How to clean “no clean” flux residues from printed circuit boards and do it right

Cleaning no clean flux residues remaining on a printed circuit board assembly (PCBA) is one of the most difficult cleaning processes.

By definition no clean flux residue are harder to clean than conventional rosin or modified resin based residues.  This is because the residue is not meant to be cleanable.

However, there are times when the residues do need to be cleaned.

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

Whether a flux residue can be cleaned effectively depends on the cleaning materials saponification factor and its compatibility with the no clean 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. Also, if the saponifier is not compatible then it won’t work as well.

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


What happens if the residues are only partially dissolved by the saponifier?

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

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

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 and 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 without problems?

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

  • The ability of the residue to be cleaned is determined?
  • The cleaning chemistry is matched to the relative degree of difficulty and the available process.
  • The success of the whole process is validated by careful testing

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.


Need to find out more?

For further information on cleaning circuit boards for conformal coating then review our cleaning section or contact us directly.

If you are new to Nexus and our work on conformal coatings then a good place to go is our Start Here page or our free conformal coating ebook.

 

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Author: nexus3c

A global resource for the Conformal Coating industry providing information, solutions, connections and advice on all areas of Conformal Coating including Conformal Coating Service, Equipment, Materials, Training, Documentation and Consultancy.

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