Learn to solder in five joints

Hard rules — read once, every session

1. Iron temperature. 320–350 °C for 60/40 leaded solder on whoop-scale pads, on a responsive temperature-controlled station. For lead-free (SAC305), check your wire’s own data sheet before turning the dial — the useful set point is station-specific, and some products cap hard. MG Chemicals sets an upper limit of 350 °C for its 4925–4926 SAC305 wire, so with that product start near 340 °C and do not exceed 350 °C. The dial setting is not the alloy’s melting point: tip size, station recovery, flux chemistry and the joint’s thermal mass all move it. Start low and bump up if the solder doesn’t flow. Too hot is the more common beginner failure mode. (Chapter 7 carries the sourced detail.)
2. Tip cleaning before every joint. Wipe on brass wool or a wet sponge; re-tin with a fresh dab of solder; wipe again. A clean tip transfers heat. A dirty tip burns flux.
3. Flux first, solder second, heat third. Apply flux to the joint. Tin the iron. Touch the tip to the joint to transfer heat; the solder follows the heat. If you’re chasing solder around the joint, the joint isn’t hot enough.
4. Hand control. Both hands free. The wire is held by helping hands or by the perfboard itself. Your dominant hand holds the iron; the other holds solder. Trying to hold the wire with one hand while soldering is how you make cold joints.
5. Eye protection. Splashed solder doesn’t happen often, but when it does it’s bright and unforgettable. Safety glasses beat a trip to the emergency room.

Joint 1 — Tinning a wire end

What it teaches: heat transfer from the iron to a stranded wire. This is the precondition for every other joint. If you can’t tin a 22 AWG silicone wire end cleanly, the rest of the progression will fight you.

Before you start: - 5 cm of 22 AWG silicone wire, stripped 5 mm at one end. - Helping hands holding the wire horizontal. - Iron at temperature, tip clean and freshly tinned. - Flux pen or flux paste touched to the exposed strands.

Steps: 1. With the wire horizontal and the strands exposed, twist the strands clockwise gently between your fingers. The wire should stay round, not splay flat. 2. Apply flux to the twisted strands. 3. Touch the iron’s tip to the underside of the wire strands. Count to 2. 4. Apply solder to the top of the wire (not the iron). The solder should melt and flow through the strands. If the solder doesn’t flow, the wire isn’t hot enough — count to 3, not 5. 5. Remove the solder. Count to 1. Remove the iron. Don’t move the wire for another count of 2.

What good looks like: - The tinned section is shiny silver, with the strand pattern just visible underneath the solder. - The solder forms a smooth, slightly tapered coating, not a blob. - The insulation isn’t visibly melted or pulled back from the strands.

What bad looks like: - Blob. Too much solder. Reduce the amount you feed. - Dull grey, lumpy surface. Cold joint — iron wasn’t hot enough, or you moved the wire while cooling. Re-tin: flux, heat, solder, hold. - Insulation melted back 3+ mm. Too hot, or held too long. Drop the iron temperature 10 °C; reduce the heat-transfer count. - Strands visibly black. Way too hot. Cut the end off and start over with a fresh strip.

Redo criterion: tin five wire ends in a row. Three need to be “good” by the standards above before you move on. If you’re under 50 % good after ten tries, the iron is too hot or the tip is dirty — stop, troubleshoot, restart.

Joint 2 — Wire-to-wire splice

What it teaches: joining two tinned wires with both pieces held in place. This is the soldering equivalent of cooking pasta — fundamental, fast, and the one you’ll do most often in repairs.

Before you start: - Two 8 cm pieces of 22 AWG silicone wire, both ends stripped 5 mm, both stripped ends tinned (Joint 1 already passed). - 2 cm of 2 mm heat shrink slid onto one of the wires before you solder. (This is the step beginners forget twice and remember forever after that.) - Helping hands holding the two wires end-to-end, with the tinned sections overlapping about 4 mm. - Iron at temperature, tip clean.

Steps: 1. Confirm the heat shrink is on the wire, slid back at least 5 cm from the joint area. Yes, look at it now. (See? You forgot. Slide it back to the wire.) 2. Press the two tinned ends together so they touch along the 4 mm overlap. 3. Apply flux across the overlap. 4. Touch the iron to the joint along the side. Count to 2. The solder on both wires should melt and flow together. 5. If needed, add a tiny amount of fresh solder — just enough to bridge any visible gap. Most of the solder is already on the tinned ends. 6. Remove the iron. Hold the wires still for a count of 3 while the joint solidifies. 7. Slide the heat shrink over the joint. Apply heat from the iron’s shaft (not the tip) at a distance, or use a heat gun. The shrink should grip the wire tight at both ends.

