Soldering guide

What is soldering?

Soldering is a technique for joining metal parts by means of a metal alloy with a lower melting point than the parts to be joined. The connection for soldering is made by melting the joining metal alloy and allowing it to flow between the parts. The difference between soldering and welding is that soldering is carried out at a relatively low temperature, whereby the material of the workpiece does not (yet) melt. In welding, the temperature is so high that, in addition to the welding wire or electrode, the metal of the workpiece itself also liquefies, in contrast to soldering.

For soldering in electrical engineering, a soldering station with an operating temperature of about 350°C is generally used.

What do you need?

  • Soldering iron, soldering gun or soldering station
  • Soldering wire (soldering tin)
  • Sponge or steel wool
  • The parts to be soldered

Optionally, the following is also useful

  • Desoldering wire or desoldering pump
  • Smoke extraction
  • Safety glasses
  • Gloves

Soldering iron, soldering gun or soldering station

There are different tools for soldering. A soldering iron is perhaps the best known. It is also the cheapest solution. The disadvantage is that these soldering irons usually have no temperature control. So for sensitive electronics, they are often not recommended. However, for soldering the CO2 sensor, they can be used quite well. We will mainly be soldering the less sensitive detectors. Make sure you use a soldering iron with a fine tip (2 mm is a good guideline).

Soldering iron

Some soldering irons have a soldering point at an angle. A straight soldering tip is often more convenient for this kind of work.

Angled soldering iron is less useful for this work

A soldering station has a temperature control and is preferred. However, they are slightly more expensive. So if you are going to solder once, a soldering iron is recommended (to reduce costs). Do you want to do more soldering? A soldering station is then a good investment.

Soldering station

In addition to the above, there are also soldering guns. These are rather for heavier work and are not recommended for this kind of work.

Soldering gun

We recommend a soldering iron or soldering station of minimum 30 W. 60 W is recommended.

A temperature of 260 ℃ is often sufficient for soldering. 350℃ is considered the maximum temperature. The higher the temperature, the more stressful it is for your soldering tip (shorter lifetime).
With a soldering station, you can easily adjust the temperature to the correct value. With a soldering iron, you usually do not have this option. Most soldering irons will easily reach 350 ℃ or more. However, for soldering the CO2 sensor, this is not a big problem.

USB soldering irons also exist. Seems handy and cheap. However, keep in mind that these are usually not powerful enough for this kind of soldering. Do not use battery powered soldering irons either.

Always make sure you have a soldering iron holder! A soldering iron can get very hot and you should never put it directly on the table. Normally, such a holder comes with your soldering iron. A soldering station usually comes with a holder.

Soldering wire (soldering tin)

Always use solder without lead (Pb)! Solder without lead will typically require higher temperatures. It also gives a duller soldering result. The solder joint will therefore not "shine" which is normal (with lead-containing solder wire a dull solder joint indicates a poor solder joint).

Use quality soldering tin. Low quality soldering tin can contain impurities that can remain on your soldering tip and make soldering more difficult. Good quality solder will always melt well at the specified temperature.

Do not use solder wire that is too thin. Soldering wire that is too thick easily results in too much tin on the solder joint. Thinner soldering wire will sooner result in too little tin on the solder joint. However, it is easier to add extra solder than to remove it.
We recommend soldering wire 0.7 mm.

Sponge or steel wool

A soldering tip must be cleaned before, during and after soldering. A clean soldering tip makes soldering easier and better. It also extends the life of your tip.

The easiest way to do this is with a damp sponge (it can be a standard kitchen sponge). Go with your soldering tip over this damp sponge to clean your tip. Make sure that the sponge remains damp (if necessary, moisten it again).
The sponge does not have to be completely wet. Simply damp is sufficient.

Never leave the soldering tip on the sponge for a long time!

The advantage of a simple sponge is that it is cheap. The disadvantage is that a damp sponge lowers the temperature of your soldering tip. Depending on the power of your soldering iron (or station) it can take a while for the tip to get back to temperature.
Also, the continuous cooling and heating of the soldering tip is not good for a long service life.

A better way to clean your soldering tip is to use a brass or stainless steel wool (sometimes called a soldering sponge). Put your soldering tip a few times in the steel wool to remove the solder and clean the tip.
The temperature will also drop less, which is good for a longer lifetime of the soldering tip.

Soldering sponge

After the tip has been cleaned, you must always put soldering tin on it. This protects the tip from oxidation. This is also very important when you stop soldering. Before you stop soldering always clean your tip and add soldering tin. Then turn off the soldering iron/station and let it cool down (with the soldering tin on the tip). This soldering tin will protect your tip from oxidation.
If you want to solder again, let your tip heat up. Then clean the tip and add tin solder to the tip. You are now ready to start soldering.

