If you are building or repairing an FPV drone, you will be doing a lot of soldering! The quality of your soldering determines how reliable your build is going to be, and it’s important that you know how to do it properly.
Soldering is simply joining two pieces of metal with molten solder, but it can take tons of practice before you master the skills. There are a few key points to mention before we get underway:
- Steady hands (and patience) are essential – so keep your coffee intake down :)
- Use a good quality solder iron with interchangeable tip (see my recommendations below)
- Patient and decisive – the best solder joints are done quickly, but without rushing
- Finally, try not to burn yourself!
Table of Content
- Soldering Tools
- How to Solder
- Soldering Temperature
- Cleaning Solder Joints
Getting some quality soldering equipment is just as important as acquiring good soldering skills. Check out this post for my recommended soldering iron, solder, and soldering related tools.
Preparation Before Soldering
Solder can spit when heated, sending tiny molten balls of solder flying, so eye protection is advisable. These can also cause shorts between traces and components, damaging your hardware.
While we are on the subject, we take no responsibility for any scorch marks, injuries or damage to priceless tapestries :) Joke aside, extended exposure to the fumes from heating solder is a health concern, so work in a well ventilated area.
The components we are dealing with are very small, so a 1.5x magnification pair of reading glasses or a magnifying glass (sometimes comes with helping hand) might help, and working in a well lit area is also essential.
Heat up the iron, and clean the tip to make sure it’s shiny.
Dull grey and black residue can build up on the tip, which affects the ability of your iron to transfer heat to your work. Simply wipe with a tip cleaner, damp sponge or aluminium scouring pad.
Right after cleaning, add a tiny bit of solder to the tip, this will help heat transfer to the solder joint.
Consider covering up areas on your flight controller where you are not soldering, with some masking tape or electrical tape.
Pro Tip: When you are done with soldering, you can clean the tip and add a decent amount of solder to the tip before turning soldering iron off. This adds a protective layer and will extend the life of the tips.
Always “tin” the wires and pads first before attempting to solder them together, this will make joining them much easier later on.
Tinning or pre-tinning basically means covering the wires and solder pads with an appropriate amount of solder prior to joining. Tinning allows you to solder more quickly as you have already saturated your pad and wire with solder.
If you are not using solder with rosin core, or if the solder isn’t sticking to the wire, then you might want to apply some solder paste (flux) to the wire first.
Pro Tip: It takes a few seconds for the molten solder to cool down – do not move the parts during this time, because it can cause fractures in the joint that might fail more easily due to vibrations and crash impacts!
Soldering Small Wire to Pad
The most common soldering job is soldering a wire to a pad on a PDB or flight controller.
First, ensure the wire and the pad have been tinned and your iron is up to temperature. Then heat the pad to melt the solder on it and place the wire onto the pad.
Once the two are joined under a smooth dome of solder, gently remove your soldering iron, and hold the wire in place for a few seconds until the solder joint cools down and hardens completely.
Pro Tip: Don’t be afraid to use a bit more solder. When you are not using enough, the joint can come off easily. When a solder joint doesn’t have enough solder, it will look bumpy, and the strands of wire are visible.
The finished joints should look round, shiny, solid and completely covered by solder. Here is another example of how it should look.
Remember not to leave the iron on the pad for too long to avoid overheating. If you heat a pad for too long, it can detach from the circuit board, this can involve some frustrating fixing later on or even spell the end of an expensive component!
If you let go of the wire too early before the solder completely sets, or didn’t use enough solder, you might end up with a cold solder joint or just an unreliable solder joint.
A cold solder joint looks bumpy and dull, it makes bad contact between the components and is unreliable. If this happens, or if you are just not sure, simply solder it again until you are happy with how it looks.
If the solder sticks to the iron when you remove it from the joint, it’s a sign of flux burning out. Adding some mroe solder flux or use fresh solder can fix that.
Soldering Large Wire to Pad
When soldering large wires, for example, a XT60 pigtail (12AWG to 16AWG) to the PDB (power distribution board), make sure your iron is set to a high temperature, and a larger soldering tip will usually work better too. (See my advice on soldering temperature later in this guide)
When tinning the wires and pads, use as much solder as the pad can take (see example below).
And apply a decent amount of flux paste on the pad before joining the wire, this will help tremendously!
It’s going to take longer to melt the solder on the PDB and thicker battery wires, because of the bigger chunk of metal. When melting solder on large wires, try not to press too hard, as this can bend, flatten and deform the wire strands.
The resulting solder joints should be round and shiny, and the wire strands should be completely hidden by the solder.
