What Soldering Bit Size do I Need?

“What size of tip should I get for copper foil?”

The size of the tip is less important than the amount of solder applied. I can use my 200 watt iron with a 12mm tip in copper foiling. The iron power and large bit mean that there are no cool times to slow the progress of making the bead. I admit the 12mm bit is over large, but a 6 or 8mm bit will work well.

Do not go for very small bits intended for electronic soldering, because they cannot hold the heat as long as bigger ones, nor as long as is needed to run a long bead of solder. 

The amount of power and tip size are relevant to temperature recovery time. The soldering tip is the heat sink of the iron. With certain limits, the larger the tip is, the fewer times the iron has to heat back up to the appropriate temperature. The greater power also helps maintain the temperature with fewer drops below the right temperature.

Note that rheostats do not control the temperature, they control the amount of power reaching the iron, making the recovery time longer. 

General information on soldering irons.

Choosing a Soldering Iron

The iron used to solder must be of a high enough wattage to readily melt the solder and be able to reheat fast enough to maintain the necessary melting temperature. The tip can't be so small it can't maintain the heat nor so big it covers more area than wanted.

For example a 75 or 80 watt iron is sufficient to begin soldering with, but it will continue to get hotter, as it has no temperature control. An iron of this type should be used with a rheostat in order to prevent overheating while it is idling.

Most temperature controlled irons seem to be produced in 100 watts or higher. These internally temperature controlled irons maintain a constant temperature. They are normally supplied with a 700F° bit (number 7) and is sufficient to melt the solder without long recovery times. You can obtain bits of different temperature ratings, commonly 800F° and 600F°. You can also several sizes of tips for different detail of work.

  For volume work you can obtain temperature controlled irons of 200 watts and more.

It is also possible to obtain a Japanese made soldering iron with the rheostat built into the handle.  It has a ceramic heating element which allows it to recover temperature quickly, even though it does not have a high wattage.  There are a number of imitations available at much lower prices, but are only suitable for infrequent use or electronics.  There are a number of irons in the 600 series, so make sure you get the right voltage and tip size.