Vacuum desoldering a through-hole part
A spring-loaded vacuum pump removes a through-hole joint in one pass when the alloy is fully molten and the nozzle seals against the pad. Most failed attempts share the same cause: the pump is fired while the alloy is still pasty, so it sucks a partial plug and leaves a ring of solder welded to the barrel wall.
Procedure
- 1
Add a small bead of fresh Sn63Pb37 to the joint and flux it with RMA so heat spreads through liquid alloy.
- 2
Press the 3.2 mm chisel tip across the pad and lead at 380 degrees C and hold until the fillet drops flat.
- 3
Hold one additional second, then place the vacuum nozzle over the pad and press it down to seal.
- 4
Trigger the pump and hold vacuum for a full second after the solder disappears.
- 5
Inspect the barrel for a remaining ring, wick it out if present, and free the lead with a light pull rather than by levering with the iron.
Workshop notes
Heat until the joint collapses, then one more second
The tell that a through-hole joint is ready is not colour, it is movement: the lead becomes free in the hole and the fillet drops to a flat pool. At a 380 degrees C chisel with a 3.2 mm face, that takes about two and a half seconds on a 2.6 mm pad. Wait one more second so the alloy at the barrel bottom is molten too - the pad surface is always hotter than the barrel, and firing the pump early leaves the plug you were trying to remove.
Nozzle selection and sealing
A nozzle 1 to 1.5 mm larger than the lead works best: it must sit over the pad and seal against it, with the lead disappearing up the bore. If the nozzle is too wide, air enters from the side and the vacuum never develops. If it is too tight, the lead blocks the bore and solder cannot travel. Keep two spare nozzles of each size on the bench, because a clogged nozzle looks identical to a weak pump.
Add fresh solder before removing old solder
Counter-intuitive but reliable: feeding a little fresh Sn63Pb37 onto the joint before desoldering improves heat transfer and lowers the effective melting range of the mixture. Old lead-free joints, particularly ones that have been reworked before, have a pasty range widened by copper dissolution from the barrel. Adding fresh eutectic alloy brings the mixture back toward 183 degrees C and the joint clears in a single shot.
Practice drill
Desolder a 16-pin DIP from a scrap board, then re-solder it, and repeat five times on the same footprint. Time each pass. A clean operator does 16 pins in under 90 seconds; if you are over 150 seconds, the pump is being fired too early.
Failure modes
| Symptom | Cause | Treatment |
|---|---|---|
| Solder ring left welded inside the barrel | Pump fired before the alloy at the barrel bottom was molten | Re-flux, add fresh eutectic solder, reheat for a full second past the collapse point, and wick out the ring with braid |
| Pad lifts off the board | Mechanical leverage applied with the iron or pump instead of waiting for full melt | Never lever: keep heating until the lead falls out under gravity, and add solder rather than force |
| Vacuum seems weak and clears nothing | Nozzle bore clogged with solidified alloy, or tip seal worn flat | Clear the bore with a 1.0 mm drill bit by hand, and replace the nozzle if the sealing face shows a bright wear ring |
Supplies
- 0.8 mm Sn63Pb37 solder for the pre-charge bead
- RMA liquid flux
- Isopropyl alcohol and swabs
- Replacement vacuum nozzles
Tools
- Temperature-controlled station at 380 degrees C
- 3.2 mm chisel tip
- Spring-loaded desoldering pump with spare tips
- Desoldering braid
Safety
A 380 degrees C tip plus a spring-loaded pump is the classic burn combination: the solder spray from the nozzle leaves at speed and sticks. Point the nozzle away from your face and wear safety glasses, not just a loupe.
Neighbouring cards
- Soldering a pad tied to an internal ground plane 400 °C / 4.0 s
- Drag soldering a 0.1 in header 360 °C / 1.2 s
- Wick cleaning a through-hole pad without lifting it 350 °C / 1.5 s
- Diagnosing and repairing a cold joint 340 °C / 2.5 s