8 Different Types of Solder

Soldering is a central and critical group of techniques in electronics, copper-based plumbing, and the engineering of delicate parts. It involves joining two or more metal components together by melting a lower-melting-point filler metal, the solder, and allowing it to flow into the joint. This creates a strong and (where relevant) electrically conductive bond when it cools and solidifies. This process is key to connecting electrical components onto printed circuit boards (PCBs) and assembling intricate mechanical and jewelry parts.

This article will review the eight commonly used forms of solder and their applications and necessary specializations in processing.

1. Lead-Based Solder

Lead-based solder is the earliest type of soldering alloy that historically contained a significant amount of lead. It often contains small quantities of antimony and tin to affect various properties. For example, tin improves the wetting quality of solder as well as its mechanical strength and fatigue resistance. Antimony in small quantities also improves the fatigue properties of lead solder, particularly at higher operating temperatures.

The most common alloy, less used now, is 60/40 tin/lead, which has a melting point of 180–190 °C.

Lead-based solder was commonly used in electronics, plumbing, and other applications due to its low melting point, ease of use, and reliability. Concerns about the environmental and health impacts of lead exposure have led to restrictions and regulations limiting its use. In many regions, such solder has been largely phased out, replaced by materials that are safer for both operators and the environment. Lead-free solder alloys offer similar performance while reducing the risks associated with lead contamination and toxicity, but can be considerably harder to work with.

2. Lead-Free Solder

Lead-free solder is the lower-toxicity alternative that has largely displaced traditional solder in common use, particularly in PCBA (printed circuit board assembly) soldering and plumbing. It offers greater safety to health and lower environmental impact. Lead-free solder alloys typically consist of a combination of metals, such as: tin, silver, copper, and small amounts of other elements like bismuth or zinc. These alloys have a higher melting point compared to lead-based solder but still provide effective and reliable bonding, though it requires higher skill levels. The most common class of lead-free solders are tin-copper, plus trace quantities of other metals.

These alloys commonly have melting points of around 217 °C. SAC (Sn-Ag-Cu tin-silver-copper) is a blend popular in Japanese electronics factories, although the higher silver content does raise the price. Lower silver and silver-free alternatives are more widely used.

Lead-free solder eliminates most toxicity risks, making it safer for workers and reducing the environmental impact during manufacturing and disposal. Although these materials have become industry-standard, they require more advanced and controlled soldering techniques at higher temperatures, and they are less tolerant of process variation and contaminants.

3. Rosin-Core Solder

Rosin-core solder is a type of solder that is in decreasing use, although still popular. The metal alloy is formed as a tube, with its hollow core filled with rosin. Rosin is the common name for natural pine tree resin, and it was the earliest form of flux used in soldering.

A flux serves as a very mild, naturally occurring acid to remove slight surface oxidation on the parts to be soldered, allowing wetting. It also helps to reduce joint impurities to make a clean and high gloss finish on the joint, which can improve strength, corrosion resistance, and aesthetics.

Rosin-core solder is popular due to its convenience, as it combines both the soldering material and flux in one, simplifying the soldering process. It was commonly used for electrical connections in electronics, circuit boards, and various small-scale soldering applications, but rosin flux is of low efficacy in lead-free soldering. Since it is unreactive (chemically passive) when cooled/solidified, both pure rosin and modern, additive-rich rosin fluxes are safe for electronic use.

4. Acid-Core Solder

Acid-core solder is a specialized type of solder wire primarily used in plumbing applications, especially for soldering copper pipes and fittings. The aggressive acid acts as a powerful flux to clean the oxides from the copper surfaces. In this way, the solder can wet and flow effectively, penetrating copper-to-copper joints and forming a clean, fully coupled meniscus when the solder is molten.

This flux core is typically a mixture of zinc chloride or ammonium chloride and serves a crucial purpose in plumbing. Without this chemical attack, copper joints are almost impossible to make effectively.

Due to its corrosive nature, acid-core solder is not suitable for electrical applications, in which other minimally corrosive flux core solders are used to prevent damage to sensitive electronic components.

5. Flux-Core Solder

Flux-core solder is a generic term for any type of solder alloy that has one or more internal galleries running through it that are filled with flux that assists in making clean and efficient joints.

The flux can be natural rosin, though pure rosin is rarely used now — it is likely to contain various additives for oxide cleaning, plus isopropyl alcohol as a diluent. Flux can also be organic, mild-acidic flux, suitable for electronics, but not strong enough to solder heavily oxidized copper surfaces. Inorganic-acid flux is more strongly acidic and can clean more intractable copper surfaces but leaves residues that will damage electronics (so it is not used in that area).

