5-step method for correct soldering with electric soldering iron
1. One scrape
Scraping means cleaning the surface of the metal to be welded before welding. You can use a knife, scrap steel saw blade, etc. to scrape off (or sand it off with fine sandpaper, or erase it with a coarse eraser) the oxide layer, oil stain or insulating paint on the welding surface. until the new metal surface is exposed. The self-made printed circuit board also needs to carefully polish the side of the copper clad foil with fine sandpaper or water sandpaper before soldering. "Scraping" is a key step to ensure the quality of welding, but it is often ignored by beginners. If the scraping is not in place, the tinning and welding will not be good. It should be noted that some component leads have been plated with silver, gold or tinned. As long as they are not oxidized or peeled off, there is no need to scrape them. If there is dirt on the surface, it can be wiped off with a rough eraser. The choice of rough eraser works best with a large eraser for drawing. Some gold-plated crystal triode pin leads, etc., will be difficult to plate with tin after the coating is scraped off. No matter what kind of "scraping" is adopted, it is necessary to pay attention to constantly rotating the component pins to clean all around the pins.
2. Second plating
Plating is to enamell the parts to be soldered. After "scraping" the component pins, wire heads and other soldering parts, an appropriate amount of flux should be applied immediately, and a thin layer of tin should be plated with an electric soldering iron to prevent the surface from re-oxidizing and improve the solderability of the components. sex. The plated solder layer is required to be thin and uniform, so the amount of tin on the tip of the soldering iron should not be too much each time. For components such as crystal diodes and transistors that are afraid of heat, you must use tweezers or needle-nose pliers to clamp the root of the lead pin to help dissipate heat, and then tin-plate it. Tinning of components is an important process step to prevent hidden dangers such as false soldering and false soldering in soldering technology, and it must not be sloppy.
3. Three tests
The test is to check the components that have been tinned to see if the components have scalding, deformation, lap welding (short circuit), etc. under the high temperature of the electric soldering iron. For components such as capacitors, transistors, and integrated circuits, a multimeter should be used to check whether their quality is reliable, and components that are found to be of unreliable quality or damaged must not be used again.
4. Four welding
Soldering is to solder the "test" qualified components to the printed circuit board or the designated position as required. When soldering, you must master the temperature and soldering time of the electric soldering iron. If the temperature is too low and the time is too short, the soldered tin surface will have burr-like tails, the surface is not smooth, or even bean curd residue. After all evaporated, a certain amount of flux is left between the solder and the metal. After cooling, the solder and the metal surface are stuck by the flux (rosin), and can be pulled apart with a little force. This is the so-called false soldering.
Furthermore, when the temperature of the electric soldering iron is too low, it is eager to solder, and the tin on the solder joint melts very slowly. Components are damaged (such as melting of capacitor plastic package, resistance change of resistors due to heat, etc.), especially transistors, which will be damaged when the core is heated above 100°C. Conversely, if the temperature of the soldering iron is too high and the soldering time is longer, the solder surface will be oxidized, the solder flow will spread out, and the solder joints will not eat enough tin. Only a small amount of solder will connect the component leads to the metal surface, and the contact resistance will be very low. If it is large, it will be disconnected as soon as it is pulled. This is the so-called virtual soldering. Whether the temperature of the electric soldering iron is suitable can be judged by experience according to the length of the tinning time of the soldering iron head and the amount of solder attached to the head. The length of welding time should ensure that the solder joints are smooth and bright, generally 2 to 3s, and the slightly larger solder joints should not exceed 5s. Welding transistors and other vulnerable parts are still the same as when tinning, use tweezers, needle-nose pliers, etc. to clamp the root of the pin to help heat dissipation.
In addition, the amount of solder should be appropriate. Do not use a large mass of solder to paste the solder joint. The outline of the lead wire can be vaguely distinguished from the tin surface of the solder joint, and the volcanic shape seen from the side of the solder joint is a qualified solder joint. point. When soldering with a hand-held soldering iron, do not use the soldering iron tip to rub the soldering surface back and forth or touch it hard. In fact, as long as the contact area between the tinned part of the soldering iron tip’s bevel and the soldering surface is increased, the heat can be effectively transferred from the soldering iron tip to the solder joint. part. It should be noted that after the soldering is completed and the soldering iron is removed, wait until the solder on the solder joint is completely solidified (4 to 5 seconds), and then loosen the tweezers or hands that fix the component, otherwise the lead wire of the welded part may come out, or the surface of the solder joint may It is like tofu residue. After welding, if you find that the tail of the solder joint is pulled out, use the tip of the electric soldering iron to dip the rosin, and then repair the welding to eliminate it.
If there are dross edges and corners, it means that the welding time is too long, and it is necessary to remove the debris and re-weld. The components on the printed circuit board should be suspended and then soldered. There should be a gap of 2 to 4mm between the component body and the circuit board surface, and it should not be close to the board surface. The transistor should be higher. For larger components, after inserting into the hole of the circuit board, the lead wire can be bent 90° along the direction of the circuit copper foil strip, leaving a length of 2mm and flattened before welding to increase the firmness. When welding high input impedance devices such as integrated circuits, if the reliable connection between the electric soldering iron shell and the ground cannot be guaranteed, you can use the residual heat to weld after unplugging the electric soldering iron power plug. When soldering printed circuit boards, it is also possible to insert the resistors first, and after point-by-point welding, use pliers or nail clippers to cut off the excess length of the leads, and then solder larger components such as capacitors, and finally solder them on. Heat-resistant and vulnerable transistors, integrated circuits, etc.
5. Five checks
The inspection is to check the welding quality of the welded circuit. There should be no false welding, virtual welding, open circuit and short circuit in the solder joints, especially whether the pins of polar components such as electrolytic capacitors and transistors are welded correctly. The quality of welding can be judged by the color and gloss of the solder joints, the degree of diffusion, and the amount of solder. Good welding, the solder joints have a unique bright white luster, which can be seen at a glance based on experience; if the color and luster of the solder are stained or the surface is uneven, it indicates poor welding.
Excessive solder accumulation not only fails to achieve the expected purpose of increasing mechanical strength, but also creates the risk of virtual soldering and collision with nearby solder joints (short circuit). It is the case that the amount of solder is insufficient. In this case, it is not easy to see any defects at the initial stage of welding, but after a period of time, it may fall off due to vibration or shaking. For problematic bad solder joints, repair welding measures should be taken to make the welding quality satisfactory.
