Why is cleaning "no clean" flux residues difficult and how to get it right?

No clean flux residues on printed circuit boards are not designed to be cleaned easily. Fact.

Therefore, cleaning the residues left behind by a no clean flux process is one of the most difficult tasks when considering cleaning.

How do you “clean” no clean flux residues if you have to?

Whether a flux residue can be cleaned effectively depends on the cleaning materials saponification factor and its compatibility with the residues.

Saponification is the ability of the no clean residues to be softened to the point of being able to be dissolved by the alkali content (the saponifier) of the cleaning chemistry. The higher the saponification factor of the cleaning fluid the easier it is to clean the residues.

So the key here is to ensure that the saponifier completely dissolves the residues.

Cleaning residues left behind by a no clean flux process is one of the most difficult tasks when considering cleaning.  After all, the residues left on the circuit board are not formulated to be cleaned away easily.

Cleaning residues left behind by a no clean flux process is one of the most difficult tasks when considering cleaning.  After all, the residues left on the circuit board are not formulated to be cleaned away easily.

What happens if the residues are only partially dissolved?

A no-clean residue that is only partly cleaned away may be far worse for a printed circuit board assembly (PCBA) than a no-clean residue left untouched.

One of the reasons is because lead free flux activators are more active than those in earlier leaded flux formulations. 

In a no clean flux, when un-cleaned, the residues are locked up in the carrier resin matrix. They are stable (benign) at normal operational temperatures and therefore will not leach out dangerous residues and cause corrosion problems.

However, if the protective matrix around the residue is partially removed by an inadequate cleaning regime, then the activators could be exposed.

This may lead to a corrosion process starting on the circuit board. Further, this process could be accelerated in the presence of heat, power on the boards in service or high relative humidity. 

So how do you clean “no-clean” residues?

It is important when considering cleaning “no-clean” residues on a circuit board that you consider three points:

  1. Can you actually clean the residue to be cleaned effectively?
  2. Have you matched the cleaning chemistry with the relative degree of difficulty and the available process?
  3. Have you validated the whole process by careful testing?

Consider these three points and it may help you be successful. Not considering these three points could easily lead you to having real problems in the long term.


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