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Originally Posted by unl3a5h3d /img/forum/go_quote.gif
Well I might just get a cheap one and buy a nice one when I start doing PCB stuff. What wattage is the best for cables?
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If I only had one cheap iron for cables, I'd want at least 50W. But that's very simplistic.
Wattage is only one factor. There's also the efficiency of heat transfer, temperature, and a variety of factors that affect how well the iron maintains a given temperature.
Wattage is the amount of power consumed by the device, including any power and heat lost in the electronics.
Heating elements can be more or less efficient in converting power to heat. Ceramic elements are more efficient than coils by about 1/3, so a 35W iron with a ceramic element will typically produce as much heat as a 50W coil.
As soon as you touch the tip to a colder material, heat flows from the tip to the material. The loss of heat begins to lower the tip temperature immediately. To maintain a constant tip temperature, you need to add heat to the tip as fast as it's being transfered to the other material. Faster, in fact, because there are losses all around.
The larger the piece of material, the more heat is required to raise its temperature a certain number of degrees. A connector has more metal than a resistor, so you need more heat to raise its temperature to the point where solder melts. It's actually not so straight forward because heat flows at different rates depending on the structure or configuration of the material, as well as the type of material. If you can heat a small point quickly, the heat won't have much time to dissipate through the material, so the small point will come up to a higher temperature. This is one reason people use a high temperature iron and solder quickly.
A simple iron without temperature control simply supplies the same amount of energy to the heating element at all times. The switch controls the amount of energy. The temperature of the tip when idle (i.e. once the temperature stabilizes) comes about from the balance between various losses and efficiencies, and particularly radiation of heat from the tip to air. Touch that tip to a connector and you can guess what happens. Heat is transferred to the connector, but the incoming heat doesn't change, so the tip temperature drops, usually considerably. Without feedback to tell the iron to crack up the heat, the temperature will stay lower all the time it's in contact with the connector.
In a temperature controlled iron, there's a sensor that detects temperature. This is wired to a switch that controls the heating element. The dial controls the temperature at which the switch triggers. When it triggers, energy is applied to the element heating the tip. When the temperature is higher than the setting, the energy is cut off. Touch this tip to a connector, and the same thing happens at first. Heat immediately begins flowing into the connector, and the tip temperature drops, but the sensor detects this and more energy is applied to the heating element to compensate for the heat loss.
Additional complications arise in any iron, but that are more important in a temperature controlled iron. The ability of any element to heat the tip is affected by the distance between the element and the tip as well as the conductivity of the material. There's also less heat loss when the element is closer to the tip, because such designs typically have less material between the element and tip from which heat can radiate. Because of this, you want the heating element as close as possible to the tip. For the same reason, the closer the sensor is to the tip, the faster it will detect the heat loss and compensate. With both very close to the tip, the whole system will compensate faster. The initial temperature drop when you contact some material will be smaller, and the tip will have a better chance to remain at the predetermined temperature. Some irons have more sophisticated temperature controls. Metcal, for example, uses a small high frequency coil very close to the tip to generate heat, and relies on special materials that become magnetic or nonmagnetic at a certain temperature to pass or block the magnetic field. This is a very efficient system that makes a 50W Metcal competitive with a 70W hakko which uses ceramic elements, and the Metcal has better stability because the feedback system is so fast.
While temperature controlled irons are usually more efficient, and can maintain temperatures better, they can still only generate as much heat as their wattage and efficiency allow. With a large enough or cold enough block of material, even a high powered iron can lose more heat than it can generate. Irons with higher power ratings are capable of generating more heat, so you do want more wattage when soldering larger components. But you can't just compare wattage ratings without taking into account the efficiency of the heating element and the other losses in the design.
