Soldering Paste vs. Flux vs. Wire

Soldering is a fantastic activity that allows people to create and repair all kinds of circuits or work with metals to make wonderful art. As you enter this wonderful world, you will read about soldering paste, wire, and flux. In this article, I will tell you everything you need to know about them to help you choose which one to use, depending on your soldering task.

Quick Comparison

Soldering Wire

The soldering wire is probably one of the most heard soldering elements, along with the soldering irons. However, it’s important to highlight some facts that will help you understand more about it and its uses.

What is Soldering Wire?

Soldering wire, as its name suggests, is a type of solder that is made in long, tubular sections resembling a wire. It comes in rails or dispensers that bring a hole on one end where you pull the wire as you use it. Soldering wire comes in different alloys and thicknesses to suit various applications and comes with a rosin core (containing flux) or solid. You can find more information about soldering wire here.

Pros and Cons

Pros:

• Precision: With the correct soldering technique and tools, solder wire is excellent for creating accurate and steady joints.
• Clean application: Wire solder often requires less post-soldering cleaning than soldering paste because it produces fewer residues. When using rosin core soldering wire, it’s always recommended to clean the board after soldering to remove any flux residue, but still, it’s a cleaner process than soldering with paste.
• Versatility: Soldering wire can be used in several soldering tasks, from electric and electronic applications to crafts like jewelry.
• Variety: Wire solder is available in different alloys, thicknesses, with or without rosin flux, etc.

Cons:

• Not SMD Friendly: You can’t solder all kinds of SMD (Surface Mount Devices) using soldering wire, and the ones you can become complex and tedious to solder.
• Not Ideal for Small Components: When you need to solder small components with a gap between their pins too small, soldering wire can lead to circuit bridges if not applied with care.
• Potential Splattering: During soldering and unsoldering, solder wire may splatter if not used carefully.

Common Applications of Soldering Wire

Below, you will find a list of soldering wire’s most common applications. It’s important to highlight that you will need to use different types of soldering wire to perform these tasks.

1. Electronics: Solder wire is extremely practical for electronic work, especially for working with through-hole PCBs (Printed Circuit Boards), connecting wires, repairing circuits, assembling connectors, etc.
2. Electrical Jobs: Solder wire is the most practical way to create strong mechanical and electrical connections to join cables, assembly control boards, and other applications.
3. Roof Repairs: Thick, solid solder wire is excellent for building, repairing, and sealing metal roofs and flashings.
4. Plumbing: Solder wire is used to join and repair metallic pipes and fittings.
5. Soldering Metal Gutters: Solid solder wire is practical for joining, sealing, and repairing metal gutters.
6. Hobbies and Crafts: Solder wire is practical for stained glass, mosaics, and other hobbies and crafts.
7. Jewelry Making and Repair: Wire solder can be used for creating or repairing metallic jewelry pieces.

Soldering Paste

Soldering paste is an alternative to solder wire that has its particularities. Keep reading for more information about it.

What is Soldering Paste?

Soldering paste is a sticky product that serves the same purpose as soldering wire, creating strong and reliable joints. You can find it in plastic or metallic containers, and also syringes, for easy and practical application. Solder paste shouldn’t be confused with flux solder paste or products like plumbing soldering paste, which serve a different purpose, as you will discover in this article.

Composition of soldering paste

Soldering paste is a mixture of metal particles, flux, and additives that regulate the compound’s viscosity, thickness, flow, and other characteristics. The metal particles come as tiny grains or balls made of materials like lead, tin, copper, or silver, depending on the paste’s intended use. Solder pastes come in different viscosities, affecting how they spread after application. This property is commonly known as ‘slump.’ While you can find rosin-free soldering paste, most products contain a significant proportion of flux, which enhances their wetting and flowing properties.

