# Intermittent fault from a cold solder joint: how to spot and redo it

> Spot a cold solder joint \(dull, grainy, cracked or blobby\), reheat it with fresh solder and test again.

Canonical: https://roboticsengineering.in/fix/soldering-cold-joint-dull-grey-intermittent/ | Published: 2026-11-03 | Updated: 2026-11-04

**A good solder joint is shiny and smooth, shaped like a small cone around the lead.** A dull, grainy or blobby joint may be cold.

## What the message means

A cold joint did not get hot enough, or the part moved while it cooled. It conducts badly or only sometimes.

## Fix it in this order

1. Look at the joints with good light and a magnifier. Check for a dull, crackled or ball-shaped surface.
2. Reheat the joint with the iron on both the pad and the lead for a moment and feed a little fresh solder.
3. Hold the part still until the solder sets.
4. Use solder with flux core and keep the tip clean and tinned.
5. Check the iron temperature is suitable for the solder you use.
6. Wiggle the part gently while the circuit runs to find the loose one.

## Why it happens

Solder forms a good joint when both the lead and the pad are hot enough for it to flow and bond to each surface. If either stays too cool, or the part moves while the solder sets, the bond is weak and the surface turns dull and grainy instead of smooth and shiny.

A weak bond can conduct well enough to pass a quick test and then fail when the board is knocked or warms up, which is why the fault comes and goes. Reheating the joint, with a little fresh solder, gives it a second chance to flow.

## If you see this, do this

| You see | Do this |
| --- | --- |
| Fault changes when you touch the board | Check the joints. |
| Joint will not take solder | Clean the pad and the lead. |

## Related

[loose contacts](https://roboticsengineering.in/fix/loose-breadboard-contacts-intermittent-robot/), [check for a short](https://roboticsengineering.in/fix/check-short-circuit-with-multimeter-before-power-on/).

Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.

## Why this happens

Solder joins metal by wetting it: the molten alloy has to flow over the clean surface of both the lead and the pad and form a thin bonded layer. If the surfaces are oxidised, too cool, or move while the solder is setting, the alloy only sits on the surface. Electrically it can look fine on a bench, but the contact is poor, so the resistance changes when the board flexes or warms.

That is why the symptom is intermittent. A joint with high resistance passes a small signal current but drops voltage when a load current flows through it, which is why a cold joint on a power pin or a motor connector is far more noticeable than one on a signal line.

## How to confirm the cause

- Inspect every joint of the suspect part under a magnifier and good side light. Dull, grainy, ball-shaped or cracked rings around the lead are the usual signs.
- With the circuit running, press or tap the board near the part. If the fault appears and disappears, the joint is the likely cause.
- Measure the voltage drop across a joint on a power path while the load runs. Place the meter probes on the pad and on the lead, not the whole board. A good joint reads a few millivolts.
- With power off, set the meter to continuity and wiggle the part gently while watching for interruptions.
- Reflow the joint and test again. If the fault disappears, the cause is confirmed.

## Worked example

A walk-through of the voltage-drop check on a connector pin that carries 0.5 A:

```
Meter on DC volts, probes on the connector pin
and on the track pad right next to it.

Reading 0.002 V -> R = 0.002 / 0.5 = 0.004 ohm (good)
Reading 0.300 V -> R = 0.300 / 0.5 = 0.6 ohm (poor joint)

A 0.6 ohm joint at 0.5 A also wastes
P = I x I x R = 0.5 x 0.5 x 0.6 = 0.15 W as heat.
```

## Common mistakes

- Melting the solder on the tip and carrying it to the joint instead of heating the pad and the lead together.
- Moving the part while the solder is still setting.
- Adding more solder to a bad joint without reheating it, which only hides the problem.
- Using a dirty or oxidised tip, so heat does not transfer well.
- Turning the temperature too high, which can lift pads and burn off flux before it cleans the metal.

## If this did not fix it

- A cracked track or broken wire inside its insulation near the connector, which acts like a bad joint.
- Headers or pins on a module that were never soldered, only pressed in.
- A loose connector contact or a corroded breadboard row if the fault only appears on a prototype.