In my first attempt at soldering, I concluded that it was extremely difficult and that I'd have to practice for years before I could gain mastery. All I had was a wooden-handle 60-watt soldering iron, which was a gift from my dad, and a 70% lead solder. Later, I discovered that some materials, beyond my soldering iron and solder wire, would make soldering a lot more exciting for me. One of these materials is solder flux.

This article will answer all your questions about solder flux and how you can use it to improve your soldering and PCB assembly experience.

What is Flux?

Flux is a chemical substance used in the soldering process. Its primary assignment is to get rid of oxide or any impurity on the metal surface to be joined (soldered). Although it comes in different textures and mediums, including paste, liquid, and dry form, between the hollow of the solder and even on the PCB or surface of the copper board to be soldered, the job description remains the same.

Electrical conductivity is higher, and the joint is stronger when the soldered surface is free from impurities. This is another reason why the use of flux in the soldering process is vital. Moreover, the presence of flux on a PCB or metal surface prevents any further formation of oxides, making it relevant for preserving a PCB or any copper board.       

Solder Flux

Different Types of Flux and Applications

When an average engineer or technician thinks about flux, they think of a gelatinous or paste-like substance used in soldering. However, in reality, flux used in soldering generally has three main classifications: rosin (type R) flux, no-clean flux, and water-soluble (aqueous) flux.

Rosin Flux: The most common type of flux used in the electronics industry is rosin, obtained from natural resin or pine sap. Rosin-based fluxes, specifically the types R and RMA (non-activated and mildly activated rosin), are not in any way corrosive when they are not active and are only activated when heat is applied to them. Their non-corrosive nature, when not activated, makes them suitable for use in most PCB assembly processes, and they do not need to be cleaned from a board after application.

There is also the activated rosin flux (type RA), which must be cleaned from the board when used. Type RA contains stronger chemicals for more effective oxide removal, and if not cleaned, it can subsequently cause corrosion in the circuit it's used in.

No-Clean Flux: When used during the soldering or desoldering process, no-clean flux leaves only a little residue behind. The residue is non-corrosive and safe to leave on a PCB. It's mostly used in modern electronics, and it has proven to be effective over the years. It does a good job removing oxides and impurities, ensures strong soldered joints, and eradicates the need for any extra cleaning after soldering or PCB assembly.

Although the residue left behind by the no-clean flux is little, it may be sticky, attract dust and other impurities, and cause corrosion or a short circuit on the PCB.

Water Soluble (Aqueous) Flux: Water-soluble flux has a higher concentration of organic acid and is super effective for oxide and impurity removal. One downside is that it cannot be left on a board. It has to be cleaned immediately after application, or the board and the components placed on it will suffer corrosion. But then cleaning a water-soluble flux from a board is quite easy, as the flux is made soluble in water and can adequately be removed by rinsing with water.

What Does Solder Flux Do?

The solder flux is a chemical substance made to prepare the metal surface to be soldered for effective soldering, and it does this by:

·       They remove every oxide layer formed on the surface when exposed to air. Over time, when metal is exposed to either dry or moist air, it undergoes oxidation, and metals such as copper start changing color due to the oxide layer formed. This oxide layer makes it difficult for the molten solder to stay if not properly removed.

·       Cleaning the metal surface by removing other impurities such as oil, dust, dirt, and other contaminants.

·       Reduces the surface tension between the solder and the metal surface, consequently allowing the solder to spread easily and stay on the metal surface.

What Is the Purpose of Flux in Soldering?

Soldering flux plays a vital role in achieving neat and effective soldering. But how does it achieve this when it just basically removes impurity? Below are a few of the reasons why the use of soldering or PCB assembly is important.

·       By removing impurities and contaminants, flux improves the electrical connectivity between the solder and the metal surface. 

·       A clean metal surface facilitates a strong bond when solder is applied, ensuring a reliable and durable connection in PCB assembly.

·       During manual soldering, applying flux makes the solder flow easily where it is used, consequently making the process easier and faster.

·       Applying flux before desoldering helps the solder melt faster, reducing the impact of heat on the component that needs to be desoldered.

·       Solder flux also helps the soldering tip stay clean and oxide-free. This reduces the time spent cleaning the tip and facilitates fast and stressless soldering.

How to Use Soldering Flux

The use of flux depends on several factors, such as the technique used or the size of the component to be soldered. Although most solders available in electronics come with an embedded flux, many people still prefer to add extra flux for a better soldering experience.

For SMD PCB assembly, solder paste (a mixture of flux and dissolved solder) is added to the PCB surface; when heat is applied, both the flux and solder perform their functions, resulting in a perfect solder joint. However, when desoldering a component, extra flux is directly added to the solder joints, further aiding in easy desoldering.

Stripboards, for instance, come with a rosin flux coating that melts when heat is applied, but you might need to use extra flux, especially if you want to prevent molten solder from bridging copper strips and causing a short circuit on the board. When the component placement on the stripboard is less complex, there is no need to worry about a short circuit, and you can proceed with the soldering without applying any extra flux。

Solder Paste Vs. Solder Flux

Most people sometimes find it difficult to differentiate between solder flux, solder paste, and solder itself. To explain the difference between the three, let's first address the question, What is Solder Paste? 

What is Solder Paste?

Solder paste

Solder paste is a mixture of flux and fine solder particles. It's mainly used in surface mount technology (SMT) for electronics assembly. It comes in paste form, earning it its name. When heat is applied, the flux does its job, and the solder does its own, making them a perfect duo for SMT assembly application.

The Difference Between Solder Flux and Solder Paste

When the solder flux is applied to a board, depending on what type of flux it is (either heat activated or not), when activated, it reacts with the metal surface, removes oxide and any other impurity, and then solder is separately administered to make the joint happen.

On the other hand, soldering paste does not need any extra solder to facilitate the joint. The solder is applied to the board just around where the solder pad for the SMD component is positioned. The component to be soldered is then placed on the PCB pad and held by the sticky paste. As soon as heat is applied (or the board is placed in the reflow oven), the solder melts and flows to create the solder joint between the component and the pad. The flux in the paste makes the flow possible and ensures no solder is left on the solder mask.

The difference between solder flux and paste is in their job descriptions. Solder flux is focused on preparing the metal surface to be soldered, while solder paste not only readies the surface but also plays a key role in making the joining process happen.

Suppose there's anything I have learned in over a decade of active soldering. In that case, it is important to realize that regardless of how skill you handle a soldering iron, the quality of your soldering materials and how you appropriate them still remains the major determinant of the result you get. The main skill lies in selecting suitable materials and tools and not just holding your soldering iron at a particular angle.

If you have patiently followed this article to this point, you should not have any problem selecting the right flux for your PCB and should know exactly how to appropriate whatever flux you selected. You now know which type is safe to leave on your PCB and which one to clean immediately. So next time you are getting a flux, pay attention to the description and apply it as recommended.