What is a Flux Residue?

Flux residues are some of the most common and damaging residues that afflict printed circuit board assemblies. Field product failures occur regularly due to flux residues that have not been fully volatized from the board during assembly processes. Because today’s circuitry is becoming smaller and more complex, there are more opportunities for flux residues to become entrapped in critical areas of circuitry or beneath low standoff components. In addition, finer pitch results in circuitry being more sensitive to detrimental residues.

 

Common Flux Residues

The most common flux residues found are chloride and weak organic acids. Weak organic acids include substances like adipic and succinic acids. Flux residues are a result of flux activators, and if allowed to fully complex and volatize from boards, do not usually cause issues. Flux deposition is also very important in preventing harmful flux residues from being left on boards. If too much flux is deposited, thermal energy during preheat is spent driving off solvents rather than complexing the flux residues.

 

Negative Effects of Flux Residues

When flux residues are left on PCBs, they have the potential to react with moisture in the presence of an applied voltage to cause electromigration and electrical current leakage. It is therefore important to make sure that flux deposition is controlled, flux residues are volatized, and the cleaning processes are optimized.

 

Flux Residue Photos

 

For more examples, visit our Photo Gallery.