As an operation technique, manual soldering can only be mastered through practical training, but following the basic principles, learning from the accumulated experience of predecessors and using the correct methods can get twice the result with half the effort. The following points are very important for learning welding technology.
Basic conditions of tin welding
1. Weldability of weldments
Not all materials can be connected by welding. Only some metals with good weldability (strictly speaking, they should be weldable) can be connected by soldering. Generally, copper and its alloys, such as gold, silver, zinc and nickel, have good weldability, while aluminum, stainless steel and cast iron have poor weldability.
Generally speaking, special flux and method are needed for welding.
2. Solder is qualified
Lead-tin solder whose composition is not up to standard or whose impurities exceed the standard will affect the quality of solder, especially some impurities, such as zinc, aluminum and cadmium. Even the content of 0.00 1% will obviously affect the wettability and fluidity of solder and reduce the welding quality. Obviously, no matter how clever a chef is, it is impossible to process delicious food with inferior raw materials.
3. Appropriate flux
Different fluxes should be used to weld different materials. Even for the same material, when different welding processes are used, such as manual soldering iron welding and immersion welding, different fluxes are often used, and different fluxes are also needed for post-welding cleaning and no-cleaning. For manual welding, rosin and active rosin can meet the assembly requirements of most electronic products. It should also be pointed out that the amount of flux must also be paid attention to, too much or too little is not conducive to tin welding.
4. The solder joint design is reasonable
Reasonable geometry of solder joints is very important to ensure the quality of solder joints. As shown in figure 1 (a), it is difficult to ensure sufficient strength of solder joints due to the limited strength of lead-tin materials, and the joint design in figure 1 (b) has been greatly improved. Fig. 2 shows the influence of different leads and hole sizes of through-hole mounting components on the printed circuit board on the welding quality.
Key points of manual welding
The following points are derived from the welding mechanism and proved to be universally applicable by practical experience.
1. Grasp the heating time.
When welding, we can use different heating rates, such as the shape of the welding head is not good, and when welding large weldments with a small soldering iron, we need to extend the time to meet the requirements of solder temperature. In most cases, extending the heating time is harmful to the assembly of electronic products because
The bonding layer of (1) solder joint is heated for a long time and exceeds the appropriate thickness, which leads to the deterioration of solder joint performance.
(2) Printed boards, plastics and other materials will be deformed and deteriorated if heated too much.
(3) The performance of the element changes or even fails after being heated.
(4) Because the flux volatilizes and loses protection, the surface of solder joint is oxidized.
Conclusion: The shorter the time, the better, on the premise of ensuring solder wetting the weldment.
2. Keep proper temperature.
If high-temperature soldering iron is used to weld solder joints in order to shorten the heating time, it will bring another problem: the flux in solder wire does not have enough time.
Welding surface overflow and premature volatilization failure; The fast melting speed of solder affects the function of flux; Because the temperature is too high, although the heating time is short, it also causes overheating.
Conclusion: The welding head should be kept in a reasonable temperature range. The general experience is that the welding head temperature is 50℃ higher than the melting temperature of solder.
The ideal state is to shorten the heating time at a lower temperature. Although this is contradictory, we can get a satisfactory solution through operation in practice.
3. It is harmful to apply force to the solder joint with the welding head.
The welding head mainly transfers heat to the solder joint by increasing the contact area, and it is futile to apply force to the solder joint with a soldering iron. In many cases, it will cause damage to the welded parts. For example, the welding points of potentiometers, switches and connectors are often fixed on plastic components, and the result of force is easy to cause the original failure.
Key points of welding operation
1. Surface treatment of weldments
The weldments encountered in manual soldering iron welding are all kinds of electronic parts and wires. Unless the electronic components within the "warranty period" are used in mass production, the surface of the weldment often needs to be cleaned to remove rust, oil stain, dust and other impurities that affect the welding quality. Manual operation often uses mechanical scraping and alcohol and acetone scrubbing.
2. Pre-welding
Pre-welding refers to wetting the lead or conductive welding part of the component to be welded with solder in advance, which is also commonly known as tin plating, tin plating and tin lining. It is accurate to call it pre-welding, because its technological mechanism is the whole process of welding-solder wets the surface of the weldment, and forms a bonding layer through metal diffusion, so that the surface of the weldment is "plated" with a layer of solder.
Pre-welding is not an essential operation for welding, but it is almost essential for manual soldering iron welding, especially for maintenance, debugging and development.
3. Don't use excessive flux.
Appropriate flux is essential, but don't think that the more the better. Too much rosin not only causes a heavy workload of cleaning around the solder joint after welding, but also prolongs the heating time (rosin needs to be melted and volatilized to take away heat), which reduces the working efficiency; However, when the heating time is insufficient, it is easy to mix with solder to form the defect of "slag inclusion"; For the welding of switching elements, too much flux tends to flow to the contacts, resulting in poor contact.
The appropriate welding dose should be that rosin water can only wet the solder joint to be formed, and it cannot flow through the printed board to the component surface or socket hole (such as IC socket). For the welding wire with rosin core, there is basically no need to apply flux.
4. Keep the welding head clean
Because the welding head is in a high temperature state for a long time during welding, it is in contact with substances decomposed by heat, such as flux, and its surface is easily oxidized to form a layer of black impurities, almost forming a heat insulation layer, which makes the welding head lose its heating effect. So always wipe off the impurities on the soldering iron frame. It is also a common method to wipe the welding head with a wet cloth or sponge at any time.
