- Soldering requires heating the solder alloy to its melting point. The melting point temperature should be much lower than that of the metals to be joined. The melting point of tin-lead solder depends on the proportions of tin and lead in the alloy. It ranges from about 350 degrees to about 600 degrees Fahrenheit. Generally, a tin-lead solder having a melting point of under 400 degrees Fahrenheit is used to solder electronic components. This temperature is much lower than the melting point of copper used in electronic circuitry.
- Because tin-lead solder is commonly used to solder electronic components, one of its desirable properties is its low resistance to the flow of an electric current. Resistivity can be described as the electrical resistance faced by current as it flows through a unit distance of the material. Resistance is measured in ohms, while resistivity is measured in ohm-meters. The electric resistivity of tin-lead solder alloys is less than a millionth part of one ohm-meter, which is as small as for a copper wire. Therefore soldering with tin-lead solder does not affect flow of current.
- It is important that the solder provides sufficient mechanical strength to the joint so that it does not break away when subjected to expected amounts of mechanical shock and vibration. The solder alloys are therefore designed to withstand moderate amounts of mechanical stress. Soldering is not used in situations where high impacts or strong shear forces are expected. In such cases, the pieces are welded, not soldered. The impact strength of tin-lead solder ranges between 8 and 20 joules, while the shear strength is less than 40 megapascals (MPa). Impact strength measures how much of a mechanical impact the material can withstand while the shear strength determines the amount of pressure the material can withstand before its mechanical form gets deformed in the direction of the applied force. The above mentioned values for tin-lead alloy are not very large, which means that the alloy can essentially be mechanically deformed with a moderate amount of force, such as bending with hands.
- Thermal conductivity of a material represents how efficiently heat can flow through it. It is generally measured in watts per meter per Kelvin, where Kelvin is a unit of temperature. The thermal conductivity of most tin-lead solder alloys is on the order of 50 watts per meter per Kelvin. This value is high enough for proper conduction of heat through the material. In general, a material that has high thermal conductivity also has high electrical conductivity, which is a desirable property for materials used in electrical circuits.
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