Header Pin to Through-Hole
When soldering header pins to through-holes, don’t pre-tin the contacts or you won’t be able to get the header pin through the hole.
- Insert the header pin through the hole, hold it at the desired angle with your hand or blue-tack
- Heat up both the header pin and the ring on the through hole for a couple of seconds
- Bring the solder to the joint and it should make a solid, shiny, “volcano” like bond
Wire to Through-Hole
There are two ways to approach this, you can either treat the through-hole like a solder pad (solder from the top), or do it like soldering a header pin (solder from the bottom). It depends on which side of the board you can access more easily.
Here is how to solder a wire directly on top of a through-hole:
Here is how to insert a wire through the hole, and solder from the bottom just like a header pin.
Joining Two Wires
It’s easier to use “helping hands” for this job, if you don’t have a set, a rubber band around the handles of a pair of pliers can make a handy holding tool. Remember, when using heatshrink, put it on the wire before you solder, we will all forget more than once!
For small wires and quick jobs, you could simply solder one wire directly next to the other. You could also twist them before soldering to increase mechanical strength.
When joining 2 larger gauge multi-strand wires together, I normally spread the strands first, and push the 2 wires together head to head, then twist them so they don’t come off easily. Now apply some solder flux, and solder them together.
This way maximizes the contact area of the metal and the benefits are the overall smaller and stronger solder joint. Either way works just fine as long as the joint is solid.
Header Pin on a Pad
You don’t tin the header pin nor the solder pad in this job.
- Bring the header pin to the pad and hold it there with a Third Hand or blue tack
- Heat up both the pin and pad for a couple of seconds, and bring solder to the joint
- Remove solder iron and let it cool down
When soldering multiple header pins, make sure the first pin is straight. The whole job will be easier when it is secured firmly in the right place by the first pin.
- Secure the XT60 connector with a helping hand or bench clamp
- Very slightly tin the inside of the XT60 connector. Do not apply too much solder otherwise you might have difficulty inserting the wire into the holes
- Tin the electrical wire, insert the wire into the hole, and heat up both the wire and the connector
- Bring solder to the joint, until the wire is buried in solder
- Remove solder iron, and allow 10+ seconds for it to cool down
Pro Tip: Connect the female XT60 to the male during soldering, this can prevent the plastic from deforming with the heat, and the gold connectors moving around.
Adjust soldering iron temperature for what you are dealing with. DO NOT leave the iron on the soldering pad for too long, overheating can cause the following issues:
- Heat can build up and damage components on the board
- Copper pads can fall off due to overheating (possible fix)
You want to get your iron in and out FAST! To do that, I personally prefer using a slightly higher temperature, as it melts the solder more quickly.
Just remember, high temperature and make it quick!
Using low heat can actually increase the risk of overheating. It takes longer to melt the solder, so you have to keep the iron in place for longer, which can eventually damage the pad.
This is the soldering temperatures guidelines I personally follow using good quality 60/40 solder:
- 300C° (580°F) – extremely delicate jobs
- 350C° to 390C° (662-734°F) – soldering signals wires on a flight controller or ESC’s
- 400C° to 450C° (752-842°F) – large connectors, ESC’s power and XT60 pigtail to PDB
Since 63/37 solder has a lower melting point, you can use 10C°-15C° lower temp from the above guideline.
If you are having difficulty soldering large connectors or wires, do not blindly keep increasing temperature. You should check:
- the quality and type of solder you are using. You should really try the solder we recommend in this guide
- your soldering setup, maybe use a bigger tip which can help transfer heat more efficiently
- lastly, apply flux to the joint before soldering!
Temperature vs. Power
Higher temperatures and bigger tips can help transfer heat faster. Higher power iron will also heat up faster and drop fewer degrees when applied to your work, making soldering easier.
When the iron is making contact with the joint, the temperature is going to drop as heat is being dissipated through the metal, which is effectively a heatsink. A larger tip holds more heat and will melt larger areas of solder faster than a smaller tip.
Cleaning Up Solder Joints
You might find residue around solder joints after soldering, this is burnt flux. While it’s not conductive, nor has any negative impact on the performance of your solder joints, it’s a good idea to clean it nonetheless.
Simply wipe the residue off with a cotton swab dipped in alcohol.
Some Other Resources
- How NOT to solder by IntoFPV – http://intofpv.com/t-how-not-to-solder
- General soldering by Adafuit – https://learn.adafruit.com/adafruit-guide-excellent-soldering
- Sep 2017 – Tutorial created
- Dec 2018 – Added more pro tips and uploaded more example images
- Mar 2019 – Added “cleaning up solder joints”, and some more tools
- Dec 2019 – Updated