Cold joints and bridges

The reason inspection is a separate skill from soldering is that a bad joint does not usually announce itself. It conducts. The board works. Six weeks later it is in a warm room or somebody knocked the enclosure, and it stops — and now you are debugging firmware.

Look at the shape, not the shine

The word every guide uses is "shiny", and it is a weak test: lead-free solder is duller than leaded even when the joint is perfect, so half the readers of that advice reheat good joints looking for a gloss that was never coming.

The shape is the reliable tell. A finished joint is concave — it curves inward as it climbs the lead, like the side of a volcano — and you can still make out the edge of the copper pad through it. Anything convex, rounded outward like a droplet, has not wet the pad: the solder is holding its own surface tension instead of being pulled flat by the copper.

The cold joint is the expensive one

It is a grey, rounded ball with a faint seam where it meets the pad. It happens when the lead got hot and the pad did not, which is almost always the iron sitting on one side rather than in the corner between them.

It usually conducts at first, through a thin mechanical contact rather than an alloy. Then the two metals expand and contract at different rates through a few dozen warm days, and the contact opens. That delay is the whole problem: by the time it fails, nothing about the symptom points at soldering.

To fix one, reheat it until it visibly collapses into the pad, and add a touch of fresh solder while you do — the flux in the old joint burned off the first time and there is nothing left to pull it flat.

Bridges are the easy one

Two adjacent pads joined by solder. They are easy to fix — lay wick across the pair, iron on top of the wick, and it lifts the excess away — and easy to miss on anything fine-pitch.

Hold the board up against a lamp and look along the row of pins from a low angle. A bridge that is invisible from directly above shows immediately as a filled gap when the light is behind it. That takes five seconds and finds more faults than any amount of continuity testing.

A checklist that fits on one line

Before you power anything up: no bridges, no bare pad, nothing round, nothing grey, no lead left long enough to touch its neighbour.

Then power it up on a bench supply with the current limit set to something the board should never need. A short you missed trips the limit instead of removing a track.