The PCB  tin-lead alloy spraying process, is a traditional surface finishing technique that applies a layer of tin-lead solder to the copper pads of a printed circuit board. Historically popular for its excellent solderability and cost-effectiveness, this process uses a eutectic alloy of 63% tin and 37% lead, which melts at a low temperature (183°C) and forms strong, reliable solder joints.

The process begins with cleaning the PCB to remove contaminants, followed by preheating to prepare the copper surface. The PCB is then either immersed in a molten tin-lead bath or sprayed with the alloy using a specialized nozzle. Immersion ensures full coverage of the copper pads, while spraying allows for precise control over the coating thickness. After application, excess alloy is removed using hot air knives or mechanical leveling, resulting in a smooth, uniform layer that covers the pads and protects them from oxidation.

Tin-lead alloys offer several advantages. Their low melting point simplifies the soldering process during component assembly, reducing the risk of thermal damage to the PCB. The alloy also provides excellent wetting properties, ensuring that solder flows evenly and forms strong bonds with both the PCB pads and component leads. Additionally, tin-lead coatings are relatively inexpensive compared to precious metal finishes like gold or silver, making them suitable for high-volume, cost-sensitive applications such as consumer electronics and automotive parts.

However, the use of lead in electronics has been restricted by environmental regulations like RoHS (Restriction of Hazardous Substances), which limits the application of tin-lead alloys in most regions. Lead is toxic, posing health risks during manufacturing and disposal. As a result, lead-free alternatives (such as tin-silver-copper) have largely replaced tin-lead alloys in mainstream electronics. Despite this, the tin-lead alloy spraying process is still used in specialized applications where its unique properties (like low melting point and high reliability) are irreplaceable, such as in certain military, aerospace, or legacy equipment where regulatory exemptions apply.