Soldering Flux for soldering and repair electronics PCB.
Author Dmitrii KhramtsovPosted 19 November 2024
Updated 14 December 2024
Introduction about soldering fluxes.
This article is not about comparing different soldering fluxes. There are enough reviews like this without me. We will talk about a lot of confusion in the choice of fluxes and attempts to use them for various soldering technologies.
The key concept is that different reflow technologies use different fluxes, and based on this, it is necessary to work out the subsequent cleaning technology.
You immediately need to separate manual installation from all others, since only with manual soldering there is no soldering profile as such, except for soldering microcircuits at the station. That is, there is no consistent heating. The entire temperature from the tip hits the soldered joint at once. Therefore, for manual soldering you need a flux that will not boil and evaporate at lightning speed, and the flux should also ensure the fluidity of the alloy and the removal of oxide films.
You also need to clarify the point with tubular solders, which already contain dry flux. Many people use tubular solders, and I, too, can say that the difference between fluxes in tubular solders can be very large. Even expensive solders can do a poor job of soldering the simplest components, and we’re not even talking about repairs, but about soldering new printed circuit boards and products. In such cases, fluxes are additionally used, which also need to be selected correctly or the tubular solder must be changed. It is ideal to use the same flux in the tubular solder and in the syringe/jar/pencil. For example, class ROL0 Noclean.
The key concept is that different reflow technologies use different fluxes, and based on this, it is necessary to work out the subsequent cleaning technology.
You immediately need to separate manual installation from all others, since only with manual soldering there is no soldering profile as such, except for soldering microcircuits at the station. That is, there is no consistent heating. The entire temperature from the tip hits the soldered joint at once. Therefore, for manual soldering you need a flux that will not boil and evaporate at lightning speed, and the flux should also ensure the fluidity of the alloy and the removal of oxide films.
You also need to clarify the point with tubular solders, which already contain dry flux. Many people use tubular solders, and I, too, can say that the difference between fluxes in tubular solders can be very large. Even expensive solders can do a poor job of soldering the simplest components, and we’re not even talking about repairs, but about soldering new printed circuit boards and products. In such cases, fluxes are additionally used, which also need to be selected correctly or the tubular solder must be changed. It is ideal to use the same flux in the tubular solder and in the syringe/jar/pencil. For example, class ROL0 Noclean.
What characteristics should a soldering flux for repair and manual soldering have?
- Gel-like consistency. Necessary for flux concentration at the soldering surfaces. Gels spread less intensively when exposed to high temperatures.
- Resistance to high temperatures. During repairs, the heating curve from the tip of a soldering iron or hot air gun increases instantly and the flux should not evaporate, maintaining the wettability of surfaces.
- Wettability. A corollary of the first two points. At high temperatures, the flux should wet the soldered surfaces and also effectively eliminate oxide films, provided that in the sharply growing profile there is no stabilization zone, where fluxes usually work with oxide films.
High-quality fluxes are not just big words. Thanks to their stable characteristics, high soldering quality is achieved.
Choosing a flux that meets these criteria can significantly improve manual soldering outcomes, ensuring strong soldering joints, and less time spent.
- Resistance to high temperatures. During repairs, the heating curve from the tip of a soldering iron or hot air gun increases instantly and the flux should not evaporate, maintaining the wettability of surfaces.
- Wettability. A corollary of the first two points. At high temperatures, the flux should wet the soldered surfaces and also effectively eliminate oxide films, provided that in the sharply growing profile there is no stabilization zone, where fluxes usually work with oxide films.
High-quality fluxes are not just big words. Thanks to their stable characteristics, high soldering quality is achieved.
Choosing a flux that meets these criteria can significantly improve manual soldering outcomes, ensuring strong soldering joints, and less time spent.
What are the classes of soldering fluxes?
It's amazing question!
There are several classes of fluxes for soldering: Low activity, medium activity and high activity.
- Low activity fluxes ROL0 are the most common, my choice is Diamond Flux FN231, as they are the safest for electronics operation if they are not washed off and left on the printed circuit board.
- Medium and high activity fluxes must be washed off after soldering, and at the same time you must be sure that no residues should remain under the microcircuits. Yes, this is a big problem, because it is very difficult to wash out all the flux residues even in the most advanced cleaning system. That is why manufacturers and repairmen try to use no-clean classes of fluxes.
But we must pay tribute to soldering fluxes with medium and high activity, since they are the ones who are able to remove large areas of oxidation of copper and other metals from surfaces and solder what is desired.
ROL0 fluxes made according to the standard will not cause oxidation on metals. This is very important as a high acid number will cause corrosion on the copper and oxide spots. Also, the higher the acid number, the lower the resistance of flux residues and this can cause leaks between conductors and contacts. These are very important characteristics, but unfortunately, many cheap fluxes from the Middle Kingdom do not meet the ROL0 criteria. You probably didn't know about this, but keep it in mind in the future. Proven soldering fluxes are the key to long-term operation of your printed circuit board. Understanding soldering flux classification helps in selecting the right flux for manual soldering tasks, ensuring preventing corrosion or electrical leakage issues.
There are several classes of fluxes for soldering: Low activity, medium activity and high activity.
- Low activity fluxes ROL0 are the most common, my choice is Diamond Flux FN231, as they are the safest for electronics operation if they are not washed off and left on the printed circuit board.
- Medium and high activity fluxes must be washed off after soldering, and at the same time you must be sure that no residues should remain under the microcircuits. Yes, this is a big problem, because it is very difficult to wash out all the flux residues even in the most advanced cleaning system. That is why manufacturers and repairmen try to use no-clean classes of fluxes.
But we must pay tribute to soldering fluxes with medium and high activity, since they are the ones who are able to remove large areas of oxidation of copper and other metals from surfaces and solder what is desired.
ROL0 fluxes made according to the standard will not cause oxidation on metals. This is very important as a high acid number will cause corrosion on the copper and oxide spots. Also, the higher the acid number, the lower the resistance of flux residues and this can cause leaks between conductors and contacts. These are very important characteristics, but unfortunately, many cheap fluxes from the Middle Kingdom do not meet the ROL0 criteria. You probably didn't know about this, but keep it in mind in the future. Proven soldering fluxes are the key to long-term operation of your printed circuit board. Understanding soldering flux classification helps in selecting the right flux for manual soldering tasks, ensuring preventing corrosion or electrical leakage issues.
Soldering Fluxes Classes Table
This reference table can guide technicians and engineers in choosing a soldering flux class that aligns with their manual soldering strategy and post-solder cleaning procedures.
Cleaning the printed circuit board from flux residues after soldering.
In fact, washing off the flux is the key to cleanliness of the printed circuit board. Dust and various debris can stick to any residue. This in turn can cause current leakage and cause conduction. Therefore, it is better to wash it. In my opinion, the ideal solution is to use FluxOff spray in cans. It will definitely clean off any residue and make your PCB clean.
I hope that I have helped you a little in understanding the details of choosing a flux for soldering and wish you excellent and high-quality work! By selecting the right soldering flux class and cleaning the PCB, you can achieve professional quality PCB soldering.