Properties of lead-tin alloys

The most important lead-based alloys are lead-tin alloys. This alloy has a light gray metallic luster, is softer, and has a lower porosity than a single layer of lead or tin. The standard electrode potential difference between lead and tin is only 10mV, and the overpotential of hydrogen is high, so it can be co-precipitated in a simple strong acid plating solution. It only needs to control the concentration ratio of lead and tin ions and the current density to obtain any alloy composition. Alloy plating, its current efficiency is close to 100%.

Pb-tin alloy plating solution, in addition to the most used fluoroborate plating solution, sulfamate, phenolsulfonate, alkanolsulfonate and citrate plating solutions have been used in production.

The metallographic structure of lead-tin alloy is supersaturated solid solution, but phase transformation will gradually appear during storage. In some special cases, the components will also form intermetallic compounds with the substrate, such as tin and copper. When this characteristic has special requirements, an intermediate isolation layer should be added between the substrate and the lead-tin alloy plating layer.

The melting point of tin-lead alloy is lower than that of pure tin and pure lead, and its outstanding advantages are that its porosity and solderability are better than that of single metal. If the pure tin coating is placed for a long time, needle-like crystals (tin whiskers) will grow on the surface, which can cause short circuits; it can also produce "tin plague" at low temperatures. As long as 2% to 3% of lead is added to pure tin, this phenomenon is not easy to occur. Therefore, this coating is the most important solderability coating in electronic components.

Lead-tin alloys with different tin contents have different properties and uses. Usually the use of various lead-tin alloys is determined according to the different alloy compositions they contain.

The composition of lead and tin at the lowest eutectic point (183°C) is 61.9% tin and 38.1% lead. At this time, the alloy has the greatest welding strength and wetting ability. Therefore, most of the current solder and welding coating use 60% tin and 40% lead. Alloy coating.