Soldering, Desoldering and SMD Rework
The Soldering, Desoldering and SMD Rework domain covers hand soldering of small parts, hot-air rework and the inspection that proves a joint is good. Questions ask for temperatures, sequence and fault recognition.
Consumables and settings
- Lead-free SAC305 solder 0.3-0.5 mm for fine work, no-clean or gel flux, desoldering wick and a pump.
- Iron tip 320-360 °C for lead-free, 300-320 °C for leaded; hot air 350-380 °C with moderate airflow for small ICs, plus a board preheater at 100-150 °C for BGAs.
- Keep the tip tinned, wipe before each joint, and never use acid plumbing flux on electronics.
Technique and rework
- Heat the pad and the lead together, feed solder, remove the iron, and let the joint freeze without movement.
- To remove: add flux, wick the old solder, lift the part with tweezers. To fit: align, tack one end, solder the rest, inspect.
- Clean flux with IPA and a brush, then check under a microscope for bridges, cold joints and lifted pads.
Worked example
You must replace a 0402 resistor. Reasoning: apply flux, wick the old solder from both pads, place the new part with tweezers, tack one pad, check alignment, then solder the other pad. A joint that looks dull and balled is a cold joint; add flux and reflow it. Moving the part while the solder is liquid gives a tombstone.
Traps that cost marks
- Too much heat or a long dwell, which lifts pads and delaminates the board.
- Soldering without flux, or adding solder to a joint that is already bridged.
- Using a metal tool on a battery or shield while the board is powered.
- Not cleaning flux, which later causes leakage and corrosion.