What defines a "solder fillet" in IPC-A-610?

A solder fillet is characterized by its meniscus-shaped bead that forms around the lead connection, specifically where the solder meets both the lead and the surface of the circuit board. This shape is crucial because it provides a good mechanical and electrical connection. The fillet’s geometry can reflect the quality of the solder joint, ensuring sufficient volume and surface area necessary for creating a reliable bond.

In the context of soldering, the fillet serves to secure a component in place while also facilitating electrical conductivity. A well-defined fillet indicates proper solder application and heat management during the soldering process. This shape is essential for the mechanical stability of components and impacts the long-term reliability of electronic assemblies.

The other descriptions, such as a flat iron surface, a smooth coating, or reinforcing materials, do not accurately convey the specific role and shape of a solder fillet, which is fundamentally associated with the junction of solder, leads, and substrates in electronic assemblies.