Difference Between Solder Paste And Flux

Soldering is a common process used to join metals and electronics components. To achieve a strong and reliable joint, it’s essential to use the right materials, including solder paste and flux.

However, many people often confuse these two terms or assume they are interchangeable. In reality, solder paste and flux have different functions and are used in different ways during the soldering process.

In this blog post, I have covered the differences between solder paste and flux and help you understand which one to use for your specific soldering needs.

Whether you’re a professional soldering technician or a DIY enthusiast, understanding the distinctions between these two materials can help you achieve the best results in your soldering jobs. So, let’s dive in and explore the differences between solder paste and flux!

What is Soldering Paste?

Solder paste plays a crucial role in the manufacturing of printed circuit boards (PCBs) as it facilitates the connection of surface mount components to the board. This particular type of solder is composed of tiny spheres of solder and flux, which are mixed together.

The flux is a chemical substance that is used to clean and prepare the surface of the metals or components that are being joined together, making it easier for the solder to adhere and create a strong bond. The solder spheres are typically made of an alloy of tin, lead, and sometimes other metals.

When applied, the paste initially holds the components in position due to its adhesive properties. The next step involves the application of heat to melt the paste, creating both a mechanical and electrical bond.

To apply the paste to the board, it is typically printed onto the board through a stencil, and then the components are placed either manually or through the use of a pick-and-place machine.

What is Solder Flux?

Solder flux is a material used in the process of soldering to facilitate the bonding of two metal surfaces.

Related Reading: Main Types of Soldering Flux

It is applied to the surfaces to be joined and helps to remove any oxides or other impurities that may be present, allowing for a stronger and more reliable bond to form.

The primary function of solder flux is to promote the wetting of the metal surfaces by the solder. This means that it helps the solder to flow more easily and evenly across the surfaces, creating a strong and uniform bond.

Solder flux also helps to prevent the formation of voids or other defects in the solder joint, which can weaken the bond and lead to failure over time. There are many different types of solder flux available, each with its own specific properties and uses.


Some types of flux are designed for use with specific metals or alloys, while others are more versatile and can be used with a wide range of materials.

Difference Between Solder Paste And Flux

AspectSolder PasteFlux
DefinitionMixture of small solder spheres with fluxMaterial added during soldering to promote bonding
CompositionSolder spheres and fluxActive ingredients, solvents, and binders
Primary functionProvide mechanical and electrical bondPromote wetting of metal surfaces and remove impurities
Application methodStencil printing and pick and place machine or by handApplied directly to metal surfaces using a brush or syringe
FormsPastePaste, liquid, gel
ClassificationJ-STD-005 classification based on ball sizeJ-STD-004 classification based on flux type and activity
ImportanceEssential for creating strong and reliable solder jointsEssential for ensuring strong and uniform bonds

What types of solder paste are there?

Solder pastes are commonly classified according to J-STD-005, which uses ball size as a key distinguishing factor. This classification system helps to standardize the industry and ensure that solder pastes are used correctly and consistently across different applications.

J-STD-005 classifies solder pastes into several different categories based on the size of the solder balls that are included in the paste. These categories range from very small (Type 1) to very large (Type 7), with each category representing a specific range of ball sizes.

The smallest category of solder paste, Type 1, includes solder balls that are 20-38 microns in diameter.

Type 2 solder paste contains slightly larger balls, ranging from 25-45 microns in diameter. The largest category of solder paste, Type 7, includes balls that are 106-150 microns in diameter.

Related Reading: Solution: Why solder not sticking to soldering iron tip?

Can I use the soldering paste as flux?

Soldering paste typically contains both flux and solder, so it is possible to use it as a flux when soldering.

However, it is important to note that the effectiveness of the flux in the paste may not be as strong as a dedicated flux, and using too much soldering paste may result in excessive solder buildup.

If you decide to use the soldering paste as a flux, it is recommended to use a minimal amount and to clean the excess solder off of the joint after soldering.

Additionally, it is important to make sure that the soldering paste is compatible with the type of metal you are soldering and that you follow the manufacturer’s instructions for proper use.

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