What is Solder?

Solder is the adhesive material used for connecting two metals together. Most often it is used to connect electronic component leads to pads or holes on a PCB to form electrical connections between the components. To make the connection, solder is heated to its melting point and then applied on the leads or pads/holes to make a connection. Solder materials are selected to have a melting point below the metals that it is being used to join. This way it melts and solidifies to provide a good metal-to-metal joint without damaging the components or the PCB after cooling.

A good solder material must also have good electrical and mechanical properties.  The IPC J-STD-006 standard provides guidelines regarding the use of different types of solder material on PCB.

Solder is available in multiple formats – Solder Bars, Solder Powder, Solder Wire, Solder Paste etc. Each format is used in different cases. For example, solder wire is used in the manual soldering process, where the technician, uses the wire and melts it over a joint where the connection needs to be made. Solder bars are used in the wave soldering process, where the molten solder wave is soldering the components on the PCB. Solder powder and solder paste are used in reflow soldering, where the solder paste holds the surface mount components in their place and the paste melts when past through a reflow oven.

An important part of solder materials is flux. Flux is a chemical substance that is used to support and make the solder joint better. It is usually sprayed on or applied on the metal surface to prepare it before the soldering process. It prevents the surface from getting oxidized and removes other impurities to ensure a better and longer-lasting connection.

Composition of Solder

Solder is usually made up of two or more materials. The type of materials used and their proportions determine various factors like the melting point, joint strength and various other properties.

Broadly, solder is divided into Lead Free Solder and Lead Based Solder. The use of lead-based solder is prohibited by the RoHS directive (2002/95/EC) which became effective on July 1, 2006. This was banned as lead as a material is harmful to human health. As a result, most of the solder used today is lead free.

Lead Free Solder

The main element of Lead-free solder is tin followed by copper, indium, bismuth, zinc, antimony, and other metals. Eutectic lead-free solder is also available. The proportion of materials (copper, indium, bismuth, zinc, antimony, and other metals) used along with tin determines the properties of lead-free solder. For example, antimony increases the strength without affecting the wettability, copper improves the wetting properties of molten solder, indium improves ductility. The composition is usually selected based on the application.

Lead-free solder composition

Lead-based Solder

The use of lead-based solder has been prohibited by the RoHS directive (2002/95/EC) which became effective on July 1, 2006.

Lead-based solder composition

In the above table, we can see three different types of solder alloys. For example, Tin 63% / Lead 37% solder, which melts and freezes at 183 °C (see figure 4). This melting point is lower than the melting point of pure metal like lead (327 °C) and tin (232°C). 

An Eutectic Solder material is one that melts and freezes at one single temperature. 

Phase diagram of tin and lead

If the composition of the solder material changes from Tin 63% / Lead 37%  to  Tin 60% / lead 40%, then the melting point becomes a melting range. For example, the composition Tin 60% / lead 40% has the melting range of 183 to 191 °C. When heated, this composition (Tin 60% / lead 40%) starts to melt at 183 °C, becomes slushy between 183 °C and 191 °C, and becomes fully liquid at 191 °C. The eutectic solders have a good appearance as compared to non-eutectic solders that have a dull and grainy appearance.

Eutectic tin-lead solder was popularly used so many years for electronic devices. The eutectic tin-lead has a lower melting temperature than the components that it is joining. Tin/lead solder provides good electrical and mechanical strength and corrosion-resistant those are the need for electronic devices. But, the use of lead is dangerous to human health. Hence, nowadays, lead-based solder is not in use.

Lead-free solder material has replaced lead-based solder materials. Most lead-free solder is tin based and consists of silver, copper, bismuth, and additive elements. The additive elements are added to lead-free solders to modify the properties of lead-free solders to become more suitable for electronics applications.

There are two other types of solder materials - Hard Solder and Soft Solder. Hard Solder is another type of solder material that is used for high-temperature applications, such as brazing (also a process to join the metals). The most common composition of hard solder is copper with either silver or zinc. Soft Solder is used in the electronics soldering process.