Soldering, Desoldering and Component Replacement
This domain tests heat control and workmanship when replacing connectors, passive components and ICs. The exam focuses on safe technique, correct materials and avoiding damage to pads and neighbouring parts.
Soldering basics
Use an adjustable temperature-controlled iron with a clean, tinned tip. Lead-free solder typically melts around 217–227 °C; set the iron hot enough to form a shiny joint quickly, often around 320–360 °C, and use flux to improve wetting. Heat the pad and lead together, apply solder, then remove the iron. A good joint is smooth and concave. For surface-mount parts, tweezers and a magnifier help; for connectors, add flux and use hot air or a specialised tip.
Desoldering and rework
Remove solder with desoldering braid or a pump, then lift the component without tearing pads. Hot-air rework for small ICs commonly uses 350–400 °C with controlled airflow; preheat large boards to reduce thermal shock. BGA replacement requires a stencil, paste or preformed balls and a reflow profile. Never force a component while solder is solid.
Worked example
A micro-USB charging port is loose. The technician powers down, removes the battery, applies flux, desolders the shield tabs and pins with hot air and an iron, cleans the pads with braid, aligns the new port and solders the tabs first. After cleaning, the technician checks for bridges with a magnifier and tests charging current before reassembly.
Traps
Too much heat lifts pads; too little heat gives cold joints. Do not use acid plumbing flux on electronics. Do not reflow a board with a lithium battery attached. A joint that looks dull may be cold, not necessarily defective-check with a gentle tug and continuity.