Precautions for using electric soldering iron

Precautions for using electric soldering iron
1、 Selection of electric soldering iron
There are many kinds and specifications of electric soldering iron, and the size and requirements of the electronic components to be welded are different. Therefore, the reasonable selection of power and type of electric soldering iron is directly related to the improvement of welding quality and efficiency. In the welding process, the heat is provided by the electric iron. Only when the solder joint absorbs enough heat to make the temperature of the welding area melt the solder and the flux volatilizes well, can the solder joint be firm and smooth. If the power of the electric iron is too high, too much heat will be transferred to the welding workpiece, and the solder joints of the components will be overheated, which will cause damage to the components, and the copper skin of the printed circuit board will fall off and other welding defects.
In actual use, special attention should be paid not to think that the smaller the power of the soldering iron, the less components will be burned. Take the welding of high-power triode as an example. If a low-power electric soldering iron is used, it can not supply enough heat quickly after contacting with the components. When the solder joint can not reach the welding temperature and the residence time of the soldering iron is prolonged, the heat will be transferred to the whole triode, which is very easy to damage the core temperature.
When welding different components, the electric soldering iron can be selected according to the table below.
1. General printed circuit board, installation wire: 250 ℃ ~ 350 ℃, 20W internal heating type, 30W external heating type.
2. Integrated circuit: 250 ℃ ~ 350 ℃, 20W internal heating type, constant temperature type, energy storage type.
3. Solder pad, potentiometer, 2 ~ 8W resistor, high power tube: 350 ℃ ~ 450 ℃, 30 ~ 50W internal heating type, temperature regulating type, 50 ~ 75W external heating type.
4. Larger components such as 8W or above large resistance, 2A or above conductor: 400 ℃ ~ 550 ℃, 100W internal heating type, 150 ~ 200W external heating type.
5. Metal plate: 500 ℃ ~ 630 ℃, 300W or above external heating or flame soldering.
6. Maintenance and debugging of general electronic products: 250 ℃ ~ 350 ℃, 20W internal heating type, constant temperature type, induction type, energy storage type and dual-purpose type.
2、 Precautions for using electric soldering iron
1. When the electric iron is not used for a long time, turn off the power in time. In order to avoid the accelerated oxidation of iron head and iron core and shorten the service life;
2. Before turning off the electric iron, tin should be hung on the iron head. To protect against accelerated oxidation.
3. The common faults and maintenance of electric soldering iron, the common faults of electric soldering iron are: the electric soldering iron does not heat after being electrified, the soldering iron head does not "eat" tin, the soldering iron is electrified, etc.
A. No heating after the soldering iron is electrified:
Measure the two ends of the plug with the ohm gear of the multimeter to check whether there is an open circuit fault;
If there is no plug open circuit fault, use a multimeter to measure the leads at both ends of the iron core. If the needle does not move, replace it with a new iron core;
For example, the resistance of the two leads of the iron core is 2.5k Ω If the soldering iron core is in good condition, it is very likely that the lead wire is broken and the connector in the cutting head is broken.
B. The iron head is charged
The power line is wrongly connected to the terminal of the grounding wire;
After the power line falls off from the connecting screw of the iron core, it touches the screw of the grounding wire, causing the iron head to be charged;
Leakage caused by winding of power leads; There is leakage in the power ground wire itself.
C. Soldering iron does not eat tin
After a long time of use, the iron head will not eat tin due to oxidation, so it should be replaced with a new one;
The soldering iron head does not heat but does not eat tin. For the treatment, please refer to "electric soldering iron does not heat after being electrified".
