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.

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 and Wires

Brand new flight controller pads or freshly stripped silicone-insulated wires are usually oxidation-free. The internal flux easily prepares the surface for a smooth solder bond.

2. Quick Tinning

When pre-tinning pads or wires before installation, flux-core solder is fast and efficient. You don’t need to flood the area with flux—just heat, touch, and you’re ready to solder.

3. Small, Low-Current Joints

Signal wires, UART connections, and receiver pins don’t require high heat or large amounts of solder. The internal flux is more than capable of handling these joints on its own.

4. Field Repairs

When you’re at the flying field and need a quick fix, carrying a separate flux pen or paste isn’t always practical. Flux-core solder makes on-the-go repairs faster and cleaner.

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.

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.

Final Thoughts

Flux-core solder is the go-to choice 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 all season.

Remember: in FPV, your quad is only as reliable as your weakest solder joint. Using flux strategically is one of the easiest ways to make sure your build survives every crash, dive, and throttle punch.