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What is the pcb and pcba virtual welding( cold solder joint/dry joint) and how to solve this problem during smt

First, what is the virtual welding Virtual welding is a common type of line fault, there are two kinds, one is in the production process, due to improper production process, the unstable state of the time pass; the other is the long-term use of electrical appliances, some heat For more serious parts, the solder joints at the solder joints are prone to aging peeling. Both false and false soldering means that the surface of the weldment is not sufficiently tinned, and the weldments are not fixed by tin because the weldment surface is not cleaned or the flux is used too little and the welding time is too short. The so-called "post-failure of solder joints" means that the solder joints on the surface are of acceptable quality, and there are no solder joints such as "lap welding", "semi-spot welding", "tip", "bronze", etc. There is nothing wrong with the finished machine. However, after the user has used it for a period of time, the failure due to poor soldering and poor electrical conductivity has occurred from time to time, which is one of the reasons for the high rate of early repair.

 

Second, the harm of virtual welding

The virtual welding is mainly caused by the oxide and dirt on the surface of the metal to be welded. Its solder joint becomes a connection state with contact resistance, which causes the circuit to work abnormally. It is unstable when it occurs, and the noise increases without regularity. Sexuality brings serious hidden dangers to the debugging, use and maintenance of the circuit. In addition, there are also some solder joints that remain in contact for a long period of time at the beginning of the circuit operation, so it is not easy to find. However, under the conditions of temperature, humidity and vibration, the contact surface is gradually oxidized, and the contact slowly becomes incomplete. The contact resistance of the solder joint causes local heat generation, and the local temperature rise causes the solder joint condition of incomplete contact to further deteriorate, and eventually the solder joint is even detached, and the circuit is completely unable to work normally. This process can sometimes take up to one or two years.

According to statistics, nearly half of all electronic product failures are caused by poor soldering. However, to find faulty solder joints from an electronic device with thousands of solder joints, This is not an easy task. Therefore, virtual soldering is a hidden danger of circuit reliability and must be strictly avoided. Pay special attention when performing manual soldering operations.


Third, the main cause of virtual welding 1. Poor solder quality; 2. The flux has poor reductive properties or insufficient dosage; 3. The surface of the welded surface is not cleaned beforehand, and the tin plating is not strong; 4. The temperature of the soldering iron tip is too high or too low, and the surface has an oxide layer; 5. The welding time is too long or too short to master well; 6. When the solder has not been solidified during welding, the welding elements are loose; 7. Component leads are oxidized.


Fourth, analyze the reasons and steps of the virtual welding (1) First check whether the weld joint surface is free of rust, oil and other impurities, or unevenness and poor contact, which will increase the contact resistance, reduce the current, and the temperature of the welded joint surface is not enough. (2) Check whether the weld amount of the weld is normal, and whether there is a decrease in the amount of overlap on the drive side or cracking. The reduction in the amount of overlap causes the joint area of the front and rear steel strips to be too small, so that the total force receiving surface is reduced and cannot withstand a large tension. In particular, the crack on the driving side causes stress concentration, which causes the crack to become larger and larger, and finally breaks. (3) Check whether the current setting meets the process regulations, and whether the current setting does not increase correspondingly when the thickness of the product changes, so that the welding current is insufficient and the welding is poor.

(4) Check if the welding wheel pressure is reasonable. If the pressure is not enough, the contact resistance will be too large, and the actual current will decrease. Although the welding controller has a constant current control mode, the resistance increases beyond a certain range (generally 15%). , the current compensation limit will be exceeded, and the current cannot be increased correspondingly with the increase of the resistance, and the set value cannot be reached. In this case, an alarm will be issued when the system is working normally. In actual operation, if the exact cause of the virtual welding cannot be analyzed at one time, the head and tail of the steel strip can be cleaned up, the welding lap amount is increased, the welding current and the welding wheel pressure are appropriately increased, and the welding is performed once again. Pay close attention to the formation state of the weld, and in most cases, the problem can be handled urgently. Of course, if there are control system problems, or grid voltage fluctuations, etc., the weld seams must be cleaned and other measures must be taken to solve them.

Fifth, the method of solving the virtual welding

1) Judging the approximate fault range based on the occurrence of the fault phenomenon.

2) Appearance observation, focusing on larger components and components with large heat generation.

3) Magnifying glass observation.

4) Pull the board.

5) Shake the suspicious component by hand while observing whether the solder joints of the pins are loose.

Sixth, virtual welding prevention method

Take AZ1084D-3.3E1 as an example:

  1. Keep the tip cleaned AZ1084D-3.3E1

Because the energized electric iron tip is in a high temperature state for a long time, its surface is easily oxidized or burned, which deteriorates the thermal conductivity of the soldering iron tip and affects the welding quality. Therefore, the impurities on the iron head can be wiped with a damp cloth or a wet sponge. When the temperature is too high, the plug can be temporarily pulled off or the rosin is cooled down, and the tin on the soldering iron head is good at any time.

  2. Precautions on tin

If there are rust, dirt or oxide on the surface of the weldment and the solder joint, it should be scraped before soldering until the bright metal is exposed to tin the weldment or solder joint.

3. Welding temperature should be appropriate

In order to make the temperature appropriate, the appropriate electric soldering iron should be selected according to the size of the component. When the power of the selected soldering iron is constant, the length of the heating time should be controlled.

When the solder is automatically scattered from the soldering iron tip onto the soldered object, the heating time is sufficient. At this point, the tip is quickly removed and a smooth solder joint is left behind. If the soldering iron is removed, the solder is not left or left very little, indicating that the heating time is too short, the temperature is not enough or the solder is too dirty; if the solder is turned off before the soldering iron is removed, it indicates The heating time is too long and the temperature is too high. Generally, the temperature of the soldering iron tip is controlled to be the optimum soldering temperature when the flux is melted quickly and does not smoke.


4. Appropriate amount of tin According to the size of the required solder joints, the amount of tin solder of the soldering iron is determined, so that the solder is enough to wrap the soldered toughness to form a solder joint of suitable size and smoothness. The solder joints are not as good as tin and tin. On the contrary, the solder joints are more likely to be soldered. It is possible that the solder is deposited on top instead of soldered on it. If the amount of tin is not enough at one time, it can be repaired again, but it is necessary to remove the soldering iron after the tin on the previous one is melted together; if the amount of tin is too much at one time, use the tip to take the appropriate amount.


5. Welding time should be appropriate Proper use of welding time is also an important part of welding technology. If it is a soldering of printed circuit boards, it is generally 2 to 3S. The soldering time is too long, the flux in the solder is completely volatilized, the flux is lost, and the surface of the solder joint is oxidized, resulting in defects such as rough, black, non-bright, burr or flow of the solder joint. At the same time, the soldering time is too long, the temperature is too high, and it is easy to burn the copper foil of components or printed circuit boards. If the welding time is too short and the welding temperature is not reached, the solder cannot be fully melted, which affects the wetting of the flux and easily causes the solder joint.


6. Do not touch the solder joint during solder joint solidification Even if there is a small vibration before the solder joint is completely solidified, the solder joint will be deformed and the solder joint will be caused. Therefore, the weldment should be fixed before the tip is removed, such as by tweezers, or by quickly blowing the tip after the tip is removed. The purpose of these practices is to shorten the time for the joint to solidify.

What is the pcb and pcba virtual welding( cold solder joint/dry joint) and how to solve this problem during smt

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