Rework, Desoldering and Repair
Wick, pump and hot air, removing chips and connectors, lifted pads, jumpers and how many cycles a board survives.
Desoldering tools
Rework is removing a part and replacing it; repair restores a damaged board; desoldering is taking out the solder. All three use heat and a way to remove or hold the molten solder, and they all benefit from the same rule: add fresh flux, because the flux in old joints has been burnt off.
| Tool | How it works | Best for |
|---|---|---|
| Desoldering braid (wick) | Copper braid with flux soaks up molten solder by capillary action | Pads, bridges, small excess |
| Solder pump (sucker) | Spring piston pulls molten solder into a barrel | Through-hole pins, one at a time |
| Desoldering station / gun | Heated hollow tip with a vacuum pump | Many through-hole joints, large parts |
| Hot-air station | Heated air melts many joints at once | SMD, QFP, SOIC, connectors |
| Low-melting alloy | Adds an alloy that melts below 100 °C to keep all the solder molten | Multi-pin parts with an iron |
| Hot tweezers | Two heated tips pinch a chip part | Resistors, capacitors and small SMDs |
| Pre-heater | Warms the whole board from below | Large or multilayer boards |
| Flux and cleaner | Gel flux, isopropyl alcohol, brush | Everything |
Worked example: how much braid does a joint need?
- A 2.0 mm wide braid is about 0.25 mm thick; if half its volume is voids (typical), each millimeter of braid holds 0.25 mm³ of solder.
- A through-hole joint holds about 2.7 mm³ (see Through-Hole Soldering), so a single joint uses 10.8 mm of braid, about a centimeter; a wick that is full of solder must be cut off.
- Clearing the excess 0.5 mm³ per pin on a 20-pin row of a QFP needs 40 mm of braid.
Through-hole parts
- Wick or pump each pin: add flux, heat the joint (adding a little fresh solder helps heat flow), then put the braid on the joint and the iron on the braid, or put the pump next to the tip and trigger it as the solder melts.
- Cut the leads of an unwanted part (resistor, capacitor), remove each lead separately with the iron and tweezers, and then clear each hole: much less risk of lifting a pad.
- For ICs and connectors with many pins, a desoldering station or a pre-heater with a pump is best. Do not twist or pull on a part that has not released.
- Clear the hole: heat it and push a wooden toothpick or a small drill through, or clean with braid; refill from the other side with flux and fresh solder.
- Fill each joint with fresh solder before removal if they are old or lead-free, because fresh leaded solder lowers the melting point and carries the heat.
Always check that a pad has not been torn off or the barrel pulled out, which can happen at 2 s to 3 s per pin if the iron is too cool.
Surface-mount parts and hot air
Chip resistors and capacitors come off with two irons or hot tweezers, or hot air at low flow; small ICs (SOIC, TSSOP) with hot air or by flooding the pins with solder and low-melting alloy. The principle with hot air is to melt all the joints together and lift the part with tweezers or a vacuum pen, and then clean the pads and fit the new part.
Worked example: how long does a chip take to release?
- The hot-air tool in the example blows 15 L/min at 350 °C, so its power is 98 W; about 15 % reaches the part.
- The energy to bring a package of mass m (c ≈ 0.9 J/g/K) from 25 to 237 °C is the heating energy shown below; dividing by 14.7 W gives the time for the package alone. The board and its copper absorb heat too, so the real time is typically 3 times longer (illustrative).
| Package | Mass | Energy to 237 °C rise | Time (part only) | Time with board (×3) |
|---|---|---|---|---|
| SOT-23-5 | 0.02 g | 3 J | 0.2 s | 1 s |
| SOIC-8 | 0.08 g | 17 J | 1.2 s | 3 s |
| TSSOP-20 | 0.08 g | 17 J | 1.2 s | 3 s |
| QFN-32 (5 × 5 mm) | 0.06 g | 13 J | 0.9 s | 3 s |
| LQFP-100 (14 × 14 mm) | 0.65 g | 139 J | 9.4 s | 28 s |
| BGA-256 (17 × 17 mm) | 1.00 g | 213 J | 14.5 s | 44 s |
- Pre-heat the board to 100 to 150 °C from below; it removes thermal shock and shortens the hot-air time.
- Shield neighbors with Kapton tape or foil; keep the nozzle moving in small circles.
- Lift only when all the pins have melted: wiggle the part gently with the tweezers; if it resists, wait.
- Clean the pads: add flux and drag a clean braid across to flatten them (do not scrub).
- Fit the new part: add fresh flux or paste and align the part carefully, then solder as for new work (see Surface-Mount Soldering).
Fine-pitch, QFN and BGA rework
Leaded fine-pitch parts (QFP) can be removed by flooding all the pins with solder or low-melting alloy and sweeping the iron along the rows, then lifting. Leadless parts (QFN, DFN) and BGAs have hidden joints, so they need hot air or a rework station over pre-heat, with the board supported underneath so that it does not warp. BGAs must be reballed or replaced with a new part, and the result checked by X-ray or an electrical test. They are not a hand job.
Worked example: low-melting alloy
- A bismuth-based low-melt alloy melts at about 58 °C (typical), against 217 °C for SAC305. Mixed into the joints, it keeps the whole row of pins liquid long enough, at an iron temperature far below the solder’s.
- The mixed alloy has a lower melting point and is weaker, so the pads must be cleaned of the alloy (with braid and fresh solder) before the new part is fitted. A residue of low-melt alloy in a lead-free joint can fail early.