If your soldering tip is very black (due to oxidation), soldering can be very difficult. The oxide prevents the tip from heating up enough to melt the solder wire. Never use a file to remove the oxide layer! You will damage your tip even more. Use a soft sandpaper or steel wool to remove the oxide layer (let the tip cool down first!). If necessary you must replace the soldering tip.

In the picture below you see an oxidised soldering tip (left) and a well-tinned soldering tip (right). The right one is what you want. The tin on the soldering tip creates a barrier between the air and the metal of the tip, preventing the tip from oxidising.

Desoldering wire or desoldering pump

Desoldering wire can be used to remove excess solder. The desoldering wire will suck up the tin.

Desoldering wire

It is easy to use:

  • Set the soldering temperature to 350℃ (if adjustable).
  • Place the end of the desoldering wire on top of the solder joint with the excess tin.
  • Press your soldering tip on top of the desoldering wire, at the location of the excess tin.
  • When the solder becomes liquid it will be sucked up into the desoldering wire.
  • Remove the soldering tip and desoldering wire in one smooth motion.
  • The excess tin is now gone.
  • If necessary, cut the desoldering wire shorter (remove the part with the absorbed tin).
  • Check the solder joint. If necessary, add more solder (if too much tin has been absorbed by the desoldering wire).

A desoldering pump can also be used.

Desoldering pump

Here, too, the use is simple.

  • Push the piston down with the spring.
  • Heat the tin you want to remove with the soldering tip.
  • Hold the desoldering pump above the molten tin.
  • Press the button of the desoldering pump. The piston shoots up and sucks the soldering tin.
  • Clean the soldering pump if necessary.

Smoke extraction

The fumes released during soldering should be inhaled as little as possible. If necessary, make sure you have a fume extractor. This can be done easily with a fan and carbon filter.

Fan with carbon filter
Using the fan with carbon filter

Of course, you can easily make such a solution yourself using a fan, carbon filter and some 3D printing 😉 .

Safety goggles and gloves

During soldering it is possible that solder jumps out. Optionally, you can wear safety goggles and work gloves to prevent possible burns.

How to solder?

Always solder in a well-ventilated room! If necessary, solder near an open window or use a powerful solder fume extractor.

Always make sure that you work in a place with as little material as possible nearby. This is to prevent a possible fire hazard. Never solder near animals or small children.

Bear in mind that a soldering tip can become over 300 ℃ hot (or rather hot)! Never touch the metal part of the soldering iron! This will cause serious burns.

Always use the handle!

Always make sure there is an adult around!

Make sure that nobody can fall over the wire of the soldering iron or station.

Wash your hands after soldering.

Not only the soldering tip is hot (or warm) but also the soldered parts can be hot. Let the parts cool down sufficiently before touching them with your hands.

Good soldering is an important basis for every electronics enthusiast. But as with so many things, "practice makes perfect". So do not be frustrated if you do not succeed the first time.

With soldering, the right tool is important (see above) but also the timing. And the more you practise, the better your timing will become. It is like riding a bicycle. In the beginning, you have to think about everything. But now cycling comes naturally.

Set the temperature of your soldering iron to 350℃. More is normally not necessary (unless you have a damaged or oxidised tip). Setting the temperature is typically only possible with a soldering station. Do you have a normal soldering iron? No problem.

Once the soldering tip is hot you must ensure that your soldering tip is clean. Clean your tip with a damp sponge or even better with steel wool. Do not hold the sponge or steel wool in your hand! Leave it on the table and rub the tip with the sponge or steel wool.

Place the soldering tip at the point where the joints to be soldered meet and do not move the soldering tip. Wait for about 2 seconds so that the two contact points have the right temperature. Hold the tip of the soldering wire against the soldering point and the contacts to be soldered.

As soon as the tin begins to melt and flow, remove the tin. Hold the soldering tip on the surface for about 2 seconds and then remove it.

Allow the tin to solidify and check the joint visually. If necessary, repeat the procedure and add more tin. If too much tin is applied, remove with desoldering wire or a desoldering pump.

Thanks to the resin core of the soldering wire, the liquid tin flows quickly over the parts to be soldered and immediately creeps up along the connecting wires of the component.

Limit the soldering time to about 5 seconds. If the soldering time is too long, there is a risk of parts melting or overheating and damaging them.

A perfect solder joint looks like the picture above. Below are two examples of a bad solder joint.

Bad solder joint
Bad solder joint



It is possible to mount the temperature/humidity sensor flat on the PCB. However, due to the heating of the display board (TTGO board) this mounting of the sensor will give a higher temperature value (and lower humidity value) than the actual environment. The sensor measures the warm air under the display board.
If a correct measurement is required, it is recommended that the temperature/humidity sensor to be mounted upright.

Soldering instructions ControlCo2

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