Historically, heating the host metals and melting the solder was done by firing in a forge or by pouring molten solder onto the metals and wiping it into place with leather pads. Later, heat was applied by means of an iron bit that was heated in the forge. The name soldering iron has carried forward to this day. Today, heat is applied by various electrically heated soldering tools called irons or pencils or guns. Figure 3 shows two examples. The majority of electronic soldering done during electronic repair, for example, is completed with a low-wattage soldering pencil.
- Some common soldering tools
- Soldering tip shapes
Regardless of the shape, size, or design of the iron, the tip must be tinned. Tinning the tip is the process of applying a thin layer of solder to the tip to keep atmospheric oxygen and other contaminants off the soldering surface and help with the flow of the solder. A poorly tinned tip will make it virtually impossible to achieve a sound solder joint.
Soldering iron manufacturers specify an operating temperature range for each type of tip. This requires mating the heating capability of iron and tip. Insufficient iron capability will result in tip temperatures that are lower than needed for quality solder work or in rapid drop in tip temperature during soldering. Excessive heat will quickly deteriorate the tip and may possibly damage the components and printed circuit board being soldered.
You can estimate tip temperature by following this two-step process:
- Apply a small amount of solder to the flat surface of the tip and immediately wipe the tip with a damp cellulose sponge or paper towel.
- Observe tip colour immediately after wiping.
- If the color of the surface is silver, the temperature is between 315°C and 370°C (600°F and 698°F).
- If the tip shows gold streaks, the temperature is approaching 425°C (797°F).
The copper tips of irons and pencils are progressively dissolved by solder, and they soon become pitted and corroded. This is particularly true of the continuous-heat types. It is virtually impossible to do quality soldering with a corroded tip. Corroding can be slowed down by keeping the tip clean and well tinned.
Clean the tip by wiping it frequently with a damp cloth or cellulose sponge. The damp wipe will shock the built-up burned flux from the tip. Immediately re-tin the tip and leave a thin coating to keep atmospheric oxygen off the soldering surface. Wipe the excess solder off the tip before the next use. Reclean and recoat with solder when you finish each soldering task.
When pitting becomes significant, you should dress the tip and reshape it to clean metal with a fine file. This can be done with the tip hot so that the refreshed tip can immediately be re-tinned. Excess solder should be wiped away after re-tinning. Steel-clad tips suffer much less corrosion and should never be dressed with a file. Like copper tips, however, they should be cleaned frequently when hot.