Learn how to Solder like a Champion!

Equipment

Picking the right equipment is hugely important to learn how to solder easily, and get reliable and professional results. Soldering is used in many industries from heavy industrial to micro-electronics and the gear used is very different. The soldering iron the linesman uses on the powerlines outside your house will not play nice with that brand-new FC you have on your desk!

Soldering Iron

FPV components are on the smaller end of the common soldering spectrum and therefore the soldering Iron you choose should be sized accordingly. Quality soldering equipment used to be prohibitively expensive but recently high-quality gear has become much more affordable.

A suitable soldering Iron for use with FPV equipment should be in the 20-40W range, preferably have adjustable temperature, and the ideal iron will have interchangeable tips. If you cannot afford an adjustable soldering iron choose one in the 40W range, and you will be able to comfortably solder 99% of components and joints that you will encounter in this hobby. Some larger connections will take longer, but they are often the ones where adding a bit more heat into the part is not going to cause issues (more explanation on this later in the article)

I’m going to jump right in here and say that if you are looking to buy your first soldering iron for FPV use you should look no further than the TS-100 by Miniware. (See the link in the image carousel at the bottom of the article). This tiny little iron is capable of 17-65W (depending on input voltage), has built in temperature control software and even has accelerometers like the FC in your quadcopter so it can tell when it has been sitting idle and reduce the temperature to preserve the heating element and the soldering tip.

I have used extremely high-end soldering stations for work in the past (500-1000 USD) and the TS-100 gives a similar result for less than a 10^th of the price. Only drawback is that you will have to provide a power supply. Pretty much any 12-24v power supply will work, the higher the voltage the faster the Iron will heat up. I use an old laptop PSU and the Iron is ready to solder in less than 20s from cold!

Click the Image above to buy your own TS-100!

Stand

Having a reliable stand for your soldering iron is a must, you don’t want a 700°F (380°C) piece of metal rolling around on your workbench or falling on your carpet!

Most soldering Irons will come with a very cheap folded metal stand, while this will keep the hot part of the iron away from your desktop, the slightest bump will usually knock the iron loose and lead to a risky game of ‘Hot Potato’ as you try and catch it before it does too much damage.
It is definitely worth spending a few extra dollars on purchasing a decent stand. Not only will it make the iron much easier to pick up and put down, you won’t have to constantly be mindful of the iron next to you and will be able to concentrate much more on what you are working on. A good stand usually has a nicely weighted base so it is much more stable.

Sponge/brass wool

Often overlooked, this part is absolutely critical to soldering well. Your soldering iron tip should always be shiny silver to provide the best heat conductivity and clean solder joints. To keep the tip clean you should be wiping it across a sponge or brass wool every time you remove it from the stand.
This is another reason to invest in a decent soldering iron stand, as often they will have a sponge or wire wool holder built in, which will reduce clutter on your desk and make cleaning the iron super simple.

Solder

An important part of soldering! There are two main types of solder, leaded, and lead free. Make sure you buy leaded solder. Leaded solder has a lower melting point than lead free, and flows much nicer which is key to achieving good results. There are various alloys of solder that you can buy, but the best and most common is 60% Tin/40% lead. Also look for a solder that has a ‘rosin core flux’ this is a oxide inhibitor that prevents the solder from oxidising to quickly and allows for nice shiny smooth solder joints, almost all good quality solder will have the flux core.

Solder comes in a vast range of thicknesses. Like soldering irons, bigger is not better, most connections on FPV equipment are very small and trying to solder a small connection with a massive piece of solder is extremely challenging. Look for solder in the 0.7-0.8mm range, this will be thin enough to solder almost any connection but is not too thin that larger connections like power connectors become difficult.

Third hand – Helping hands

Soldering is a two-handed operation and you will usually be trying to solder two parts together, it’s not hard to do the math and realise that it’s going to get challenging to hold everything and solder at the same time. Luckily ‘third hands’ or ‘helping hands’ are common.

