Quick Answer
A cold solder joint is a connection where the solder hasn't properly bonded to both surfaces. In FPV builds, vibration and current spikes cause these weak joints to crack within dozens of flights. We see more builds fail from bad joints than from component defects, and most are preventable with the right technique and a decent iron.
The Joint That Fell Apart at 200 Metres
A customer sent in a "dead" quad last summer. The motor cut mid-flip and it spiralled into grass. The positive battery pad on the ESC had a hairline crack through the solder joint. Grainy, dull surface. Classic cold joint. The solder had never wetted the pad properly, and 40 flights of vibration finished it.
We see this weekly. Pilots spend weeks picking motor KV and props, then solder everything with an iron that cannot hold temperature on a ground plane. The weak link in most builds is the joint, not the component.
What a Good Joint Actually Looks Like
A proper joint is shiny, smooth, and concave, flowing from the pad onto the wire. If the surface is dull, grainy, or rounded like a ball, it is cold. Cold joints have poor contact and no mechanical strength.
Three causes: iron too cold, insufficient flux, and moving the wire before the solder solidifies. Insufficient flux is the most common culprit we see. Flux strips the oxide layer so solder bonds to bare metal. Without it, solder sits on top like water on a greasy plate.
The Joints That Fail Most Often
Battery pads on ESCs and PDBs carry the highest current and are the hardest to solder. The large copper planes absorb heat fast, so your iron needs thermal mass and temperature to bring the pad up to solder melt before heat dissipates into the board. Our beginner soldering guide covers the basics, but the key point is: if solder does not flow within two seconds, your iron is the problem.
Motor wire joints fail from vibration more than poor soldering. The stiff wires transmit vibration directly into the joint. We always secure motor wires with hot glue or a zip tie for strain relief. Signal wires on FC UART pads are easy to solder but the pads are tiny. Too much heat lifts the pad off the PCB. We use 300°C with a fine tip and count to three.
Our Workshop Soldering Setup
Every bench in our workshop runs a Sequre SI012 Pro at 350°C for power pads and 300°C for signal pads. We use 60/40 leaded solder (0.8mm). Lead-free melts at higher temperature and is more brittle under vibration. Our 10-piece tool kit covers field soldering, but for bench work, a temperature-controlled iron makes the biggest difference. See our full tools range for everything we use on the bench.
The Five-Second Pre-Flight Solder Check
Before every session, we do a quick visual pass over the critical joints. We look for discolouration around battery pads, cracks in the solder surface, and any wire that moves when gently tugged. A multimeter continuity check takes five seconds and catches intermittent connections that visual inspection misses.
FAQ
Q: What temperature should I solder FPV components at?
A: 340-370°C for battery pads and motor wires. 280-320°C for small signal pads. If the solder does not flow within two seconds of contact, your iron is too cold or the tip is oxidised.
Q: Do I need separate flux or is flux-core solder enough?
A: For battery pads and large ground planes, always add extra flux. The flux core in solder wire is not enough for these high-thermal-mass joints. A small flux pen or paste makes a visible difference in joint quality.
Q: How do I know if my existing joints are safe?
A: Inspect each joint under good light. Shiny and smooth means good. Dull, grainy, or cracked means rework it before your next flight. Gently tug each wire. If it moves at all, re-solder it. Also read our build mistakes guide for other common errors we catch in customer builds.
Q: Can I rework a cold joint without removing the old solder?
A: Yes. Add fresh flux, heat the joint until the existing solder flows, then add a tiny amount of fresh solder. The new flux and solder will alloy with the old joint and create a proper bond. This is faster and less risky than desoldering completely.
Q: Leaded or lead-free solder for FPV builds?
A: We use 60/40 leaded solder for everything. It flows at a lower temperature, creates shinier joints that are easier to inspect, and is more resistant to vibration cracking. Lead-free is mandatory in commercial manufacturing but for personal FPV builds, leaded solder produces more reliable joints with less effort. Wash your hands afterwards.