Cold Solder Joint: Symptoms, Causes and Fixes
A circuit worked yesterday and doesn't today; or it recovers when you press on the board and fails again when you let go. The most common cause of these "ghost" faults is a cold solder joint — a joint where the solder solidified without properly melting and flowing, forming a poor connection. This article isn't about how to solder; it explains how to diagnose and fix a bad joint.
Why does a cold joint happen?
A solid joint needs heat, cleanliness and stillness; if any one is missing, the connection stays weak:
- Insufficient heat: the iron doesn't heat the pad and lead enough; the solder freezes before it flows.
- Dirty/oxidized surface: solder won't stick to a flux-free or oxidized surface.
- Movement while cooling: the part shifting as the solder solidifies creates an internal crack.
- Too little solder or wrong sequence: touching the solder to the iron instead of the pad is a classic mistake.
Symptoms: how to spot one
| Clue | Good joint | Cold joint |
|---|---|---|
| Appearance | Shiny, smooth cone | Dull, grainy, lumpy |
| Shape | Spread over the pad, concave | Ball-like, not touching the pad |
| Surroundings | Integrated | Cracked ring around the lead |
| Behavior | Stable | Intermittent / changes when pressed |
Diagnosis steps
- Eye + magnifier: flag joints that look dull, cracked or lumpy; good lighting is essential.
- Continuity test: measure between the two ends of the connection with the multimeter's continuity range; an unstable beep points to a cold joint.
- Press/push test: if lightly pressing the joint with an insulating stick changes the circuit's state, that's likely the culprit.
- Verify with heat: if reheating the suspect joint makes the fault disappear, the diagnosis is confirmed.
Similar intermittent faults sometimes come from unwanted bridges instead; in that case, PCB short-detection methods are a useful complement.
The fix
Fixing a cold joint is usually simple; the right order is:
- Add flux: applying a little flux to the joint eases reflow and wetting.
- Reheat with enough heat: touch the iron to both the pad and the lead and wait until you see the solder flow fully.
- If it's badly damaged, remove the solder: clean off the old solder with wick/pump and redo it with fresh solder.
- Don't move it while cooling: hold the part still until the solder solidifies shiny and concave.
Prevention
A clean iron tip at the right temperature, flux-cored solder and the habit of securing the part prevent the majority of cold joints in the first place.
Cold joint, or a different defect?
Not every bad-looking joint is a cold joint; to pick the right remedy, distinguish the defect:
| Defect | Appearance | Root cause |
|---|---|---|
| Cold joint | Dull, lumpy, cracked ring | Insufficient heat / movement |
| Dewetting | Solder not adhering to the pad, balled up | Dirty/oxidized surface, no flux |
| Bridge | Solder touching between two pads | Too much solder |
| Tombstone | One end of an SMD part lifted in the air | Uneven heating (reflow) |
Even if the appearance is similar, the solution differs: with dewetting you first clean the surface and add flux; with a bridge you remove the excess solder. For reflow defects like tombstoning, the profile recommendations in our reflow SMD soldering article help.
Common mistakes
- Touching solder to the iron and dripping it: the solder should melt from the pad's heat; otherwise it won't adhere.
- Too-short contact: pulling the iron away before the pad heats up produces a cold joint.
- Not using flux: solder won't flow on an oxidized surface; it balls up.
- Adding excess solder: if the problem is heat, more solder hides it but doesn't solve it.
Frequently asked questions
Does a cold joint damage the circuit?
It usually causes unstable/intermittent operation rather than permanent damage. But on a connection carrying high current it can cause resistance and heating, and thus further damage.
Are cold joints more common with lead-free solder?
Lead-free solder has a higher melting point and a naturally slightly duller appearance; so applying enough heat and reading the appearance correctly matters more.
Is every dull joint a cold joint?
No. Lead-free solder can be somewhat dull even when normal. The real symptoms are a lumpy/ball shape, a cracked ring and unstable behavior.
Conclusion
The cold joint is the prime suspect in intermittent faults; it's found quickly with a visual check under good light, a continuity test and a press test, and fixed permanently with flux + enough heat. When repairing a board, our guide on salvaging components may also help with the parts you recover. You can review supplier offers for the solder, flux and consumables you need with search.