Advantages of Using Soldering Paste

Soldering paste stickiness makes it easy to apply when cold. This feature is beneficial because it allows you to position the parts without rushing, providing an extra margin for error. This characteristic also enables the use of stencils and, when combined with reworking soldering stations and hot air guns, allows you to solder hard-to-reach areas that would be impossible to join using soldering wire. Another great advantage of solder paste is its ability to cover large areas, allowing you to solder multiple components at once, making it ideal for automated soldering processes. These benefits ensure excellent results and reduce the odds of common soldering issues, such as “solder bridges”, especially when working with small or fine-pitched components.

Common Uses of Soldering Paste

Solder paste is mainly used to assemble and repair SMT (Surface Mount Technology) boards and fix SMD (Surface Mount Devices) into PCBs (Printed Circuit Boards). As mentioned above, solder paste’s characteristics make it ideal for high-precision applications, automation, and mass-production soldering processes.

Soldering Flux

Soldering flux is an excellent companion of soldering paste and wire. Below, you will find all about it so that you can determine when to use it and what type of flux you need for your soldering projects.

What is Soldering Flux?

Soldering flux is a cleaner that enhances the wetting and flowing of molten solder. Flux comes as a paste, cream, or liquid, and its function is to remove oxide, dust, and debris from the surfaces to be soldered and improve the solder’s efficiency. When soldering with solid solder wire, for example, which doesn’t contain flux, applying flux before soldering is essential to ensure that solder effectively creates a solid joint. Flux is also indispensable for cleaning and tinning he soldering iron tips, keeping them in good shape and ready for use.

Remember that even when flux is essential for most soldering tasks, you still need to use solder to create electrical connections and/or metallic bonds.

Types of Flux

Below, you will find a list of the different types and forms of flux available:

1. Rosin Flux: This is the most common type of soldering flux. It’s an organic product, mainly made of pine tree rosin combined with a solvent like isopropyl alcohol. Some formulas contain other chemicals that work as activators to increase the product’s ability to penetrate and remove heavy oxide and rust generated by high soldering temperatures. Activators are divided into two main types: acidic and non-acidic. It’s important to highlight that even the mildest ones are corrosive, so it’s crucial to know your flux before soldering, depending on the application, and always clean the surface after soldering to remove any residues and prevent future damage.
2. No-Clean Flux: This type of flux leaves fewer residues than standard rosin flux. You can find natural (rosin-based), semi or fully synthetic compositions. This type of flux is ideal and practical for electronic applications or extended soldering sessions because it doesn’t leave dangerous substances behind. However, no-clean fluxes may be a little sticky and attract dust and other particles, so it’s always best to do some cleaning after using them.
3. Water Soluble Flux: As the name suggests, this type of flux can be easily cleaned with water. They are more resistant to high temperatures and are usually less corrosive than other flux types.

Pros and Cons

Whether you can’t avoid using flux on most occasions, it’s important to be aware of its pros and cons.

Pros:

• Oxide Removal and Cleaning: Flux effectively eliminates oxide, dust, and debris from metals, helping to create strong and reliable solder joints.
• Corrosion Protection: Because flux removes previous oxidation, and because of its chemical properties, it helps to prevent corrosion on the soldered joints.
• Enhances Solder Flow: Flux helps molten solder flow more efficiently, improving the quality of joints and reducing soldering times.
• Improves Solder Adhesion: Flux improves solder adhesion, ensuring secure and reliable connections.

Cons:

• Corrosive Residues: Some flux types contain corrosive chemicals that require proper cleaning to prevent damage to the soldered pieces.
• Needs Cleaning: Even the least aggressive formulas, like no-clean flux, may leave residues that must be cleaned.

Common Applications

Below, you will find a list of soldering flux’s most common applications. It’s important to highlight that you will need to use different formulas depending on the task.

