Tin-Lead Solder
Elemental Tin
Tin is a silvery-white, lustrous metal that retains its luster when exposed to air. It has good ductility and a relatively coarse grain structure. When a solder rod is bent, the friction between the grain interfaces produces a distinctive sound known as "tin snore." Tin is a soft, low-melting-point metal with a phase transformation point of 13.2℃. Below this temperature, it becomes powdery gray tin (q tin), which has a diamond-like crystal structure. Above 13.2℃, it becomes white tin (B tin), exhibiting a body-centered cubic lattice and high ductility. α-tin atoms can accelerate the formation of α-tin in untransformed materials. Adding certain metallic elements can significantly reduce the risk of this tin blight; for example, the US National Standard QQ-S-571 requires the addition of 0.25% antimony to solder to prevent the formation of α-tin.
Tin has good corrosion resistance in the atmosphere and retains its metallic luster; however, it is not resistant to corrosion by substances such as chlorine, iodine, caustic soda, and caustic potassium.
In tin-lead solders, tin will alloy with the base metal during the soldering process due to metallurgical reactions, forming solder joints.
Element Lead
Lead is a bluish-gray metal with a bright metallic luster on its freshly exposed surface. Normally, this surface quickly turns dark gray due to deterioration and oxidation in air. This oxide film has very strong adhesion, protecting the underlying metal from further environmental corrosion, giving lead its unique resistance to various chemical and environmental corrosions. Lead is a soft metal with a face-centered cubic lattice, making it easy to process and shape. Lead is a toxic metal harmful to humans, and special care should be taken when handling it. Tin-lead solder (hereinafter referred to as solder) contains tin, which forms an alloy with the base metal during the soldering process due to a metallurgical reaction. Lead, however, hardly reacts under any circumstances. However, adding lead to tin as a component of solder allows the solder to acquire the following excellent properties that neither tin nor lead possesses:
Lower melting point, facilitating operation:
Tin's melting point is 232℃, while lead's is 327℃. Mixing tin and lead yields solder with a lower melting point than either metal (melting temperature 183℃). Because of its low melting point, it is relatively easy to handle.
01
Improved mechanical properties:
Tin has a tensile strength of 1.5 kg/mm² and a shear strength of approximately 2 kg/mm². Lead has a tensile strength of approximately 1.4 kg/mm² and a shear strength of approximately 1.4 kg/mm². If the two are mixed to form a solder, the tensile strength can reach 4~5 kg/mm² and the shear strength 3~3.5 kg/mm². After soldering, these values will become even higher. This significantly improves the mechanical properties.
02
Reduced interfacial tension:
The diffusivity (wettability) of liquid solder is improved due to the reduction in surface tension and viscosity, thereby increasing its fluidity.
03
Oxidation resistance:
Adding lead to tin increases the solder's oxidation resistance and reduces oxidation.
04
Tin-lead solder
The main reasons why tin and lead are widely used as solders in lead-based assembly processes for electronic products are:
- A narrow melting temperature range (i.e., a narrow paste-like region), which is very suitable for engineering applications;
- Good wettability and mechanical and physical properties;
- Good economic efficiency.