4. Welding operation essentials
A. Preparation before welding
Materials: including direct materials and auxiliary materials, pay attention to the polarity requirements of the welding elements, whether there are oxidation and oil stains on the element feet, and whether there are special requirements for the welding temperature and time during welding;
Tools and instruments: depending on the welding elements, there should be tin wire holder, element box, welding gun, welding table, tweezers, scissors, etc. If there are anti-static requirements, anti-static tools and appliances shall be used, and the operator shall wear anti-static wrist strap;
B. Implement welding
Prepare solder wire and soldering iron head, and keep them clean;
Heating weldment (simultaneously heating element pin and pad);
Melt solder: when the solder is heated to a temperature that can melt the solder, the solder wire is placed on the solder joint, and the solder begins to melt and wet the solder joint;
After adding proper solder to the solder joint, remove the solder wire;
When the solder is completely wetted, the temperature is about 45 ℃ ° Remove the soldering iron at the right angle.
The above process for general solder joints is completed in about 2-3 seconds. It should be noted that before the solder is completely solidified, do not shake the connecting element to avoid false soldering.
C. Treatment after welding
When the welding is finished, check whether there are missed welding, wrong welding (reverse polarity welding), short circuit, false welding and other phenomena, and clean up the residues on PCBA board, such as tin slag, tin chips, component feet, etc.
5. Welding of several vulnerable components
A. Welding of cast plastic shell components
Electronic components made of thermoplastics. Such as micro switch, connector, etc., can not withstand high temperature. If the heating time is not controlled, it is easy to cause plastic deformation, resulting in component failure or performance degradation, resulting in hidden failure. Therefore, the following matters should be paid attention to when welding such components in practice:
In order to facilitate welding and shorten the welding time, tin is hung on the component feet in advance;
When welding, it needs to weld one foot at a time, which shortens the heating time of the element foot;
The soldering iron head shall not exert pressure on the component foot;
The tin wire with small flux content and the diameter of the tin wire is 0 ф 0.6-0.8mm is suitable;
Do not touch the pressure element before the shell is cold.
B. FET and IC welding
Because of its high input impedance, MOSFETs, especially insulated gate elements, can break down and fail if they are not careful. Bipolar integrated circuit, due to the high degree of internal integration, tube isolation layer is very thin, once subjected to excessive heat is also easy to damage. None of the above circuits can withstand the temperature of 250 ℃. Therefore, the following points should be noted during welding:
The welding time should not exceed 3 seconds and should be as short as possible on the premise of ensuring wetting;
The anti-static constant temperature soldering iron is used, and the temperature is controlled between 230 ℃ and 250 ℃;
The power of electric soldering iron shall not exceed 20W for internal heating type and 30W for external heating type;
If the integrated circuit does not use socket and is directly welded on the PCB board, its safe welding sequence is ground terminal → output terminal → power terminal → input terminal.
C. Common welding defects, causes and solutions.
Serial number
Welding defects
Causes
terms of settlement
one
short circuit
1．
Pad spacing is too close.
2．
The direction of dragging tin is different from that of walking tin.
3．
The solder is not fully melted.
1．
Add white oil screen printing between pads.
2．
Change the drag direction.
three
Increase temperature, extend long time, apply pine perfume.
two
Tin point
1．
If the welding time is too long, the solder viscosity will increase.
2．
The welding technique is not skilled, and the angle of soldering iron is not right.
3．
The solder is not fully melted.
1．
Shorten the time and add proper amount of pine perfume.
2．
About 45 ° The direction of the iron.
3．
Increase temperature, extend long time, apply pine perfume.
three
Tin coating
1．
Too much solder.
2．
The pad is not heated enough to wet.
3．
The element feet are not heated enough to wet.
1．
Add appropriate amount of tin.
2．
The soldering iron head should fully contact the pad and be heated.
3．
Add appropriate amount of tin.
four
Tin cave
1．
The aperture of PCB is large, or the center of pad is separated.
2．
Oxidation and uncleanness around the pad.
3．
Insufficient tin.
1．
Replay the board.
2．
Wipe the pad with alcohol.
3．
Add appropriate amount of tin.
five
Copper skin
1．
The welding temperature is too high and the welding time is too long.
2．
The power of the soldering iron is too high.
3．
When the component is disassembled and soldered, the solder is not melted and the component is pulled.
1．
Lower the temperature and shorten the time.
2．
Choose the right soldering iron.
3．
After the solder melts, take out the component.