A soldering iron has to rapidly heat metal parts above the temperature commonly used in electrical and electronic work — 60/40 or 63/37 tin/lead melts between three hundred sixty and three hundred seventy degrees — in order to make good connections. The solder you apply to the joint will melt and flow smoothly after it has been quickly heated. If your soldering iron heats too slowly the heat will be able to transfer to your components (resistor, capacitor, etc.) which can cause them to overheat and become damaged and, if you’re soldering insulated wire, too-slow heating can cause the insulation to weaken or melt.
Soldering tip temperatures are generally set between seven hundred fifty and eight hundred fifty degrees so that the temperature of the solder will raise above its melting point. Given that most solders have melting points below four hundred degrees you might be wondering why the soldering tip gets so hot. The answer is that using a higher temperature stores heat in the tip, thus speeding up the melting process; this enables you to solder your connections without applying excessive pressure on the joint. In addition, these high temperatures allow the proper formation of intermetallic layering between the components and solder to form, which is crucial for creating reliable electrical and mechanical solder joints.
Since we’ve established that the temperature is perhaps the most important aspect of choosing a soldering iron or soldering station, the next point of confusion is that soldering irons and stations are rated in watts rather than degrees. Most inexpensive soldering irons are actually unregulated, which means that the temperature of the tip isn’t controlled; they don’t advertise a temperature because the tip’s temperature will significantly change during use. The following data on certain unregulated soldering irons (fifteen, twenty-five, and forty watt) will shed some light on why choosing the right wattage is important.
A fifteen watt soldering iron has a resting temperature of roughly five hundred forty degrees Fahrenheit. However the temperature will drop to around four hundred twenty degrees after briefly wiping the tip on a damp sponge and soldering a couple PC board pads. This happens because the iron’s fifteen watt heat-storage capacity can’t maintain its resting temperature during use — it doesn’t have the capacity or the ability to restore temperature so it quickly cools when used. You can work around this limitation by allowing some rest periods between soldering joints, but if you’re doing work that requires more heating power — e.g., tinning a stranded wire — a fifteen watt iron won’t have enough power to get the job done.
A twenty-five watt iron has a resting temperature of around six hundred forty degrees and will retain much of its resting temperature (i.e., over six hundred twenty degrees) after soldering more than ten PC board pads. With regard to tinning wire, a twenty-five watt soldering iron can handle fourteen gauge wire well, but it lacks the power to tin ten or twelve gauge wire. If you tried to tin a ten gauge wire you can get the iron’s tip hot enough to melt solder in around two minutes, but by that time the insulation is hot enough to melt as well. The goal is to heat the surfaces being soldered, so we don’t want to heat the surfaces for more than a couple seconds or we risk damaging components and wires.
A forty watt soldering iron’s resting temperature is roughly seven hundred forty degrees and will keep a tip temperature of over seven hundred degrees through repeated PC pad solders. While a forty watt iron has enough power to easily tin twelve and fourteen gauge wire, ten gauge wire will still be on the slower side. Lower wattage soldering irons and soldering stations can really slow down your work and may not be suited to the electrical or electronic work that you’ll be doing.
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