A lecture on Soldering
6. Tin soldering iron
The purpose of using a soldering iron is to heat the mother metal to be worked on and the wire soldering tin to the optimum temperature about 250℃. The temperature at a soldering iron is normally 340℃～400℃. The heat at the iron head is passed to the mother metal instantly at the moment the iron head contacts the mother metal, with temperature at the head then decreasing dramatically. The temperature decreasing multitude depends on the thermal capacities of both the iron and the mother metal.
A soldering iron with a low thermal capacity can never heat a large mother metal to the temperature to 250℃, no matter how long the heating lasts. Contrarily, if an iron with a high thermal capacity is applied on a mother metal with a low thermal capacity, the temperature will exceed 250℃ instantly and as a result, a good alloy layer cannot form properly due to the overheat. So logically, an iron with a high thermal capacity is used for working a big mother metal, while an iron with a low thermal capacity applied on a small mother metal. It is important to be aware that a big iron cannot take the place of a small one.
6.1 Thermal capacity
The term “thermal capacity” is often used in soldering. What does it mean after all? What thermal capacity means to heat is something like cubic volume to water, to make it simplified to be understood.
But normally, the temperature in an object is by no means to be seen by our eyes, say, nobody could state that he has seen the temperature of an object is 120℃. And neither is true that somebody would state, “I see the energy which can heat 30g of copper to the temperature of 200℃.” In other word, thermal capacity is a means to measure the energy and, let’s make it more simplified: it means how much heat in an object.
When we say the thermal capacity in soldering,
it refers to the heat at an iron head;
how much heat is needed to fuse mother metal and soldering tin.
Usually we use “high” or “low” to describe the capacity measurement.
So the larger the size of an iron head or a mother metal is, the higher the thermal capacity is needed, in simple direct proportion.
Now let’s compare the iron to a kettle with hot water, and the mother metal to the bath. Then the kettle is compared to an iron head and the heater to the flame to heat the water in the kettle. The contact area of the iron head with the mother metal is compared to the outlet of the hot water kettle.
It can be easily imagined from the above metaphor that if the kettle is too small, the water in the bath won’t be hot enough. However, if the kettle is too large, the water will be too hot to take a bath in. On the other hand, if the outlet is too small, the water in the bath still cannot be hot enough, even if the kettle is large enough. The work of a heater is to continuously heat water while a right size of the kettle is more important. In the same principle, the right size of an iron head is more important than the wattage (power) of a heater. And it also tells us that it is next to impossible to heat the mother metal to a suitable temperature if the temperature at the iron head is set at an invariable state regardless of the size of the mother metal.
So no matter how advanced the iron is, the optimum working result cannot be assured unless the iron head size, the mother metal size and their contact methods are taken into consideration. They are more than necessary conditions.
6.2 Kinds of tin soldering irons (heater)
The iron head is heated by a surrounding nickel-chromium coil. Its heat efficiency is not very ideal as the heat is also dissipating to the surrounding air, which, as a result, requires a heater with large wattage and, in turn, the iron must be big in size. Furthermore, as the temperature at the head is very difficult to control, it is set at 400 or above in most cases, which shortens the life expectancy of the iron head. These are often-used cheap soldering irons available in the household article.stores.
A ceramic heater is able to pass all its heat to the iron head efficiently since its heating starts inside the head. This kind of heater is commonly employed in soldering irons recently thanks to its quick heating feature.
The ceramic heater is also employed in temperature-controllable soldering irons. It is more convenient to change the head.
6.3 Kinds of soldering irons with temperature control
Heat saturated soldering irons
It is commonly believed if a non-temperature-control iron is plugged in a power socket for a long while, the temperature of the iron head is supposed to rise steadily. But it’s not true.
Take a balloon with a relief valve as an example. When it swells to a certain size after it has been filled with water, it will maintain the size because the input water is equal to the out-flowing water through the relief valve. It is like the maximum saturated temperature maintained at the iron head. The out-flowing water from the balloon is just like the heat at the iron head dissipating to the surrounding air.
A temperature-control iron
A temperature-control iron with a thermal transducer and thermal regulator controls power “ON” and “OFF” of the iron heater. It is like the above balloon example controlling the balloon size by controlling the input water volume. In the case of a temperature saturated iron, it is like a balloon with a tap at maximum volume to freely make it swelled. But, for a temperature-control iron, there is still some heating capacity margin in case the heat with the iron head dissipates and needs to be made up.
A temperature-control iron has a sufficient thermal capacity to pass the heat to the mother metal. Compared with a heat-saturated iron, a temperature-control iron is both able to control the heat and quickly restore the set temperature at the iron head, which is very helpful in continuous soldering operation. The most recently marketed soldering irons all have a thermal couple (temperature transducer) installed at the head in order to accurately control the head temperature.
However, when head shape has been changed or the head has been worn out, an error as high as dozens of degrees () in the temperature reading may occur, which may play a trick upon you.
6.4 Matters need attention
Please be aware that the temperature at the iron head is often as high as 350. Your skin will be burnt in case it touches the iron head by accident. And do not touch the just solidified solder tin. Your finger will as well be burnt if you do so, for sometimes you may be motivated to do so by thinking, “How beautifully the job is done.” In case of being burnt, go to running water to wash the burnt skin, as running water will take away some heat. Anyway it is advised to cool down the burnt skin first.
It is occasionally seen that a soldering iron is laid directly on a table or on an ashtray; sometimes, a soldering iron head is seen swinging when the electric wire is pulled, which proves to be very dangerous. The iron should be placed on a stand specially designated for placing iron.