Lifted pads, broken tracks and jumpers
A pad that lifted from the board, or a track that broke, can be repaired. The pad’s bond to the board is the weak point: it is a thin copper foil (35 µm) held by adhesive, which weakens with heat.
- Lifted pad, still attached to the track: clean, press the pad down, flow cyanoacrylate or epoxy under it, and hold. Solder the lead to the pad without letting it carry the load; fix the part mechanically (glue it) if it is heavy.
- Pad gone, track intact: scrape the solder mask from the track for 1 to 2 mm, tin it, and solder the lead (or a jumper wire) to the track. Cover with epoxy or UV mask.
- Broken track: scrape the mask on each side of the break, tin both ends and bridge them with 30 AWG wire-wrap (Kynar) wire; fix the wire with epoxy and cover the join with UV solder mask.
- Pad replacement kits (adhesive-backed copper pads) can replace a pad properly on a valuable board.
- Plated through-holes: a damaged barrel can be restored with an eyelet or a short wire through the hole soldered on both sides.
Worked example: a jumper wire
- 30 AWG wire-wrap wire has a diameter of 0.25 mm and an area of 0.05 mm². A 50 mm jumper has a resistance of 17 mΩ (copper, 20 °C) and carries about 0.5 A: enough for logic, not for a power track.
- For power use 24 to 18 AWG; keep the wire short and run it flat, with an adhesive dab every 10 to 15 mm. Jumpers on high-speed signals should be short to limit extra inductance (about 1 nH per millimeter of wire in free space).
How many cycles a board survives
Every time a joint is melted, the pad is heated and the intermetallic layer grows (see How Soldering Works). The pad’s adhesive weakens, flux burns away, the board delaminates and the plated barrels fatigue. IPC-7711/7721 limits the number of reworks; as a rule of thumb a board can survive about three reflow cycles in total (the original plus two reworks), but fewer for thin pads, multi-layer boards, lead-free and BGAs.
Worked example: cycles and layer growth
- One reflow with 62 s above liquidus. The intermetallic layer grows with √(total time), so after n cycles it is √n times thicker: 1.73× after three cycles.
- If a pad’s peel strength drops by about 7 % each cycle (illustrative), the typical 1.4 N/mm for 35 µm copper falls to 1.21 N/mm after three cycles.
| Cycles | Total time above liquidus | Intermetallic thickness (relative) | Peel strength (relative, illustrative) |
|---|---|---|---|
| 1 | 62 s | 1.00× | 100 % |
| 2 | 125 s | 1.41× | 93 % |
| 3 | 187 s | 1.73× | 86 % |
| 4 | 249 s | 2.00× | 80 % |
| 5 | 312 s | 2.24× | 75 % |
- Plan the work: do all the rework in one session if you can.
- Use as little heat as possible for the shortest time and do not reflow a board you do not have to.
- Record the rework on high-reliability products.
After rework: clean and check
- Clean with isopropyl alcohol and a brush, to remove flux that is no longer active and may corrode (see Solder Alloys, Flux and Paste).
- Inspect every reworked joint with magnification; compare with the criteria on Joint Quality, Inspection and Failure.
- Check for bridges and shorts with a meter; check the supply for shorts before power-on.
- Test the function, and re-inspect after a thermal or vibration test for high-reliability work.
Troubleshooting
| Symptom | Likely cause | What to do |
|---|---|---|
| Wick will not pick up solder | No flux, cold iron, oxidized braid | Add flux, use a hotter tip, cut off the used braid |
| Hole stays blocked | Solder cooled in the hole | Reheat with flux; push a toothpick or use the pump from the other side |
| Pad lifts while desoldering | Pulled the part while solder was still solid; too hot too long | Cut the leads; lower the force; use a pre-heater |
| Chip will not release | Pre-heat missing; ground-plane heat sinking | Pre-heat and use more heat on the ground pins |
| Neighboring parts shifted | Hot air too wide or too strong | Shield; lower flow; a smaller nozzle |
| Board scorched or bubbles | Overheated | Lower the temperature; shorter times; pre-heat |
| Part works, then fails | Cracked joint, moisture damage, ESD | Re-inspect; bake moisture-sensitive parts; use ESD care (see Soldering Safety, Fumes and ESD) |
Related
- Soldering: the overview of this guide.
- Irons, Stations, Tips and Hot Air: hot air and irons.
- Joint Quality, Inspection and Failure: inspecting the result.
- Design, Testing and Troubleshooting: locating the faulty part first.
FAQ
Braid or pump?
Braid cleans pads and bridges well; a pump removes the solder from a through-hole joint in one go. Many people use both.
How many times can I rework a pad?
A rule of thumb is two reworks on a normal board; fewer on thin pads, BGAs and lead-free work. Each melt weakens the pad’s bond.
Why does desoldering lead-free joints feel harder?
The solder melts at a higher temperature and wets less, so adding a little fresh leaded or low-melt solder and extra flux makes it flow.
Can I remove a BGA with a hot-air gun?
It is possible with pre-heat and a proper nozzle but risky; reballing and checking needs equipment. Use a rework station or have it done.
How do I repair a torn-off pad?
Expose the track, fix the lead or a jumper to it and secure with epoxy; use a pad kit for a neat repair.
References
- IPC-7711/7721, Rework, Modification and Repair of Electronic Assemblies; IPC-A-610; IPC J-STD-001.
- Lee, N.-C., Reflow Soldering Processes and Troubleshooting, Newnes.
- Manufacturer data for desoldering braid, low-melt alloys and rework stations.
Last reviewed: October 2026. Capacities, times and the strength loss per cycle are typical or illustrative.