How to Choose the Right Solder Wire for Your Projects

Solder hails from the middle english word soudur, which derives from the latin word solidare, meaning "to make solid." It's a fusible alloy (i.e. low melting point) used to bond metal workpieces together. Solder is a material that is used to bond objects, such as pipes or wires, together permanently. The primary application for soldering in the plumbing industry is leak-free connections. In the electronics industry, soldering is used to fuse wires for circuit components. Soldering is an essential skill in electronics, enabling secure electrical connections and reliable assembly of components. However, the success of a soldering project heavily depends on the type of solder wire used. Choosing the right solder wire ensures strong joints, reduces defects, and enhances the overall quality of your project.

Solder is a metal or metallic alloy that historically consisted of lead and tin in various proportions. Today, lead-free solder is available to comply with the Safe Water Drinking Act, which restricts the amount of lead used in portable water systems. This guide explores the factors to consider when selecting solder wire for various applications, whether you're a hobbyist or a professional. To know more about PCBs and how they are manufactured see our detailed guide on PCB etching process.

What Is Solder Wire?

Solder wire is a fusible metal alloy used to join electronic components by melting and creating a conductive bond. Solder forms the critical connections in electronic devices, ensuring their functionality and reliability. All types of solder act as connective tissues that glue the electronic components to the PCB substrate, ensuring connectivity to the circuit board. It comes in various compositions, diameters, and flux types, each suited for specific applications. The selection of the appropriate types of solder is a pivotal element for efficient PCB manufacturing and optimized production techniques.

Key Factors to Consider When Choosing Solder Wire:

Solder Wire is one of the most popular forms of solder that is sold and accessible to users. It is used in many of the industries in which solder is used. Solder wires are created to cater  different applications and temperatures. The solder wire can also vary drastically depending on the type of solder you are interested in purchasing, many different types of soldering wires are available.

• Composition of Solder Wire: Leaded or lead- free, conductivity and material used.
• Flux Core Type: Rosin core, No-Clean Flux or  Water-Soluble Flux.
• Solder Wire Diameter: Thin Wires (e.g., 0.4mm to 0.8mm), medium Wires (e.g., 0.8mm to 1.2mm) or thick Wires (e.g., 1.2mm and above).
• Strength and Durability: Exposure of mechanical stress or high temperatures.

Leaded and Lead-Free Solder:

On a huge ground solder wires are generally two different types – lead alloy solder wire and lead-free solder. Lead solder wire is usually made from an alloy of lead and tin. Tin is commonly used with lead as it has a lower melting temperature. It has an alloy ratio of 63/37 or 60/40. When working with electrical components 63/37 is the best. This means that this solder wire has a sharp transition between solid and liquid states when the temperature changes. This property is very useful for reducing cold joints which usually happens when the components move while cooling.

In the past lead-based solder has been an industry standard, although the health problems associated with lead had made it a less popular choice. As a result the lead solder wire and lead solder bar price have reduced over the past few years. Lead-free solder wire usually contains tin, silver and copper. Lead- free wires are more expensive than lead alloy solder wire, also their melting temperature is higher. This means that they require higher temperature flux. The typical melting point of leaded solder wire is around 183°C to 190°C, and in case of lead free solder it jumps to around 217°C to 227°C.

Other Type of Solder Materials:

Lead-based solder has been around for ages. This alloy traditionally included a high percentage of lead, often combined with small amounts of tin or antimony. Tin makes the solder flow better and creates stronger, more durable bonds. Antimony, in small doses, also improves durability, especially at higher temperatures.

Tin-antimony Solder: Contains 95% tin and 5% antimony. This type of solder is lead-free and therefore safe for water supply lines. Designed for use in plumbing applications where frequent extreme temperature changes and vibrations occur (refrigeration, cooling equipment, and heat ducts, it is also used to solder electronics. The melting range is 450°F to 464°F and has an unlimited shelf life.