1. Electronics: Flux is commonly used for preparing PCB (Printing Circuit Board), automated soldering processes, and SMT (Surface Mount Technology) to ensure clean and reliable electrical and mechanical connections.
2. Roof Repairs: Flux is used in roof repairs to clean and remove oxide before soldering and to ensure the solder’s proper bonding.
3. Plumbing: Flux is perfect for soldering copper pipes, preparing the surface, removing oxide, and acting as a temporary bond that helps to hold them in place before applying the solder.
4. Soldering Metal Gutters: Flux is essential for sealing and repairing metal gutters.
5. Hobbies and Crafts: Flux is indispensable for tasks like stained glass, mosaics, and other hobbies and crafts where solid and rosin-free solder is used.
6. Jewelry Making and Repair: As in other soldering practices where rosin-free solder is used, flux is also indispensable.

Choosing the Right Soldering Material

When preparing your project’s material list, it’s essential to consider some factors to decide whether you will use soldering wire or paste and the type of flux when needed.

First, you should consider what type of solder is best for the job and then, depending on that, the type of flux. Factors like accessibility, size of the soldering area or components, and the joint’s mechanical requirements play a crucial role when choosing the solder type.

In my experience and from what I’ve seen in different industries, solder wire is like the Swiss army knife of solders. You can comfortably use it for jobs as simple as joining two wires to mount components into a PCB. The intended mechanical strength is also fundamental when choosing a solder type; solder wire is mechanically stronger than solder paste.
Another thing to consider is what type of tool you will be using. For example, a reworking or hot-air soldering station is more practical to work with soldering paste, even when you can melt it using a soldering iron.

Once you have chosen whether you will be using soldering wire or paste, you will have to make a second assessment to select the material’s characteristics, such as the type (rosin-core or solid), the alloy, and others, and determine if you will need extra flux, etc.

If you will be using flux, the most important thing to consider is what kind of job you will be doing. As a rule of thumb, always use non-acidic or rosin-based flux for your electronic projects to avoid corrosion and component damage. Also, keep in mind that when using lead-free solders, it’s best to use flux with activators to ensure proper wetting and bonding.

Lastly, for your plumbing projects, always use a non-toxic, non-contaminating flux, especially when working with pipes carrying drinking water and drains carrying water that will go to the soil or water streams.

Recommendation

Below, I will tell you what soldering products are best for different jobs from my experience and knowledge. This is a general recommendation; you will have to check your project’s needs, such as the components’ characteristics, the workpieces’ materials, and other considerations for choosing the exact and perfect combination of products. It’s not an exact science, and plenty of practices were adopted by different industries by trial and error, experience, or engineering, and sometimes it’s a question of discovering which elements work best for you based on your personal experience and preferences.

1. Electronic circuits:

When working with electronic circuits and PCBs, it’s essential to determine the solder based on the board characteristics (through-hole or SMT), components’ size, pin location, and the gap between them.

When hand-soldering through-hole components, use a thin (between 0.3 and 0.6 millimeters thick) rosin core soldering wire.

When soldering SMT boards and components, use rosin-based type III (25µm-45µm) paste combined with proper no-clean flux for electronics.

2. Electrical Connections

For electrical connections, use a rosin core soldering wire and choose a thickness that suits the wires’ thickness or the pins of the elements you are soldering. Using flux for electrical connections is not very practical unless you need to clean the wires or ensure that the joints don’t have added resistance or noise due to dirt or small particles contaminating the joint. Flux is necessary when soldering wires carrying data or signals, for example.

3. Plumbing

For plumbing, remember to use lead-free solder when working with pipes that carry drinking water to avoid potential poisoning, contamination, and other hazards. The best solder for plumbing is a thick (1mm and up, depending on the fittings or pipes’ diameter) solid 95/5 or 97/3 tin/antimony or tin/copper. For piping, solder works best when using acid-core flux to clean and prepare the parts to be soldered and enhance the solder’s bonding properties.

4. Stained Glass and Crafts

When working with stained glass and other crafts like mosaics, it’s best to use 50/50 tin-lead solid-core soldering wire, or an allow with copper or silver if you choose a lead-free alternative. The thickness can go from 1 to 3 millimeters, depending on the workpieces’ size and the area to be covered. Additionally, you will have to use paste flux (zinc chloride is a great choice), liquid rosin flux, or special flux for stained glass.