POS-61 Solder: Characteristics, Properties, and Applications

What is POS-61 and how to interpret its designation?

The abbreviation "POS" (ПОС) is straightforward: "P" (П) stands for pripoi (solder), "O" (О) for olovyanny (tin-based), and "S" (С) for svintsovy (lead-based). The number indicates the percentage of tin. In POS-61, tin accounts for 61%, with the remaining 39% consisting of lead and a small amount of impurities. It is one of the tin-richest solders in its category—which defines its specific properties.

The labeling is regulated by GOST (state standards), so when you purchase POS-series solder that complies with GOST, you can be certain of its exact composition. This is crucial, as the ratio of tin to lead directly determines the melting point, fluidity, and the strength of the resulting joint.

Composition and key properties

Classic POS-61 tin-lead solder consists of:

• tin — 61%;
• lead — approximately 39%;
• permissible impurities (such as antimony, copper, bismuth, or iron) — within the limits set by GOST.

This composition offers several significant advantages:

Low melting point. The alloy begins to melt at 183°C and becomes fully liquid at 190°C. For comparison, POS-30 has a broader melting range—from 183°C to 238°C. This low melting point makes POS-61 ideal for soldering components sensitive to overheating, such as circuit boards, microchips, and thin conductors.

Good fluidity. When molten, the solder easily fills tiny gaps and irregularities, creating a neat, airtight joint. This is particularly valuable when assembling precision electronics.

High electrical conductivity. The joint does not create unnecessary resistance, which is critical for radio electronics and various devices.

Ductility and strength. The solder joint withstands vibrations and moderate mechanical loads without cracking over time.

Corrosion resistance. Under normal operating conditions, the joint does not oxidize and offers a long service life.

Available forms: rods, wire, and ingots

POS-61 solder is supplied in several formats to suit different tasks and soldering methods.

Solder rods. A classic format for manual soldering. These rods typically have a diameter of 8–10 mm and are convenient for working with large components, heat sinks, and thick conductors.

Solder wire. A thin wire with a diameter ranging from 0.5 to 5 mm. This is the most convenient option for soldering printed circuit boards and making precise connections. It is often produced as a hollow wire (tube) containing flux, eliminating the need for separate flux application. If you are looking for POS-61 solder, wire is the optimal choice for electronics.

Solder ingots. Large-weight blocks used in industrial settings for remelting, casting, tinning baths, or for producing custom solders and alloys. Wholesale buyers most frequently purchase these ingots.

Any of these forms can be manufactured as a tin-lead solder with various additives, depending on the specific application.

GOST standards and quality control

Tin-lead solders are produced and tested in accordance with the GOST 21931-76 standard. This standard regulates chemical composition, permissible impurity levels, melting points, and available product forms. When you purchase POS-series solder, you can be confident that the alloy has been tested and meets its stated specifications.

In large-scale manufacturing, quality control is applied at every stage—from the initial analysis of raw materials to the final inspection of the finished batch. Each batch comes with documentation, which is essential for enterprises that operate under strict technical regulations.

What is POS-61 used for?

This solder has a wide range of applications, from household soldering to industrial production.

Radio electronics and instrumentation: Used for assembling printed circuit boards, soldering microchips and connectors, and working on precision measuring instruments. Its low melting point allows for work on sensitive components without the risk of damaging them.

Electrical engineering: Used for soldering the windings of electrical machinery, cable components, and contact surfaces. Its good electrical conductivity ensures a reliable connection.

Automotive industry: Used for soldering electrical wiring, radiators, and heat exchangers, as well as for tinning parts. While antimony-containing or low-antimony solders are more commonly used here, POS-61 is also suitable for certain tasks.

Food and medical industries: Used for tinning and soldering the internal seams of cookware and medical equipment. For these applications, solders with a higher tin content—such as POS-90—are typically preferred, though POS-61 is used in some cases.

Household soldering and repairs: For wires, contacts, and small components, POS-61 remains the most convenient and accessible option.

How does POS-61 differ from other grades?

To avoid confusion among the various grades, it is helpful to understand the broader context.

• POS-30 and POS-40: These are also tin-lead solders, but they contain a lower percentage of tin. They are cheaper, but they melt over a wider temperature range and produce a less precise joint. They are used for soldering coarser connections, tinning, and applications where high precision is not required.
• POSSu (antimony-containing) solders. These alloys contain added antimony, which increases their hardness and strength. Low-antimony solders are used in electronic equipment, while those with higher antimony content are used for components subject to vibration and mechanical stress. If you specifically need this type of alloy, look for "POSSu" solder.
• Lead-free solders. An eco-friendly alternative that contains no lead. They are used in products for the food and medical industries, items intended for children, and in areas subject to regulatory restrictions.