Soldering Flux: The Definitive Guide

When joining two metals in a soldering process, like used in PCB assembly, flux is required to achieve a true metallurgic bond. That ensures the solder joint doesn’t crack or come loose even with the day-to-day wear-and-tear. This article discusses everything about flux soldering.

What is Flux Soldering

Flux soldering refers to the process of using a flux material to help join two pieces of metal with solder. Flux is a chemical compound that is applied to the metal surfaces being joined to prevent oxidation and promote the flow of molten solder. The solder is then melted and applied to the joint, forming a strong bond between the metal pieces. Flux soldering is commonly used in electronics and plumbing to create secure and long-lasting connections between components and pipes.

Types of Soldering Flux

There are several types of soldering flux, including:

• Rosin flux: This is a common type of flux made from pine tree sap and is good for general soldering work.

• Organic flux: Also known as water-soluble flux. It is made up of organic materials other than rosin. Compared to the previous type, this flux is a stronger cleaner. It is corrosive and requires careful cleaning after application. Flux residue can lead to poor component performance and even short circuits. Organic flux can be cleaned with water.

• No-clean flux: This type of soldering flux can be constructed from natural rosin or other synthetic rosins. While no-clean rosin-based soldering solutions are the same as Type R fluxes, they usually contain much lower concentrations of natural gum rosin—less than what is used in R, RMA, and RA flux solutions. No-clean fluxes were designed as a quality of life improvement, to help PCB assemblers by reducing the time and expense required to clean a board after soldering. With minimal residue, they are highly beneficial in a variety of PCB manufacturing scenarios.

• Inorganic acid flux: another solder flux type that works better with stronger metals for instance brass, copper and stainless steel. You will need to clean the surface completely so that remove any corrosive residues from the surfaces if you use the inorganic acid flux. However, it will achieve a perfect solder joint if the residues are on the surface.

Why use Flux in Soldering

The primary purpose of flux in printed circuit board assembly or rework is to clean and remove any oxide and impurities from the PCB, which can cause poor solder joints and hinder electrical conduction.

Soldering flux makes a good effect on soldering and wetting, as well as is good for de-oxidizing metals (copper tracks on the board and leads of electronic components). the activators in the flux promote wetting of the molten solder to the surface mount lands and component terminations or leads by removing oxides and other surface contaminants.

How does Flux Work in Soldering

Flux is used in soldering to clean the surface of the metals being joined, and to prevent oxidation or other contamination during the soldering process. The activators in the flux react with the metal surface to remove any oxides or other surface contaminants, creating a clean surface for the molten solder to flow into. This helps to improve the wetting of the solder onto the metal and create a strong bond between the solder and the metal.

The following is how to use solder flux:

• You should clean the metal contact with a good solvent before using the soldering flux so that remove the dust, grime as well as excessive oxidation that may be present.

• Then, it needs to apply an even coat of the flux to the surfaces where the actual soldering is slated to happen. But a tip you need to know, it can’t heat at this stage.

• It should be placed on the metal contacts covered with flux when the tip of the soldering gun is hot and ready. And it should melt the flux and spread, covering the metal surface, which will remove the oxide layer, until the flux is present.

• It should introduce and melt the soldering wire after vaporizing the flux, making sure the solder bonds before oxidizing again so that there are an effective soldering.

When to Use Flux When Soldering

Flux should be used whenever soldering to help ensure a good quality solder joint. Flux is especially important in the following situations:

• Soldering to a surface that is contaminated with dirt, oil, or other impurities. The flux helps to remove the contamination and promote good solder wetting.

• Soldering to a metal surface that is prone to oxidation, such as copper or brass. The flux helps to remove the oxide layer and prevent it from reforming during the soldering process.

• Soldering at high temperatures, where the metal being soldered may become oxidized or contaminated due to the high heat. The flux helps to prevent this and promote good solder wetting.

In summary, flux should be used in almost all soldering applications, but it is especially important when soldering to dirty or contaminated surfaces, when soldering to metals that are prone to oxidation, and when soldering at high temperatures.

The Application of Different Types of Flux

Flux can be in solid, paste, or liquid form. It can be applied on a PCB in a number of ways, based on the soldering process being used:

• Wave Soldering: The flux used for wave soldering is usually made up of more solvents than flux used for other applications, and will be sprayed on the board prior to it going through the solder wave. Once in place, the flux will clean the components that are to be soldered to remove any oxide layers that have formed. If the board is using a less corrosive type of flux, then the board will have to go through a pre-cleaning before the flux is applied.

• Solder Reflow: For boards that are going through the solder reflow process, a paste composed of a sticky flux and small spheres of metal solder is used. This solder paste holds the parts in place until the heat of the oven causes the solder particles to reflow. Not only are the metal surfaces cleaned by the flux, but the pasty nature of the flux seals out the air preventing further oxidation. The solder paste flux also contains additives to improve the flow characteristics of the solder as it melts.

• Selective Soldering: The flux used for selective soldering processes is applied either by spraying it, or by using a more precise drop jet process.

Conclusion

Solder flux is an invaluable material for preparing PCBs for soldering. It cleans the surface of the board and removes any oxides that form on the electrical contacts. Additionally, it helps to create a strong bond between the metals being joined during the soldering process.

If you have any questions, or need more information on solder mask or solder paste, please feel free to contact us, we will be happy to help you.