Soldering & Joints
Soldering is the one hands-on skill the Associate CET exam expects every technician to understand — how a good joint forms, what temperature and technique produce it, and how to spot a bad one. This guide covers the concepts and the inspection criteria you'll be tested on.
Soldering joins metal parts by melting a filler metal (solder) into the gap between them, where it bonds to each surface and solidifies into an electrically and mechanically sound connection. The key idea: the solder does not act like glue sitting on top — done right, it forms a metallurgical bond called wetting, where the molten solder flows into and alloys with the surface of the base metal.
The heat comes from the iron, but a crucial rule is that you heat the joint (the lead and the pad together), not the solder. You then feed solder to the heated joint so the joint's own heat melts it. This is what lets the solder flow by capillary action and wet both surfaces.
A textbook joint is shiny, smooth, and concave — the solder curves up the lead and feathers out onto the pad at a shallow angle (a "volcano" or "Hershey's Kiss" shape). That low contact angle is the visual proof of good wetting. You should still be able to see the outline of the lead through the solder.
- Clean and prep. Surfaces must be clean and oxide-free. Flux removes oxidation so the solder can wet the metal.
- Heat the joint. Touch the iron tip to both the lead and the pad at once, so they heat together. Hold for one to two seconds.
- Feed the solder to the joint — not to the iron tip. The heated joint melts the solder, which proves it's hot enough to bond.
- Let it flow. The solder flows around the lead and wets the pad, forming the concave fillet. A small amount is enough.
- Remove solder, then iron. Pull the solder away first, then the iron, and hold the joint still until it solidifies (about a second).
- Inspect. Look for a shiny, smooth, concave joint. Re-do anything dull, blobby, or cracked.
There's no single "correct" temperature, but there's a usable working range. Too cool and the solder won't flow (cold joints); too hot and you damage components, lift pads, or burn off the flux before it can work.
| Parameter | Typical value | Why |
|---|---|---|
| Iron tip temp | 315–370 °C (600–700 °F) | Hot enough to melt solder fast, not so hot it burns flux/pads |
| Leaded solder melt (63/37) | 183 °C (361 °F) | Eutectic — melts/freezes instantly, no plastic range |
| Lead-free solder melt | ~217 °C (423 °F) | Higher melting point → needs a hotter iron. Typically tin with silver and bismuth. |
| Dwell time on joint | 1–3 seconds | Long enough to wet, short enough to protect the part |
Shiny, smooth, concave fillet. Feathers onto the pad at a low angle. Lead outline still visible. Mechanically solid.
Dull, grainy, or lumpy. Caused by too little heat or movement while cooling. May look connected but is electrically unreliable — a top exam topic.
Solder balled up and didn't wet the pad (high contact angle). Not enough heat to the work, or dirty/oxidized surface.
Excess solder shorts two adjacent pads or pins together. Caused by too much solder. Remove with wick or a clean iron tip.
Fine crack around the lead, often from thermal/mechanical stress over time. Reflow to fix.
Frosted, slightly fractured surface from the joint being moved before it solidified. Reheat and let cool still.
Removing solder is as much a skill as applying it. The two standard tools: solder wick (braided copper that wicks molten solder away) and a solder sucker (a spring-loaded vacuum pump that slurps up melted solder). Reflow a suspect joint by adding a touch of fresh flux-cored solder and reheating — that alone fixes many cold and cracked joints.