Flux Residue Removal

Removing No-Clean Flux Residues

Suggestions of how to remove the residues from no-clean fluxes: To remove the residues of no-clean fluxes most solvents will work. The best results are achieved by removing the residues in an aqueous spray cleaner. The spray cleaner can be either a batch type or an in-line type. The addition of a saponifier (Kester # 5768) and hot water, about 130°F will completely remove the flux residues.

Removing Organic Acid (OA) Flux Residues

It is required to remove the residues of OA fluxes? You will get corrosion if you do not fully remove all the flux residues. The flux residues from organic fluxes are hygroscopic, they contain organic acids and they may contain halides. Over time the residues will absorb moisture and the halides will become mobile and corrosive. This will lead to ‘catastrophic failure’ of the assembly in question. We recommend cleaning with an aqueous cleaning system, it can be either batch type or in-line. The water temperature should be between 120-140°F. The important thing is to have lots of rinsing and flushing to completely remove all the flux residues.

White Residue

Identification and categorization of white residues often found on cleaned circuit assemblies.
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Flux Thinning

Soldering fluxes lose solvent and need additions of thinner. Flux is composed of solids dissolved in a solvent base. Over time the solvent evaporates. As the solvent evaporates the solids concentrate and eventually crystallize out. You will need to add thinner on a regular basis to prevent the flux from crystallizing. A hydrometer is used to measure the specific gravity of the flux. Normally the specific gravity (density) is checked 2 or 3 times a day. Thinner is then added to return the flux to its correct density. Hydrometers can be purchased from any scientific supply house. Kester has flux density control Data Sheets (also called nomographs) for the rosin and higher solid content fluxes. Using the nomograph you can determine how much thinner needs to be added. We sell test kits for the low residue fluxes. The test kits indicate how much thinner to add to the flux pot.

What Does No-Clean Mean?

There is some apprehension at the description, ‘no-clean’. What does it mean? The flux category called “no-clean”. Many fluxes fall into this category because the flux residues are not harmful to assemblies. It does not mean there will be no residues. All fluxes leave residues (the solids are the active portion of the flux that does all the work). Some flux residues are conductive or corrosive and must be removed. Other fluxes like the no-clean fluxes leave residues that do not need to be removed.

Flux Test Kit

The PS-22 Test Kit is an acid titration kit. As solvent evaporates, the acids in the flux become more and more concentrated. The kit is used to determine the acid number of the flux. The test is a simple drop test with an indicator for the endpoint (similar to a swimming pool test). You count the drops and then go to a chart. From the chart you determine how much thinner to add to bring the acid number back down to spec. Traditionally flux control is maintained by measuring the density of the flux and adding thinner as the flux thickens. This is a good method for fluxes with high solids contents. A slight change in density corresponds to a small change in the flux. The no-clean fluxes have low solids percents (typically 2 or 3 %) so a small change in the flux density could reflect the change in solids of 25-30%. This would put the acid level unacceptably high. The test kit is more accurate than using density measurements as a way to monitor the flux.

Rosin Flux

Rosin Flux Removal

Saponification

Saponifiers in an aqueous cleaner can be used to remove rosin flux residues. What the Bio-Kleen #5768 saponifier does is react with the outside layer of flux and soften it up, literally turning it into a soap. You need a spray cleaner so that this soap layer will be washed away which then allows the Bio-Kleen to react with the next layer. Rinse that layer off, etc. The problem with ultrasonic removal is that the outer layer is slightly dissolved and that is the end of the reaction. In the meantime the solder is being exposed to an alkaline condition. The exposed solder turns a dull gray as the surface shine is etched off by the caustic solution.

Thinner Calculation

A calculation that can be used to determine how much thinner needs to be added to the flux to maintain a specified specific gravity. The following formula can be used to determine how much Kester # 4662 thinner to add to the Kester # 2235 flux to bring it back to the recommended specific gravity (density). For your information this formula can be used for any Kester flux that utilizes the # 4662 thinner. V = ( x – y ) 128 Y – 0.783 Where V = ounces of # 4662 thinner to be added per gallon of flux. X = present flux density Y = desired flux density For example: The flux is at 0.875 and it should be at 0.856 V = ( 0.875-0.856 ) 128 or V = 2.432 or V = 33.3 ounces of thinner per gallon of flux 0.856 – 0.783 .073

Alcohol-based flux wave soldering profile

A general profile setting up a wave solder machine to optimize reflow using an alcohol base flux.
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Green Corrosion?

Rosin fluxes react with copper (especially on braided cables underneath the insulation) to form a green by-product that is not corrosion.
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Water-Soluble (OA) Flux

Thinner Calculation

A calculation that can be used to determine how much thinner needs to be added to the flux to maintain a specified specific gravity. The following formula can be used to determine how much Kester # 4662 thinner to add to the Kester # 2235 flux to bring it back to the recommended specific gravity (density). For your information this formula can be used for any Kester flux that utilizes the # 4662 thinner. V = ( x – y ) 128 Y – 0.783 Where V = ounces of # 4662 thinner to be added per gallon of flux. X = present flux density Y = desired flux density For example: The flux is at 0.875 and it should be at 0.856 V = ( 0.875-0.856 ) 128 or V = 2.432 or V = 33.3 ounces of thinner per gallon of flux 0.856 – 0.783 .073

Removing OA Flux Residues

Is it required to remove the residues of OA fluxes? You will get corrosion if you do not fully remove all the flux residues. The flux residues from organic fluxes are hygroscopic, they contain organic acids and they may contain halides. Over time the residues will absorb moisture and the halides will become mobile and corrosive. This will lead to ‘catastrophic failure’ of the assembly in question. We recommend cleaning with an aqueous cleaning system, it can be either batch type or in-line. The water temperature should be between 120-140°F. The important thing is to have lots of rinsing and flushing to completely remove all the flux residues.

How To Test Outdated Flux

This method is to test performance of flux and it is in no way a method used to re-certify, by Kester, flux that had passed its shelf life. Download Article