A third hand is usually a small clip or clamp that is attached to some type of flexible positionable ‘arm’ or stand. You can manoeuvre the clamp to the correct position to hold the parts that you are trying to solder, making it much easier to get parts in the right place and achieve a tidy result.

Some third hands have multiple arms allowing parts to be thoroughly supported and some even include extra features like a work light or small extractor fan which can be very helpful for removing the smoke generated when soldering.

Miscellaneous tools

Soldering is the final step in connecting parts together, getting the parts ready to be connected can require some other basic tools. It’s a good idea to have tweezers (preferably fine point), a sharp scalpel, small pliers, and a pair of sharp sidecutters at hand for trimming and positioning wires and components. The best type of side cutters, are known as Flush cutters and have the cutting edge flat on one side of the pliers allowing parts to be trimmed ‘Flush’. You will make mistakes at some point when soldering, or you might just need to rework a soldered joint, a ‘Solder Sucker’ or ‘Solder Wick’ is a very useful tool to have to remove excess or unwanted solder from a part.

Having the right supporting tools will help you learn to solder with a lot less hassle.

Theory

The true secret to achieving great soldering results is understanding what is happening when you solder. It seems simple, you use the hot bit, to heat up the other bits, and then you stick them together with the solder. Not quite.

Thermal Transfer

Optimising thermal transfer is key to getting great solder joints. First thing to cover is a bit of a throw back from High School physics: energy always travels from hot to cold. Keep this in mind. When soldering you are providing a pathway for that heat energy to flow from the hot iron to the colder work parts.

Oxides, the biproduct of heating metals in an oxygen rich environment (the dull grey or black build up on your soldering iron) are very poor heat conductors. A bright shiny soldering tip is much more effective at transferring heat because there are no oxides to get in the way. This is why it is so important to keep your soldering iron clean at all times.

In addition to keeping the tip of the soldering iron clean, maximising the area available for transferring heat is very important. As shown in the diagrams above holding a clean iron tip against a part, is much better than an oxidised tip, but only provides a very small contact area to transfer the heat from the iron to the workpieces. If you apply a blob of solder to the iron first then place it against the parts to be soldered, the blob will spread out against the part, increasing the contact surface area and rapidly increase the rate in which heat is transferred from the iron to the part. This is especially important when tinning large pads on PCBs or larger diameter wires like battery wires.

Heat soak

Heat soak is the enemy of any solderer. Heat soak is when the heat that you introduce to the part to create a solder connection, spreads out through the part and raises the temperature of the surrounding area/components. Many PCB mounted components are very sensitive to elevated temperatures and can be permanently damaged if they get too hot. You might think “Oh that’s simple, I’ll just turn down my soldering iron”. WRONG!

Heat soak is the by-product of how hot your soldering iron is, coupled with how long you have it in contact with the part being soldered. Solder melts at 370°F (188°C) if you have your soldering iron set to 570°F (300°C) it may take a minute or two before the contacts to be soldered get up to the melting point of solder, in the meantime the rest of the part will have heated up significantly as well.
If you set the soldering Iron to 750°F (400°C) the contacts to be soldered will heat up to the melting point of solder very quickly allowing the joint to be made, and the heat removed, before the rest of the part has a chance to heat up. This is much better for the sensitive components on the part.

Thermal Mass

Thermal mass relates to how much heat energy an object can absorb or store. This is generally linked to the size and density of an object, and in this case is applicable both to the solder joints being created as well as the hot tip of the soldering iron.

In soldering it is very important to consider the thermal mass of the parts that you are trying to solder and balance them with the equipment you are using to make the solder joint.

A small solder joint like a camera wire on an FC only requires a very small amount of energy so even a small soldering iron can do an excellent job. A large solder joint like adding an XT60 to a set of battery leads required much more heat energy and therefore using a larger soldering iron will make the job much quicker and easier.