6. Solid-Core Solder

This is a general term for solder wire that lacks any internal flux cores, requiring external fluxing suited to the application. Most solder alloys, from eutectic lead to Japanese SAC and other RoHS (Restriction of Hazardous Substances), lead-free solders are supplied in solid core forms, to suit the wide range of applications requiring solders.

7. Silver-Alloy Solder

Silver soldering is a considerably higher temperature process, as silver has a higher melting point. The liquefaction temperature depends heavily on the alloy, ranging from around 300 °C up to 420 °C for high silver content. This process is often referred to as silver brazing.

The silver-alloy solder rods are solid core and are used in conjunction with more complex and aggressive fluxes consisting of: 50 to 80% alkali metal fluoroborate, 9 to 30% alkali metal tetraborate, up to 15% boric acid, up to 10% alkali metal carbonate, and between 3 and 15% of an organic polymeric binder. These are white pastes or powders (to be wetted for use) that form an oxygen-excluding, acid, and oxide-removing glass-like layer that encourages wetting and flow.

8. Lead-Alloy Solder

Lead-alloy solders were the earliest forms of solders used and are modified with tin, antimony, bismuth, copper, silver, and other metals to enhance the mechanical and wetting properties or resistance to oxidation of the resulting joints.

A typical form of this is eutectic lead solder, 63% tin, and 37% lead, at which proportions the melting point is exactly 183 °C, allowing precision process control. Altering the proportions moves the melt temperature to a range that is harder to manage in real production.

What Is Solder?

Solder is a generic term that can be applied to any metal or alloy that is used as a molten-applied coupling agent between other metal parts. It is applied in a molten form such that the parts to be joined are wetted by the molten solder alloy, but not melted. This can include alloys ranging in melt temperature from 180 °C (tin-lead).

How To Choose a Type of Solder?

Choosing the right type of solder depends in subtle ways on the specifics of the application such as: materials to be joined, available equipment, access to specialist solders and fluxes, and safety considerations. 

Consider the metals you are joining. Solder alloys have specific suitability for target joints. Lead-free solder with low residue and low acidity fluxes are required for electronics, while lead-based solders with more aggressive fluxing may be required for plumbing. The alloy melting point must be selected to be compatible with the materials to be joined. For example, high-temperature solders can easily damage sensitive electronic components.

Determine the specific implications of the application. For electronics, one must select a solder with good electrical conductivity, whereas for plumbing the solder must comply with applicable local codes and standards. Be cognizant of local regulations regarding the use of lead-based solder in consumer products. Many regions restrict or ban the use of lead solder in certain applications, under RoHS guidelines/requirements.

Understand the functional impacts of flux and its method of delivery. Decide whether you can use a flux-cored solder or need a separate flux for cleaning and deoxidizing the joint. Flux-core solder is convenient for some applications but is only suited to hand soldering.

If sustainability (or the impression of it) is a concern, lead-free solder will reduce environmental concerns. Solders are available in various forms, such as wire, bar, paste, and solder preforms, selected according to equipment and process requirements. Skill and experience can drive the choice of solder. Expert opinion is best sought, where questions are unclear or unanswered.

How Important is Choosing the Right Type of Solder?

Solder selection can be a delicate matter, in which precision tasks impose choices, as do regulatory/import restrictions, electrical or thermal behavior, strength, and the difficulty imposed by severe oxide layers or larger gaps. Choosing the wrong solder for a particular application can lead to several faults and issues, which can negatively impact the quality, reliability, and safety of the soldered joint or product. 

Inappropriate solder selection can result in weak or incomplete joints that do not provide a secure connection between components. The selection of fluxes can be just as important and mistaken choices lead to electrical or mechanical failure. Faults in electronics assemblies in reflow and wave soldering can be complex to diagnose and have obscure reasons and solutions. Common experiences resulting from minor variations include: component “tombstone”, dry joints, short circuits, and thermal cycling failures. In high-compliance industries such as: the military, avionics, satellites, and medical equipment, failures can cost lives.

Using the wrong solder alloy can promote corrosion in the joint or on the surrounding materials, reducing the life span and reliability of the soldered connection. This can result from both solder and flux choices, which must be managed in tandem.

What Is the Best Type of Solder?

The best type of solder is highly dependent on the specifics of the application. Lead-free solders are mandatory and regulated in most consumer and industrial product sectors. However, variations in alloy constituents can be subject to strong preferences, in some sectors or process types. The Japanese preference for higher silver content is, for example, a quality choice in electronics production, but most other markets make different choices.

What Is the Most Common Type of Solder?

