What problems should be paid attention to when using soldering iron welding in the maintenance process
1. Selection of electric soldering iron
There are many types and specifications of electric soldering irons, and the size and requirements of the electronic components to be soldered are different. Therefore, the reasonable selection of the power and type of the electric soldering iron has a direct relationship to improving the welding quality and efficiency. During the soldering process, heat is provided by a soldering iron. Only when the solder joint absorbs enough heat so that the temperature of the soldering area melts the solder and the flux is well volatilized, can there be a firm and smooth solder joint. If the power of the electric soldering iron is too large, too much heat will be transferred to the welding workpiece, and the overheating of the solder joints of the components will cause damage to the components, and welding defects such as copper peeling off of the printed circuit board.
In actual use, special care should be taken not to think that the lower the power of the soldering iron, the less likely it will be to burn components. Take welding high-power triode as an example, if you use a low-power electric soldering iron, it cannot supply enough heat quickly after it contacts with the component, and when the solder joint does not reach the welding temperature and the soldering iron stays for a longer time, the heat will spread to the entire On the triode, it is very easy to make the temperature of the die reach the level of damage.
When soldering various components, you can refer to the following table to select the configured electric soldering iron.
1. General printed circuit board, installation wire: 250℃~350℃, 20W internal heating type, 30W external heating type.
2. Integrated circuit: 250℃~350℃, 20W internal heat type, constant temperature type, energy storage type.
3. Solder lugs, potentiometers, 2~8W resistors, high-power tubes: 350℃~450℃, 30~50W internal heating type, temperature adjustment type 50~75W external heating type.
4. Larger components such as large resistors above 8W, wires above 2A: 400℃~550℃, 100W internal heating type, 150~200W external heating type.
5. Metal plates, etc.: 500℃~630℃, external heat or flame soldering above 300W.
6. Maintenance and debugging of general electronic products: 250℃~350℃, 20W internal heat type, constant temperature type, induction type, energy storage type, dual-purpose type.
Precautions for using electric soldering iron
1. When the electric soldering iron is not used for a long time, the power should be turned off in time. To avoid accelerated oxidation of the soldering iron core and shorten the service life;
2. Before turning off the soldering iron, tin the tip of the soldering iron. To protect against accelerated oxidation.
3. Common faults and maintenance of electric soldering irons. The common faults of electric soldering irons are: the electric soldering iron does not heat up after being energized, the soldering iron tip does not "eat" tin, and the soldering iron is charged, etc. Now take the internal heating 20W electric soldering iron as an example to illustrate.
A. The soldering iron does not heat up after being powered on:
Measure the two ends of the plug with the ohm block of the multimeter to check whether there is an open circuit fault;
If there is no plug open circuit fault, then use a multimeter to measure the lead wires at both ends of the soldering iron core. If the needle does not move, replace it with a new soldering iron core;
If the resistance value of the two lead wires of the soldering iron core is about 2.5KΩ, it means that the soldering iron core is intact, it is very likely that the lead wires are broken, and the joint in the skewer is disconnected.
B. The tip of the soldering iron is charged
The power line is wrongly connected to the terminal of the grounding line;
The power cord falls off from the soldering iron core connection screw and then hits the grounding wire screw, causing the soldering iron head to be electrified;
Leakage caused by winding of power lead wires; leakage of power ground wire itself.
C. The tip of the soldering iron does not "eat" tin
After the iron tip is used for a long time, it will not "eat" tin due to oxidation, and a new soldering iron tip should be replaced;
The tip of the soldering iron does not heat up and does not eat tin. For the solution, please refer to "The soldering iron does not heat up after being energized".
4. Welding operation essentials
A. Preparation before welding
Materials: including direct materials and auxiliary materials, pay attention to whether the welding components have polarity requirements, whether the component feet are oxidized, oily, etc., and whether there are special requirements for welding temperature and time during welding;
Tools and utensils: depending on the welding components, there should be tin wire holders, component boxes, welding torches, soldering stations, tweezers, cutting pliers, etc. If there are anti-static requirements, anti-static tools and appliances should be used, and the operator should wear an anti-static wrist strap;
B. Implement welding
Prepare solder wire and soldering iron tip, and keep the soldering iron tip clean;
Heating the weldment (heating component feet and pads at the same time);
Melting solder: When the weldment is heated to a temperature that can melt the solder, put the tin wire on the solder joint, and the solder begins to melt and wet the solder joint;
After adding the appropriate solder to the solder joint, remove the tin wire;
When the solder has completely wetted the joint, remove the soldering iron at a roughly 45° angle.
The above process is completed in about 2 to 3 seconds for general solder joints. It should be noted that the connected components should not be shaken before the solder is completely solidified to avoid false soldering.
C. Treatment after welding
When the soldering is finished, check for missing soldering, wrong soldering (reversed polarity soldering), short circuit, virtual soldering, etc., and clean up residues on the PCBA such as: tin slag, tin chips, component feet, etc.
5. Welding of several vulnerable components
A. Welding of cast shell components
Electronic components made of thermoplastic. Such as micro switches, connectors, etc., which cannot withstand high temperatures, if the heating time is not controlled, it is very easy to cause plastic deformation, resulting in component failure or performance degradation and hidden failures. Therefore, the following items should be paid attention to when actually welding such components:
Tin the component feet in advance to facilitate soldering and shorten the soldering time;
When soldering, it is necessary to weld one foot by one foot to shorten the heating time of the component feet;
The tip of the soldering iron must not exert pressure on the component feet;
It is more appropriate to choose tin wire with a small amount of flux, and the diameter of the tin wire is ф0.6-0.8mm;
Do not touch the components before the plastic case is cold.
B, FET and integrated circuit welding
MOSFET, especially the insulated gate type component, because of its high input impedance, a little carelessness can cause its internal breakdown and fail. Bipolar integrated circuits, due to the high internal integration, but the tube isolation layer is very thin, and it is easily damaged once it is subjected to excessive heat. None of the above types of circuits can withstand a temperature of 250°C. Therefore, the following items should be paid attention to when welding:
The welding time should not exceed 3 seconds, and should be as short as possible under the premise of ensuring wetting;
Use anti-static constant temperature soldering iron, the temperature is controlled at 230 ° C ~ 250 ° C;
The power of the electric soldering iron should not exceed 20W for the internal heating type and 30W for the external heating type;
If the integrated circuit does not use a socket, when it is directly soldered to the PCB board, the safe welding sequence is ground terminal → output terminal → power terminal → input terminal.
