What is solder made of?

Solder is a metal alloy used to create a permanent bond between metal parts. This is achieved by melting the solder and using it to create a joint. This requires solder to have a lower melting point then the pieces being joined. After the solder cools down, the parts are permanently connected.

Most electronics use solder to attach components to printed circuit boards (PCBs). The solder used in electronics needs to have favourable conductive properties.

Traditional solder is made of lead and tin. For many years, this alloy has been the standard used across the electronics industry. However, concerns have been raised about the health and environmental effects of leaded products. In the recent past, the use of lead-free solder alloys has become common.

Lead alloy solders

These solders are made of an alloy of lead and other metals. The most common mixture is 60% tin, 40% lead (or 63/37). Leaded solder has a low melting point (around 180° C). This makes it especially easy to work with. Lead alloy solders flow well and form strong bonds with other metals. Because of their properties, they are also known as soft solders. The presence of lead in the alloy inhibits the formation of tin whiskers.

Unfortunately, lead is a poisonous substance. It’s very harmful to the human body. Also, it’s hazardous waste and can contaminate soil and ground water. To answer the concerns raised about the use of lead, the EU implemented the Restriction of Hazardous Substances directive (2006). The directive restricts the use of lead alloy solder in consumer electronics produced in the EU.

Lead-free solders

These solders are made from many different metals. Tin is the most common base. It provides strength to the alloy. Other metals include silver, copper, nickel and antimony. Lead-free solders have higher melting points, compared to leaded ones. This has several side effects, including an increased stress on components during assembly. Also, lead-free solders do not flow as well as lead-based ones. Joints tend to be less strong.

To prevent such problems, manufacturers have developed solders with unique properties. Tin-silver-copper alloy, for instance, has a lower melting point (217° C). Using silver reduces oxidisation, while copper improves conductivity and mechanical strength.

The formation of tin whiskers can be prevented by using conformal coatings or mixing nickel in the alloy.

Flux core solders

These solders are sold as a spool of wire. Regardless of the composition of the alloy, they all have flux at their core. Flux is a reducing agent. Its purpose is to remove oxides and impurities from metals. Using flux improves the conductivity and mechanical strength of the soldered joints. It also helps the flow of molten solder.

Flux core solders release the flux during the soldering process. There are two main types of flux. Rosin flux is used for electronics, because of its relatively low corrosiveness. Residues can be removed with isopropyl alcohol. Acid (or inorganic) flux is used for metal joining and plumbing. This type of flux is very corrosive and should not be used for electronics.

Conro Electronics offers a wide choice of solder and soldering tools from major manufacturers such as Indium and Metcal.

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