QPAD: soldering tutorial

Quentin Bolsée, 2026

This tutorial guides you through soldering a QPAD from start to finish. No prior experience in soldering is needed to follow this tutorial.

1. What to get

To assemble a complete QPAD, you will need:

Optionally, you can also use:

2. Prepare for soldering

If using the 3D printed jig, you can secure the circuit board with the latch on the right side of the jig:

You can also secure the circuit board in a vise, or use double sided tape to attach it to a flat surface.

3. Soldering resistors

In general, components can be soldered in whichever order is most convenient for you. A good rule of thumb is to prioritize small components, and to work your way out from the center of the circuit. In this case, starting with the two resistors is reasonable. This is also an easy type of component to solder, and a good way to practice your soldering skills.

Start by tinning (adding solder to) one of the pads. For each component to solder, we want to tin exactly one pad first.

Refer to your lab's guidelines on solder type and soldering iron temperature. For non-leaded solder, 380°C (716°F) should be enough.

To tin a pad, gently touch it with your soldering iron to get it hot, then add solder. You'll find that solder itself helps conduct heat, so it's a good idea to start with a very small drop of solder. Once the pad is hot, you can easily add the desired amount of solder, and it will gladly fuse with the pad. If you fail to get the pad hot, your solder risk beading up and rolling away.

Here is a video showing tinning of one pad for each resistor. Notice how the solder flows nicely over the entire pad, once it is hot:

After tinning one pad, grab a resistor with tweezers and secure it in place by remelting the solder. You want the resistor to form a good bond with the pad, which requires getting the resistor hot as well. Be patient and look for the exact moment the solder seems to flow to the resistor. You might need to partially touch it with your iron to get it hot enough. Ideally, your iron should be touching (1) the pad (2) the resistor and (3) the solder, to remelt it.

Once you think a bond has been made, remove the soldering iron, but keep the resistor perfectly still. It takes 1-2 seconds for solder to solidify again. You can then check that the resistor is held in place adequately.

If no bond has been made, simply try again. If you try too many times, you might find that the solder is starting to behave differently. That is because the flux that was included with it has been fully spent. Flux is used to make solder flow more easily. Without flux, you'll have a harder time making a good bond to a pad or a component. You can remidy this by adding a very small amount of solder to bring some fresh flux with it. You can also use a flux pen or syringe, but handle it with care; flux is often corrosive, and any excess on your circuit should be cleaned up with isopropyl alcohol (IPA). It is unlikely that you would need to add flux to this circuit, but in the future, you might find that it helps with tricky components/boards.

Once your resistor is nicely secured, solder its second pad with more solder. In this video, note how the iron is placed to make a nice contact with the resistor and the pad simultaneously. The perfect angle for this might be slightly awkward to find; don't hesitate to turn your workpiece around, and take your time until the iron placement feels natural. Once again, adding the solder also helps with heat conduction, and ideally it should be introduced exactly at the intersection of the two parts you're trying to solder. Once this initial solder has flowed and helps distribute heat, add more solder by gently advancing your wire. Be gentle and don't add too much, it often takes less solder than you think. You want a round, generous joint, but without excess.

Proceed with adding the second resistor. The follow video shows an example of a wrong placement. As long as you are still working on the first pad, you can still remelt it and correct the placement of your component. Tweezers should be used gently when doing so; you only want to move the component only once the solder is entirely melted. If you apply any force before that, you risk shearing the solder off, or damaging your component.

You should now have two perfectly soldered resistors. A visual inspection should be enough to convince yourself that they are secured, and that there is no short circuit (excess solder) between their pads. If you are feeling curious, you can use a multimeter to check the value of your resistors. At this stage, there is no other component that could skew the value of the resistance you're measuring.

4. Soldering the xiao module

The Xiao module from Seeed Studio is a handy all-in-one microcontroller module, ready to use. We will add it to our board by soldering its pads one by one. Just like with the resistors, we start by soldering a single pad. I like to start with the bottom right pad, for ease of access.

After tinning the pad, the Xiao module can be secured with the same method we used for the resistors. You might find that it takes a bit more time and skill to get its pad to bond nicely. This is normal, as the Xiao has more thermal inertia; there is more metal that needs to heat up, and it can take some time to get there.

When placing the Xiao in its final position, use large tweezers to manipulate it (not precision tweezers, as they can easily get damaged at the tips).

Make sure the Xiao is well oriented, and correct its angle if needed. There is no shame in repeating this 2-3 times, I often find that my initial placement is quite off. Look at every one of the 14 pads on the Xiao; you want the alignment to be satisfactory on all of them, and you don't want one pad on the circuit to risk touching two pads of the Xiao simultaneously: this could lead to a short circuit.

Just like with the resistor, after soldering the initial pad, we can now fully secure the component by soldering a second one. The best candidate for this is the complete opposite corner of the Xiao. After that, more pads can be soldered one by one. In the following video, you can see that it can take several seconds for solder to fully flow to the Xiao, because of how long it takes to get the area sufficiently hot. This transition is very sudden, and something you will easily notice and predict once you have more experience.

The most difficult pad to solder is the second-to-last pad (pin 13). That pad is connected to a ground plane on the Xiao board, which means there is a large thermal inertia associated with it. In other words, it takes a very long time to get this pad hot enough. Don't worry if this takes many tries to get the solder to finally flow correctly. You might need to add solder a few times. This helps both with heat conduction and with flux.

After soldering all 14 pads of the Xiao, you are almost done with the build! The hardest part is now over, and we simply need to add the OLED display.

5. Soldering the OLED display

Unlike the other components, the OLED display module is a through-hole component, which means it has pins that stick through the board and need to be soldered from the other side. If you are using the 3D printed jig, you can place the OLED display face down in the receptacle included on the jig. Don't forget to remove the sticker protecting the display, as it could interfere with correct placement.

If you are using the 3D printed support for the display, you can insert it now by sliding it from the bottom of the display.

Gently place your QPAD board face down, over the OLED display, and make sure the pins go through the board and stick out a sufficient amount. Once everything looks good, release the latch on the jig and make sure nothing will move while you are soldering the pins.

Soldering a through-hole component is somewhat easier than a surface-mount component. The trick is to simultaneously touch the pad and the side of the pin, and to introduce solder at that intersection point. Solder should nicely flow all around the pin. You can be quite generous with the amount of solder, as some of it is also needed to fill the hole (called a via) on the circuit board.

This video demonstrates soldering all 4 pins in a row. Notice the long break after introducing the solder, to make sure it flows all around the pin. Also note how some additional solder can be introduced once the intial solder has started flowing. A good joint should have a gentle curved shape, and naturally climb up the pin. A cold joint (bad joint), on the other hand, would clump up on the pad, and refuse to climb up the pin.

6. All done!

After soldering all 4 pins of the OLED display, you should be all set with the assembly of your QPAD. Congratulations! You can now test it out with some basic code, and write some code of your own. Instructions and examples are provided on the project page.