Differences and selection of soldering iron power sizes
The power of the soldering iron used is too high, which can easily burn the components (generally, the junction temperature of the diode and transistor will burn out when the junction temperature exceeds 200°C) and cause the printed wires to fall off the substrate; the power of the soldering iron used is too small, and the soldering tin cannot be sufficient When melted, the flux cannot volatilize, and the solder joints are not smooth and firm, which can easily lead to false soldering. Generally, it is used for welding integrated circuits, printed circuit boards, CMOS circuits, decoration transistors, IC radios and recorders, and televisions. It is generally used for ordinary circuit experiments. Generally, 20W is suitable for repairing vacuum tube machines, such as tube amps and old instruments. , 35W is appropriate, and the external heating type is 45W. For welding the wiring of the large transformer and the grounding trunk line on the metal base plate, the internal heating type is 50W, and the external heating type is 75W. If you want to weld metal materials, you should use an external heating soldering iron with a power rating of 100W or more. If conditions permit, amateur radio enthusiasts can equip themselves with a 2OW internally heated soldering iron, a 35W internally heated or externally heated soldering iron, and a 150W externally heated soldering iron, which can basically meet various welding needs.
The solder we use is generally divided into two types: leaded solder and lead-free solder, but the most commonly used is lead-containing solder, which has a composition of 63% tin, 37% lead, and a melting point of 183°C: while the composition of lead-free solder is 99% tin. , the flux is about 1%, and the melting point is 227°C. The advantage of leaded solder is that it has a low melting point, is easy to weld, and is cheap. However, it is not environmentally friendly and lead is harmful to the human body. Therefore, you must wash your hands carefully after soldering. During the soldering process, it is best to wear a mask or use a brightly lit place to ensure There is a certain distance between the head and the weldment. As people's awareness of environmental protection increases, lead-free solder is now used for machine welding in factories. Since lead-free solder has a higher melting point, it is not difficult to understand why it is sometimes difficult to melt the solder when repairing imported electrical appliances.
The electric soldering iron is an electric heating device that can generate a high temperature of about 250°C when energized. During the welding process of the electric soldering iron, it is actually a process of heat conduction. When it contacts the welding surface, the heat from the soldering iron head is transferred to the solder. The solder absorbs the heat, melts and flows, and forms a bright and round solder joint under the action of surface tension. . During the welding heat conduction process, because metals are good conductors of heat, heat transfers quickly. During the melting process of solder, due to the heat loss of the soldering iron tip, its temperature will drop more or less. If the solder joint area is large, it needs to absorb more heat to make the solder on it reach the melting point. If the soldering iron head is smaller in size, it stores less heat and the temperature drops quickly. Due to the small power of the soldering iron core, the heat generated is too late to replace the lost heat. The most intuitive phenomenon at this time is that the solder does not melt or the melting is incomplete. In this case, we need to use a high-power soldering iron for soldering. On the contrary, if the welding parts are small, we do not need to use a high-power soldering iron; if a high-power soldering iron is used, be sure to pay attention to the welding time, otherwise too much heat can easily cause damage to the circuit and circuit board where the current flows. , causing the printed copper foil to peel off. There are no specific quantitative requirements for the specific power of a soldering iron. The long-term work experience accumulation of maintenance personnel is the best way to choose a soldering iron that suits you.
Let us explain two common problems again to give us a deeper understanding of the principles of weldments. First of all, why use flux (such as rosin)? In fact, to put it bluntly, the purpose of using flux is to prevent the phenomenon of virtual soldering. The most direct function of flux is to make the solder flow easily. It is not difficult to understand that smooth-flowing solder will fill every A tiny hole; in addition, the flux can also remove the oxide layer on the surface of the soldering piece. Of course, some fluxes are corrosive (such as solder paste) and should be used with caution. In order to improve the welding effect and ensure quality, rosin is added to the finished solder wire, which is more convenient to use.
The second question is, why should we prevent the soldering iron tip from being burned? The soldering iron tip is burned because the soldering iron is not used for a long time, causing a black oxide layer to form on the soldering iron tip. This black oxide layer is the oxidation product of copper, solder and other metals. Metal oxides are poor conductors of heat and prevent the conduction of heat. Moreover, since the soldering iron tip is oxidized and does not stick to the solder, the solder contact surface between the soldering iron tip and the weldment is reduced, further preventing the transfer of heat. Therefore, after the soldering iron tip is burned, the welding function of the soldering iron is basically missing, and the remaining function is only the heater function. In order to prevent the soldering iron tip from being burned, it is key to control the long-term high-temperature empty burning of the soldering iron. If the soldering iron is not used for a long time during maintenance work, the power should be cut off to stop the soldering iron. Of course, it is also a good idea to install a control circuit to reduce the temperature of the soldering iron when it is idle.