What good looks like: - The joint is shiny, smooth, with the overlap fully fused — no visible boundary between the two wires’ tinning. - The heat shrink covers the joint completely with insulation extending 3+ mm onto the silicone on each side. - Tug test: pull each end gently. The joint doesn’t budge.

What bad looks like: - You forgot the heat shrink. Cut the joint, re-strip, re-tin, start over. Don’t try to slide shrink over a finished joint — it won’t fit, and even if it did, the shrink wouldn’t grip cleanly. - Joint pulls apart on a light tug. Cold joint. Re-flux, re-heat, re-flow. - Heat shrink charred or melted. Iron too close, or held too long. The shrink should be just at its activation temperature for ~5 seconds, not glowing.

Redo criterion: three good wire-to-wire splices in a row. By the third, you should feel the rhythm: shrink → strip → tin → press → flux → heat → solder → cool → slide → shrink.

Joint 3 — Through-hole header pin

What it teaches: heating a pin and a pad simultaneously, with the pad providing more thermal mass than a wire. This is the geometry you’ll meet again on a flight controller header, on an ESC signal pad, and on a receiver’s UART pad.

Before you start: - 2-pin header pins, snapped off the longer strip. - Perfboard, 4 cm × 6 cm, plated through-holes. - Push the header pins through two adjacent holes from the component side. The short legs poke out the solder side; the long pins stick up on the component side. - Optionally: a piece of tape on the component side to hold the pins flush against the board so they don’t fall through. - Iron at temperature, tip clean and freshly tinned with a small bead of solder. - Flux applied to the solder-side pad.

Steps: 1. Hold the iron’s tip so it touches both the pin and the pad at the same time. The pad surrounds the pin; touch the iron at the junction. 2. Count to 2. 3. Feed solder into the opposite side of the pad from where the iron is touching. The solder should flow toward the heat, wicking around the pin and onto the pad. 4. Once you see the joint fill — the solder forms a conical fillet from the pin up to the pad edge, not a ball — remove the solder. Count to 1. Remove the iron. Hold for 2.

What good looks like: - A small shiny cone of solder around the pin, blending smoothly into the pad surface. - No solder on top of the pin (you didn’t add too much). - No spike of solder reaching down off the pad (you didn’t add too much and the joint isn’t a bridge). - Pad next to the joint is clean — no flux residue you can see, or the flux residue is contained on the pad itself.

What bad looks like: - Ball of solder sitting on top of the pin without touching the pad. Pin was hot, pad wasn’t. Re-flux the pad, touch the iron to the pad side, count longer. - Solder bridges to the adjacent pad. Too much solder, or you moved the iron. Either re-flow (touch the iron to the bridge, the bridge wicks back into the joint) or use solder wick. - Lifted pad. You held the iron too long and the pad’s adhesive to the board gave up. Cut your losses on this hole and move to a fresh pair of holes.

Redo criterion: solder four 2-pin headers (eight joints total) across a row of perfboard holes. At least six of the eight should look like “good” above. Lifted pads count as failures even if the joint still works.

Joint 4 — Pad-to-wire

What it teaches: soldering a stripped, tinned wire onto a flat pad on a board. This is the technique you’ll repeat on every motor output on the AIO, every VTX signal pad, and every battery lead pad on Build Two’s 4-in-1 ESC.

Before you start: - A 5 cm length of 22 AWG silicone wire, stripped 4 mm at one end, tinned (Joint 1). - A clean pad on your perfboard — ideally a pad that’s not used for a through-hole header, so you have a flat unobstructed surface to work on. (Some perfboards have pads without through-holes; if yours doesn’t, use a pad where you’d planned for a header but skip the header.) - Flux applied to the pad. - A small dab of solder pre-melted onto the pad — touch the iron to the pad, add a small amount of solder, lift the iron, let it cool. This is called “tinning the pad.” - Helping hands holding the wire so the tinned end rests flat on the tinned pad. - Iron at temperature, tip clean.

Steps: 1. With the wire’s tinned end resting on the tinned pad, touch the iron to the top of the wire where it meets the pad. 2. Count to 2. The solder on both the wire and the pad should melt and merge. 3. Remove the iron. Don’t move the wire for a count of 3.

