What is a cold joint?
A cold joint is one of the most common problems in soldering and can be identified when it appears to be distorted, whitish, and swelling/bulging. A cold joint is often confused with a disturbed joint due to the similar appearance, however, they are in fact different issues with different remedies.
How is a cold joint formed?
- Faulty melting or insufficient heating. Whenever the solder does not fully melt when soldering or is heated insufficiently, it would form an improper joint causing the joint to bulge and swell.
Disturbances during solidification. When the solder is cooling and has yet to be harden, moving the solder will cause it to misshape.
Too high of a soldering temperature. An extreme temperature during soldering would cause the flux to prematurely breakdown and founder.
- Too low of a soldering temperature. A temperature that is too low during soldering would cause incomplete wetting of the solder joint.
- Mismatched geometry of the components. An inconsistent geometry between the components and the solder joint would impact the reliability of the solder joint.
How to prevent a cold joint?
- Use an appropriate reflow profile with reference to the solder manufacturers' specification
- Set the temperature at least 15°C above the melting point of the solder alloy for around a minute
- Omit any vibration source (e.g. device vibration, human error, etc.)
- Use a good quality solder paste – Hakko FS-100 is best suited for the removal of oxidation on soldering tips
How to fix a cold joint?
Cold joints can often be quickly fixed by simply re-heating the cold joint with a hot metal until the solder flows. Most cold joints are a byproduct of excess solder; therefore, the extra solder could be removed with the tip of an iron.
However, some cold joints require additional tools and steps. If the aforementioned remedies are insufficient, simply follow these steps,
1) Flux the cold joint with the FS-210 Brush Tip Flux Pen
2) Reheat the cold solder joint with a preheater or the FV-310 Heat Gun to remelt the solder. Do so until the solder shapes itself into the conventional solder fillet.
(Remember not to heat for too long)
3) Remove excess flux using the Techspray G3 Flux Remover