Appropriately increase the temperature and time of reflux preheating zone to make it fully welded.

5. The time from printing to reflow shall be strictly controlled, and integrated production shall be adopted as far as possible to reduce the direct long-term contact of solder paste with air after printing.

6. Replace the expired solder paste, and strictly control the management and use according to the validity period and first in first out of solder paste.

In summary, the difference between false welding and false welding is that false welding is connected during welding, but it is not firm, while false welding is not successfully connected during welding, which only creates a "false image" of welding. This article is introduced here. I hope it can be helpful to you. We will bring more wonderful content later. The company's testing services cover: electronic component testing and verification, IC authenticity identification, product design and material selection, failure analysis, function testing, factory incoming material inspection, tape braiding and other testing items. Welcome to call Chuangxin testing. We will serve you wholeheartedly.