What is Solder?

Solder is a key item for the electronics industry. Used in virtually every electronic item, it is often overlooked. Find out all you need to know..

Solder is an essential material for the electronics industry. Without solder the assembly methods currently used in the manufacture of electronics equipment would need to completely change.

Apart from being used in the electronics industry, solder is also used in plumbing and many other areas such as in jewellery - in fact anywhere that metals need to be joined together.

The word solder comes from the middle English word "soudur" which in turn came from the Latin "soldare" meaning to make solid.

What is solder?

Solder is a fusible metal alloy. Often it consists of tin and lead, although the use of lead based solders was banned in the European Union on the 1980s. Under the Restriction of Hazardous Substances, RoHS, directive lead free solder is required. However traditional tin lead based solders are still in widespread use in many areas.

The melting point of solder used in electronics is below 450°C, often around 180 - 250°C for electronics applications, and there are often low melting point solders for special low temperature applications. 60/40 solder melts at 188°C.

Solder is able to melt at a point much lower than either of its constituents. Lead has a melting point of 327.5°C and tin has a melting point of 231.0°C. The reason for this is that solder is what is known as a Eutectic or eutectic mixture. This is a mixture of two or more elements that has a lower melting point than any of its constituents.

The traditional form of solder used for electronics manufacture, production and general construction was a 60 / 40 mix of tin and lead respectively. This provided a sufficiently low temperature melting point consistent with a high yield of good joints. In fact a 63 / 37 alloy has the lowest melting point of 183°C.

It is also found that the greater the level of tin within the solder, the greater is its tensile and shearing strength. Higher levels of tin also improve the wetting properties of the solder.

Solder flux

Solder is often used with a flux. Flux is a reducing agent and when used within solder, its purpose is to reduce (return oxidized metals to their metallic state) metal oxides at the points of contact.

Dry joints are caused when the solder oxidises and causes a joint that has poor properties both in terms of strength and electrical conduction. Dry joints may also be intermittent.

When solder oxides it forms a complex structure on the surface. Studies have revealed a four layer structure: tin(IV) oxide on the surface; below it a layer of tin(II) oxide with finely dispersed lead; this is followed by a layer of tin(II) oxide with finely dispersed tin and lead; finally there is the solder alloy underneath.

The use of a flux which helps prevent the formation of the oxides improves the electrical connection as well as the mechanical strength.

There are various types of flux:

  • Acid flux, used for metal mending and plumbing.
  • Rosin flux, used in electronics.
  • Water soluble flux, which can be removed with deionized water and detergent, instead of hydrocarbon solvents.

The electronics industry has been moving from the rosin flux which required removal by solvents to the water soluble forms as a result of concerns regarding atmospheric pollution and hazardous waste disposal.

For manual or hand soldering the traditional format of the solder is that of a wire. This wire has a flux core. This is often supplied as a coiled wire of solder, with one or more continuous bodies of non-acid flux embedded lengthwise inside it. As the solder melts onto the joint, it frees the flux which flows onto the joint to clean the joint prevent oxidation of the solder.

By Ian Poole

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