Why cleaning "no clean" flux residues from a circuit board assembly can be difficult and how to get it right

Author: Dr Lee Hitchens

So why is no clean flux so difficult to clean?

Cleaning no clean flux residues is one of the most complex processes when considering cleaning.

No clean fluxes are by definition harder to clean than conventional rosin or modified resin based residues.  After all, the flux residues left on a printed circuit board assembly are supposed to be permanent. It was designed to be a no clean process.

So, what do you do when you want to clean the no clean residues?

Well, the reality is whether a flux residue can be cleaned effectively does depend on two factors relating to the cleaning material being used.

They are the saponification factor of the cleaning fluid and the cleaning media’s compatibility with the flux residues.

First, consider the saponification factor.

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 flux residues from the circuit.

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

Second, lets consider the compatibility of the cleaning media.

Well, this is pretty straightforward. The cleaning fluid needs to have the right chemistry components that work with the no clean residues. That’s the combination of solvents, liquids and additives that make the system work.

This concept is simple to understand but absolutely critical to the performance of the cleaning process.

What happens if the residues are only partially dissolved?

The reality is that a no-clean residue on a circuit board that is only partly cleaned away could be far worse for the reliability of a printed circuit board assembly than a no-clean residue left untouched from a corrosion point of view.

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

When the flux residues are not cleaned, the dangerous components of the residue like the flux activators are locked up in the carrier resin matrix.

Further, this carrier resin should be stable (benign) at normal operational temperatures. Therefore, the residues should not allow any leaching out of dangerous residues that could 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 and this process could be accelerated in the presence of heat and power on the boards in service and, or high relative humidity. 

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

There is no simple answer to this question.

However, it is important when considering cleaning “no-clean” residues on a circuit board that you:

  • Check the ability of the no clean residue to be cleaned is determined. Talk to the flux manufacturers and ask them what they would recommend.
  • Confirm the cleaning chemistry is matched to the relative degree of difficulty of the flux residues and the available cleaning process. You don’t need a hammer to crack a nut but you need a hammer to hit a nail. Select correctly.
  • Confirm the success of the whole cleaning process by validating with careful testing. Use the right tools like ionic testing and other systems to ensure that the residues are removed.

Following these three guidelines can 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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