For instance, the TS-100 is an excellent iron but it has quite a small tip (there are multiple tips available, but they are all pretty small) therefore the tip has quite a small thermal mass, for 99% of FPV related soldering the size of the tip is perfectly adequate and it can store enough heat energy to melt the solder required and create a nice joint.
However, on big soldering jobs it is possible for the solder joint to have a larger thermal mass than the tip of the iron. This means as heat is transferred from the tip of the iron, and the solder melts, it sucks all the heat from the tip and leaves the soldering iron element trying to catch up. This can cause the solder to re-solidify mid joint or worse can mean that you must keep the iron on the joint for a long time which increases the heat soak effect and risks damaging other components.

The best way to avoid running into issues like this, is to use a soldering iron with a larger tip and therefore more thermal mass. This larger soldering iron will no doubt take much longer to heat up, as it takes much more energy to fill up the tip but for that same reason, when you come to solder those large wires the tip will have the energy and the solder will melt easily and quickly.
Turning the heat of the tip up high has a similar effect to using a larger tip, as there is more heat energy packed into the small space.

Oxide Formation

As we have already covered keeping your soldering iron clean and oxide free is very important, but it is good to know that the hotter a part is, the faster it will oxidise. This is the catch 22 with using a higher heat to make solder connections, while it will avoid heat soak, the tip of your soldering iron will oxidise much faster and require more frequent cleaning and re-tinning.

Preparation

Luckily most the parts that you will work on in FPV are designed to be soldered manually and therefore there will need very little preparation. Make sure that all pads to be soldered are clean and shiny. In rare cases some parts are conformal coated from the factory and this can mean the solder pads are covered in a protective film, while this can be soldered through it is cleaner to lightly scrape the surface clean with a scalpel blade or similar.

First thing to do is make sure the tip of the soldering iron is clean, shiny, and well tinned. Tinning is the process of wicking solder onto/into a part that is to be soldered. This coats the surface in solder and makes it much easier to connect to another ‘tinned’ part by simply applying heat with the soldering iron. In the case of the soldering iron itself, tinning is not done to make it easier to connect but to increase the thermal conductivity of the iron and promote faster transfer of heat between the iron and parts being soldered.

Wipe the freshly heated iron on the damp sponge to clean off any oxidised solder then apply a medium sized blob of solder directly to the tip of the iron, as soon as the blob is molten, wipe the tip of the soldering iron on the sponge again, this will smear the solder over the tip and coat it evenly, if you wipe it gently you can rotate the iron and wipe the it through the same spot, using the same piece of solder to coat all sides of the tip. Otherwise simply repeat the process until the whole tip is bright, shiny, and well covered in solder.

Before Wires are soldered they should be stripped (outer insulation trimmed off) and tinned as explained above. By coating the wire first, it will be much easier to solder to its corresponding part.

That is pretty much it for preparation, some people swear that you need to use 7 different kinds of flux (flux, the oxidisation inhibitor can be bought in a range of forms including pastes and special applicator pens)  and only solder under the light of a certain moon phase to get good results, but as long as you keep your gear and parts clean, all you need is the Iron, the solder, and your cleaning sponge.

The process of soldering

So now we have covered the Equipment, Preparation, and Theory of soldering, it’s time to work through the steps of soldering two parts together.

This section is all about achieving ‘flow’. Solder has 3 states when soldering, Solid, molten and flowed.

• Solid is self-explanatory
• Molten, is when the solder is liquid but will not stick to anything else, the surface tension of the solder causes it to form into a small bubble.
• Flowed, is when the solder is molten, and the surrounding parts are above the melting temperature of the solder. The solder ‘relaxes’ and flows onto/into the surfaces. A properly flowed solder joint has the best conductivity and most strength.

For this we are going to work through the process of soldering a wire to a pad on an PCB, this is where people often have the most trouble and where a cold/loose solder joint can have the worst outcome. A good medium size soldering example would be soldering a motor wire to an output pad on a 4in1 ESC.

Step 1: Tin the motor wire and ESC pad

Prepare the motor wire by stripping back 2-3mm of outer insulation from the end of the wire.
TIP: Try make the exposed portion of wire equal to or shorter than the length of the pad on the ESC to make sure there will be no wire overhanging the pad that could cause a short on other components.