5. Heating depends on solder bridge.
In non-assembly line operation, there are many shapes of solder joints welded at one time, so it is impossible for us to change the welding head often. In order to improve the heating efficiency of the welding head, it is necessary to form a solder bridge for heat transfer. The so-called solder bridge is to reserve a small amount of solder on the soldering iron as a bridge for heat transfer between the welding head and the weldment when heating.
Obviously, because the thermal conductivity of molten metal is much higher than that of air, the weldment is quickly heated to the welding temperature, as shown in Figure 4. Pay attention to the amount of tin reserved as solder bridge.
6. The amount of solder should be appropriate.
Too much solder not only consumes more expensive tin unnecessarily, but also increases the welding time and reduces the working speed accordingly. More seriously, in high-density circuits, excessive tin can easily cause imperceptible short circuits.
But too little solder can't form a firm bond, which reduces the strength of solder joints, especially when welding wires on the board. Insufficient solder often causes wires to fall off.
7. Welded parts shall be firm.
Do not move or vibrate the weldment before the solder solidifies, especially when clamping the weldment with tweezers, be sure to wait until the solder solidifies before taking out the tweezers. This is because the solidification process of solder is a crystallization process. According to the crystallization theory, the external force (the movement of the weldment) in the crystallization process will change the crystallization conditions and produce coarse crystals, which is called "cold welding". Appearance phenomenon is dull surface and bean dregs; The internal structure of solder joint is loose, and air gap and crack are easy to appear, which leads to the decrease of solder joint strength and poor conductivity. Therefore, before the solder solidifies, the weldment must remain still, and various suitable methods can be used to fix the weldment or reliable clamping measures can be adopted in actual operation.
8. Pay attention to the evacuation of soldering iron
The soldering iron should be handled in time, and the angle and direction of evacuation have a certain relationship with the formation of solder joints.
When the soldering iron is removed, a slight rotation can keep the proper solder in the solder joint, which needs to be realized in actual operation.
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① Flux
Flux can be generally divided into inorganic flux, organic flux and resin flux, which can dissolve the oxide on the metal surface, surround the metal surface during welding and heating, isolate it from the air, and prevent the metal from oxidation during heating; The surface tension of molten solder can be reduced, which is beneficial to the wetting of solder.
② solder mask
The solder is limited to be welded only at the required welding points, covering the board surface part of the printed circuit board that does not need to be welded, so as to protect the panel from thermal shock and foam, and prevent bridging, sharp drawing, short circuit and virtual welding.
When using flux, it must be used appropriately according to the area and surface state of the workpiece to be welded. If the dosage is too small, it will affect the welding quality. If the dosage is too large, the flux residue will corrode components or make the insulation performance of the circuit board worse.
Basic requirements for welding points
1, the solder joint should have enough mechanical strength to ensure that the welded part will not fall off or loosen when subjected to vibration or impact. Too much solder can't be piled up, which easily leads to virtual welding and short circuit between solder joints.
2, reliable welding, good conductivity, must prevent virtual welding. Virtual welding means that no alloy structure is formed between the solder and the surface of the workpiece. Simply attached to the surface of the welded metal.
3, solder joint surface should be smooth, clean, solder joint surface should have good luster, there should be no burr, cracks, no dirt, especially the harmful residue of flux, to choose the appropriate flux and flux.
Basic operation method of manual welding
"Preparation before welding.
Prepare tools such as electric soldering iron, tweezers, scissors, diagonal pliers, needle-nosed pliers, soldering tin, flux, etc. Tin the electric soldering iron and weldments, with soldering tin in your left hand and electric soldering iron in your right hand, and keep them in a solderable state at any time.
"Use a soldering iron to heat spare parts.
"Send solder, melt the right amount of solder.
"Remove the solder.
"When the solder flows to the covered solder joint, quickly remove the soldering iron.
Master welding temperature and time. There should be enough heat and temperature when welding. If the temperature is too low, the fluidity of solder is poor and it is easy to solidify, forming a virtual solder; If the temperature is too high, the solder will flow, the solder joint will not be easy to store tin, and the decomposition speed of flux will be accelerated, which will accelerate the oxidation of the metal surface and lead to the pad falling off on the printed circuit board. Especially when natural rosin is used as flux, the brazing temperature is too high, which is easy to oxidize and peel, resulting in carbonization and virtual welding.
Tin welding method of aluminum parts
Aluminum is easily oxidized, and its surface is usually covered with an alumina film. Even if this film is scraped off before welding, due to the high temperature of the soldering iron during welding, an oxide film will be quickly formed on the welding surface, so that the scraped new surface will not contact with air, thus making tin adhere to aluminum. There are two ways to weld aluminum parts.
1, first polish the welding surface of aluminum parts with sandpaper, and put some rosin and iron powder. Use a soldering iron with power greater than 60W, dip enough solder on the welding surface, and rub hard. Due to the action of iron powder, the oxide layer is worn off and tin adheres to the aluminum surface. When the tin is not solidified, wipe off the iron powder on the welding surface and soldering iron with a cloth, and welding can be carried out according to the ordinary method.
2. Apply a layer of mercury nitrate solution on the welding surface of aluminum wire or aluminum plate to be welded. Due to chemical action, a layer of aluminum amalgam is formed on the surface of aluminum, which can be welded after washing with water. The newly welded tin is welded on the aluminum amalgam, and the welding strength is not high. Therefore, soldering iron of 100W should be used for welding, and welding head mostly stays on the welding surface, so that mercury diffuses in aluminum, and tin can be firmly welded with aluminum matrix, thus strengthening the welding strength.