Soldering is one of the most important skills in FPV (First Person View) drone building. Whether you’re assembling your first quadcopter or troubleshooting a video transmitter in the field, the quality of your solder joints directly impacts flight reliability. One question many pilots and builders ask is:

If my solder already has flux inside it, do I really need to add more flux?

The short answer: sometimes yes, sometimes no. In this guide, I’ll explain when flux-core solder alone is enough, and when you should still reach for extra flux.

What Is Flux-Core Solder?

Flux-core solder is solder wire with a thin channel of flux running through its center. As the solder melts, the internal flux is released, cleaning the metal surfaces and helping molten solder flow smoothly over the joint. This is the solder I’ve been using and have been happy with.

For FPV electronics work, most builders use rosin-core or no-clean flux solder in a diameter between 0.5 mm and 0.8 mm. This size and composition work well for everything from ESC pads to small signal wires.

When the solder melts, that flux:

  • cleans oxidation

  • improves heat transfer

  • helps solder “wet” the metal

  • makes the joint flow smoothly

Without flux, solder just beads up and refuses to stick.

For FPV work, I typically use:

  • rosin-core or no-clean

  • 0.5–0.8mm diameter

That size gives you control on tiny pads without flooding everything.

It’s what I use for basically every build.

This is the solder I use

When Flux-Core Solder Is Enough in FPV

From experience, there are plenty of times when the built-in flux will give you a perfect, reliable joint without any extras.

1. Fresh, clean pads

Brand new flight controllers and ESCs usually solder beautifully.

No oxidation. No grime.

Just:

  • heat

  • touch solder

  • done

The internal flux handles everything.

2. Pre-tinning pads and wires

When you’re tinning:

  • motor wires

  • receiver leads

  • small signal wires

Flux-core is perfect.

Quick touch → shiny coat → move on.

No need to overthink it.

3. Small signal wires

UART, RX, camera, VTX control wires — these joints are tiny and low-current.

They don’t require much heat or solder.

Extra flux usually isn’t necessary.

4. Field repairs

If you’re fixing something at the flying field, you probably don’t want to carry:

  • flux pen

  • paste

  • alcohol

  • wipes

Flux-core solder keeps things simple and fast.

I’ve fixed plenty of quads with just:

  • iron

  • solder

  • tweezers

When You Should Still Add Extra Flux

Click here for the flux pen I use

Even good flux-core solder has its limits. Here are scenarios where extra flux makes the difference between a “good enough” joint and a bulletproof one.

1. Oxidized or Dirty Pads

FPV gear often gets exposed to moisture, grass, and dust. This creates surface oxidation that the small amount of internal flux can’t fully remove. Adding extra flux ensures the solder wets the pad properly.

2. Large Heat-Demanding Joints

For XT60/XT30 connectors, ESC ground pads, or large capacitor leads, extra flux improves heat transfer and solder flow, making the job quicker and reducing the risk of overheating the pad.

Extra flux:

  • improves heat transfer

  • helps solder flow faster

  • reduces the time you’re cooking the pad

3. Reflowing Old Solder

If you’re reworking a joint, the old solder already on the pad won’t get much help from the flux in your new solder wire. Applying extra flux helps the old solder reflow and bond cleanly.

4. Delicate SMD Work

Small components like VTX antenna ground shields or camera connectors need precision. Extra flux helps control solder flow and avoid accidental bridges.

Click here for the flux pen I use

Best Practices for Using Flux-Core Solder in FPV

  • Buy quality solder – Brands like Kester and MG Chemicals have reliable flux that flows well and burns off cleanly.

  • Use the right diameter – 0.5–0.8 mm gives you control on small pads without overloading them.

  • Don’t rely on flux-core for cleaning – If the board looks dull or has visible corrosion, clean it first with isopropyl alcohol and apply extra flux.

  • Store properly – Keep solder sealed in a cool, dry place to prevent oxidation.

I use Flux-core solder for most FPV soldering tasks, especially when working with fresh pads, clean wires, and small joints. But when you’re dealing with oxidation, big connectors, or rework jobs, adding extra flux—whether from a pen, paste, or liquid—can mean the difference between a joint that works now and one that lasts.