Copper-tin Solder: Contains 97% tin and 3% copper. Lead-free and safe for water supply lines, this solder type is designed for sweating copper and brass plumbing joints. The melting range is 440°F to 572°F, and it has an unlimited shelf life.

Silver-bearing Solder: Contains silver, copper, bismuth, and tin. Lead-free and safe for water supply lines, this solder is ideal for low-lead brass applications. It has a wide melting range (420°F to 460°F), enabling more time for soldering joints.

Solder Types by Core:

Rosin-Core Solder: It’s a less common solder type but a viable option for its ease of use. Its solder combines the metal alloy with a core filled with rosin, a natural pine tree resin and the original flux used in soldering. Flux acts like a mild acid, cleaning slight tarnish from the joining metals, allowing the solder to flow smoothly and create a strong bond. It also helps remove impurities for a clean and shiny finish, contributing to strength, corrosion resistance, and aesthetics. The convenience of having both solder and flux in one makes rosin-core popular, especially for tasks like electrical connections in electronics, circuit boards, and small-scale soldering projects.

Acid-Core Solder: Acid-core solder is a special material for plumbing applications, particularly soldering copper pipes and fittings. Unlike rosin-core, it uses a powerful acid flux (typically zinc chloride or ammonium chloride) to aggressively clean oxide layers from copper surfaces. This allows the solder to flow effectively, penetrate the joint deeply, and form a strong, clean connection. The strong acidic action is crucial for effective copper soldering in plumbing, as it cuts through the oxidation that would otherwise prevent a good bond.  However, this very aggressiveness makes it unsuitable for electronics, where it can damage delicate components.

Flux-Core Solder: Flux-core solder is a broader category encompassing any solder with internal channels filled with flux to aid in creating clean and efficient joints. The flux itself can vary:

• Natural Rosin - While rarely used alone now, rosin often forms the base with additional cleaning agents and isopropyl alcohol for dilution.

• Organic Flux - Suitable for electronics, this flux is gentle enough not to harm sensitive components. However, it might not be strong enough for heavily oxidized copper surfaces.

• Inorganic-Acid Flux - More aggressive than organic flux, it tackles tough oxidized copper but leaves residues that can damage electronics.

Solid-Core Solder: Solid-core solder simply refers to solder wire without any internal flux. This allows users to apply the specific flux needed for the job. Most solder alloys, from traditional lead solders to modern lead-free options, come in solid-core form to cater to diverse applications requiring different flux types.

Popular Solder Wire Options for Specific Applications

1. Electronics Repair and Prototyping: Lead-based solder (60/40) or lead-free Sn-Ag-Cu alloys with rosin or no-clean flux. And a wire diameter of 0.6mm to 1.0mm for versatility.

2. Surface-Mount Technology (SMT) Work: Lead-free solder with silver content for better wetting with no-clean flux to avoid residue cleanup. And a wire diameter of 0.4mm to 0.6mm for precision.

3. Power Electronics and Heavy-Duty Connections: Lead-free solder with high copper content for strength with rosin core for better bonding. And a wire diameter of 1.0mm to 1.5mm for larger joints.

4. Hobby and DIY Projects: Lead-based solder for ease of use (if regulations permit) with rosin or water-soluble flux. And a wire diameter of 0.8mm for general-purpose soldering.

Conclusion

Selecting the right solder wire is a critical step in ensuring the success of your soldering projects. Lead-based solders are the best understood, are reliable, and preferred in mission critical applications such as aerospace or medical electronics. Lead-free solders are available for electronics that need to meet health and environmental requirements. And flux solders contain a rosin reducing agent at the core that is released during soldering removing oxidation from the bonding site. Master the design rules with our new guide on DRC.

By understanding the key factors such as composition, flux type, diameter, and application needs, you can make informed choices that improve efficiency and reliability. Whether you’re working on intricate electronics or heavy-duty connections, the right solder wire will not only enhance the quality of your work but also simplify the soldering process. Always prioritize quality and compatibility to achieve the best results in your projects.