That’s it — no extra solder. Both surfaces were pre-tinned; the joint is the merge.

What good looks like: - The wire is mechanically locked to the pad by a smooth shiny fillet of solder along its length where it sits on the pad. - The wire is in contact with the pad along its full tinned length — not lifting off at one end. - The solder hasn’t spread beyond the pad onto the adjacent board surface.

What bad looks like: - Wire lifts off at the far end of the tinned section. The iron didn’t transfer heat far enough down the joint, or the wire wasn’t held flush. Re-heat with the iron on the wire-and-pad contact line, not just the centre. - Joint looks “dry” — visible gaps between the wire’s strands and the pad’s tinning. Not enough flux. Add flux, re-heat. - Solder spread off the pad onto the board. Too much solder on either the wire or the pad. Use less next time.

Redo criterion: five pad-to-wire joints, with at least four mechanically solid (the wire doesn’t move when you tug it laterally with your fingernail). Cut the wire off cleanly between attempts so each joint starts fresh.

Joint 5 — Surface-mount capacitor

What it teaches: handling a tiny passive component on a tiny pad. The exact technique you’ll use if you ever need to replace a capacitor on an AIO. On 0805 (the practice kit’s size), the parts are pickable with tweezers; on 0402 (some real AIOs), they’re not. The technique scales.

Before you start: - Tweezers — pointy, fine-tipped. (If your hands are unsteady, a pair of locking tweezers helps.) - One 0805 SMD capacitor from the practice kit. - Two adjacent pads on the perfboard, ideally with a small gap between them about equal to the capacitor’s body length (~2 mm). - Flux applied to both pads. - A small dab of solder pre-melted on one of the two pads — only one. The other stays bare with just flux. - Iron at temperature, tip clean, with a tiny bead of fresh solder.

Steps: 1. With the tweezers, place the capacitor across the two pads. One end should rest on the tinned pad; the other on the bare-flux pad. 2. With the iron’s tip carrying its small solder bead, touch the end of the cap that’s on the tinned pad. The pre-tinned solder melts and bonds the cap to the pad. Count to 1. Remove the iron. 3. Move the tweezers to release the cap (the joint is now holding it). 4. Apply a tiny amount of fresh solder to the iron’s tip. 5. Touch the iron to the other end of the cap, against the bare pad. The solder transfers from the iron tip onto the pad-and-cap joint. Count to 1. Remove.

The cap now has both ends soldered.

What good looks like: - The cap sits flat on the board, parallel to the surface. - Both end joints are small, smooth fillets — about the size of a pinhead per side. - No solder has bridged across to adjacent pads or onto the cap’s top surface.

What bad looks like: - Cap “tombstoned” — standing on one end. One pad melted before the other, and the cap’s surface tension pulled it upright. Re- flow both ends simultaneously, or remove the cap with tweezers and start over. - Solder bridge between the two ends across the top of the cap. Too much solder. Use solder wick to clear, then re-flow. - Joint dull, dimpled. Cold. Re-flux, re-heat both ends.

Redo criterion: three SMD caps in a row that sit flat with clean fillets on both ends. Tombstoned caps and bridges both count as failures.

You’re ready for the AIO when

All five joints redo-criteria passed in one session. If you have to re-read this PDF between joints — that’s fine the first session, not fine the third.

A reader who has done all five joints to criterion takes about 30 seconds per joint on the AIO. A reader who skipped the practice kit takes 5-10 minutes per joint on the AIO and damages the board twice during Build One.

The choice is yours.

Troubleshooting checklist (if anything keeps going wrong)

Symptom	Likely cause	Fix
Solder doesn’t flow at all	Iron not hot enough, or tip dirty	Clean tip, re-tin, bump temperature 10 °C
Solder flows but joint goes grey	Joint moved while cooling	Hold the joint still for a count of 3 after iron lift
Insulation melts back several mm	Iron too hot, or too long on the joint	Drop temp 10-20 °C, shorten count
Pad lifts off the perfboard	Iron held too long	This is on you. Practice on a different pad.
Solder won’t stick to the iron tip	Tip oxidised	Brass-wool clean, re-tin with fresh solder while warm
Solder won’t stick to wire	Wire is enamel-coated, not bare strand	Strip more, scrape the enamel, re-tin
Tombstoned SMD cap	Asymmetric heat	Re-flow both ends simultaneously

If two consecutive sessions in a row hit the same problem, the problem isn’t the joint — it’s a setup issue. Re-read the Hard rules at the top of this doc and check your bench against them.