Four broad classes of solder are in widespread use, these are:

1. Wave-applied solder in a variety of RoHS-compliant, lead-free alloys, sold as bar and ingot, to be melted in a wave soldering machine, in which PTH (plated through-hole or leaded components) are soldered to PCBs.
2. Solder pastes, which are generally RoHS-compliant alloys sold in a paste with low residue and low-acidity flux for screen printing onto PCBs such that components can be adhered to by manual or automated placement. The PCB assembly is then soldered in a reflow oven that melts the alloy and forms a precise and controlled electrical/mechanical coupling.
3. RoHS-compliant, flux-cored solder wire for manual repair/rework or manufacturing use.
4. Lead-based and lead-free solder wire for copper-plumbing jointing. These are supplied both as solid-core and flux-core wires.

What Is the Type of Solder Used for Plumbing?

The most common types of solders used in plumbing are increasingly lead-free alloys, due to health and safety concerns associated with lead exposure, particularly in drinking water. Tin-copper or tin-silver-copper alloys are commonly used in plumbing applications. They have a higher melting point compared to lead-based solder and can be more challenging to use. They are known for their reliability and robust outcomes in soldering copper components.

Bismuth-based alloys are an alternative option used in plumbing. These often contain a combination of tin, bismuth, and other metals, with a lower melting point than tin-copper alloys, making them easier to work with. They are suitable for domestic plumbing but can be weak in applications involving high temperatures or pressure.

What Is the Type of Solder Used for Electronics?

In electronics, most applications now require lead-free solder. This shift away from lead results from the RoHS implementation in the early 2000s. This was driven by the associated environmental and health concerns resulting from lead exposure.

There are several common lead-free alloys used in electronics, including: Sn-Ag-Cu (tin-silver-copper), Sn-Cu (tin-copper), Sn-Bi (tin-bismuth), and Sn-Zn (tin-zinc). Tin-copper is a common choice for electronics soldering due to its low-cost, but still reliable, solder joints. On the other hand, tin-zinc is less commonly used but offers benefits for specific applications.

Can You Use Regular Solder on Electronics?

No. The restrictions in solder types used in electronics relate to RoHS (Restriction of Hazardous Substances). Commercial electronics production and all consumer, medical, and industrial products must comply with lead-free solder requirements. However, repairs and home projects do not have to meet the same restrictions and there are still plentiful sources of lead-based electronics solders available.

How Is Solder Used in Soldering?

Creating a successful solder joint requires careful preparation and skilled technique. Work in a well-ventilated area, or use a fume extractor when soldering. Ensure the work area is clean and free of flammable materials. For large area jointing of metal parts, clean the metal surfaces you will solder using a wire brush, abrasive paper, or an acid etching/cleaning agent to remove oxides, dirt, etc. 

Apply a suitable flux to the cleaned surfaces, if the solder is not flux-cored. Flux completes the removal of oxides, promotes wetting, and ensures a clean and well-formed solder joint, with a good surface finish and meniscus. The flux type must match both the solder alloy and the substrate materials. Use a soldering iron or gun, a flame, or a furnace to heat the workpieces evenly. The temperature should be just sufficient to melt the solder, while not enough to damage components or materials.

Hold the solder wire against the heated workpiece, not the soldering iron tip. Allow the heat in the workpiece(s) to melt the solder, and it will wick outwards onto the cleaned and fluxed surfaces. Ensure the solder flows evenly and fills/wets the entire joint. Avoid excess solder buildup, short circuits, and drips. The solder should create a smooth, shiny, and concave fillet at the joint line. Convex curvature can indicate excess solder, but this can conceal a “dry” joint, in which the flux has failed to enable wetting of some areas.

What Is the Melting Point of Solder?

Solder melting points vary considerably with alloy, allowing selection according to processing needs. Table 1 below shows the typical melting points of different solder alloys:

What Are the Metals That Can Be Soldered?

Most metals can be soldered, so long as: an alloy that can wet at the joint when molten and remain integral and adherent when solidified is available, and a flux that is compatible with both materials can be applied to encourage wetting and meniscus formation. Table 2 below shows the solderability of various metals:

Can Bronze Be Soldered?

Yes, bronze can be soldered with a range of alloys such as lead-based plumbing solder, lead-free plumbing solder, and even low-temperature bronze-like alloys — for higher cosmetic jointing.

Summary

This article presented types of solder, explained each of them, and discussed each of their various applications. To learn more about solder, contact a Xometry representative.

Xometry provides a wide range of manufacturing capabilities and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.

Disclaimer

The content appearing on this webpage is for informational purposes only. Xometry makes no representation or warranty of any kind, be it expressed or implied, as to the accuracy, completeness, or validity of the information. Any performance parameters, geometric tolerances, specific design features, quality and types of materials, or processes should not be inferred to represent what will be delivered by third-party suppliers or manufacturers through Xometry’s network. Buyers seeking quotes for parts are responsible for defining the specific requirements for those parts. Please refer to our terms and conditions for more information.