Take the soldering Iron (clean the tip as you remove it from the stand!) apply a small blob of solder directly to the tip of the iron. Place the small blob against the side of the exposed section of wire and give it a second or two to transfer some heat into the wire. Now apply the solder to the opposite side of the wire from where you are applying the heat with the iron. The solder should begin to melt and wick into the strands of the exposed wire. Move the iron and the solder around to make sure the whole exposed portion of the wire is coated in solder and that it has wicked into the strands nicely.
Tip: on small wires putting the solder on the far side of the wire will work almost right away. For larger gauge wire (battery wires etc) you may have to move the solder closer to the tip of the iron until it starts melting then work it to the opposite side.

Clean the soldering iron tip on the sponge! Re-apply the small blob of solder directly to the tip of the iron, now place the tip of the iron with the small solder blob against the pad on the ESC, you will see that to begin with the blob will sit on top of the pad (like a rain drop on a freshly waxed car) after just a second or so you will see it relax and ‘flow’ onto the pad, now you should add more solder until the pad is covered and there is just a slight convex shape to the solder on the pad.
Tip: Do not add too much solder to the pad, it should have just a slight convex shape to the top of the puddle. If you add too much it will look like a bubble and when you come to add the wire later, it may go everywhere and could damage surrounding components.

Step 2: Joining the wire to the pad

If you were fast and efficient in tinning the ESC pad you could complete the next step right away without worrying about too much heat soak. If it took a little while to get the solder to flow onto the pad and to cover it properly then give the esc a minute too cool before applying heat again.

Take the soldering iron (clean the tip!) and apply the small blob of solder to the tip to increase the heat transfer. Place the motor wire onto the pad of the esc and put the soldering Iron with the small blob on top of the wire, hold it there until the heat transfers through the wire and into the pad. Watch carefully you will see when the two parts get up to the same temperature the solder from the wire will ‘flow’ into the pad and vice versa. If you have tinned the pad and wire with the right amount of solder, you can now remove the iron and let the joint cool. The joint should be smooth in appearance.

• Not blobby, as that indicates too much solder which is a waste of weight (lead is heavy!)
• The wire strands shouldn’t be too visible, this indicates too little solder and may be a weak joint which could fail when flying.

TIP: Be very careful not to let the parts move as they are cooling as that can cause fractures in the joint that might fail when flying!

If you have a smooth, well flowed solder joint then you are done!

Handy Tips

There are a few handy tips that are not commonly shared that might make your soldering adventures a little more pleasant and durable.

• Temperatures to use:
  • Normal soldering of Flight controllers/ESC’s etc – Around 700°F (380°C)
  • Large connectors or items with large thermal mass – Around 780°F (415°C)
• Almost all factory prepared electrical components are now forced to use lead free solder to meet electrical standards. If you are replacing factory installed wires on an ESC for example, it is a very good idea to use a high heat to remove the wire, then remove as much of the original solder as possible. Return your soldering Iron to normal soldering temperature and re-tin the pad with leaded solder, before soldering the new wire as normal.
• If you are soldering fine wires to a PCB with through holes, do not tin the through holes. Carefully tin the wire, making sure not to make it too thick to fit through the hole. Place the wire through the hole, then solder the wire to the pad on the opposite side of the board. This will give you a very strong solder bond and will mean that in a crash if the parts are somehow separated, the wire is more likely to break than simply pull the pad off the board which can ruin the component.
• Watch out for solder balls!!! When soldering and cleaning the soldering iron, it is easy for small bits of solder to get flicked from the parts you are working on. These cause little solder balls that can roll around on your workstation, or worse sit amongst the legs of the components on your FC/ESC’s which can potentially destroy your electronics. Always carefully inspect parts after soldering and clean the solder balls off your workstation before testing components!
• When soldering XT60 connectors (or any connector that may melt) plug the opposite plug into the one that you are soldering. This way if the plastic does get soft, the terminals will still be held in the right place!