solderfillet

Solder fillet refers to the concave, triangular-shaped connection formed between a component lead or pad and a plated through-hole (PTH) or a surface mount pad on a printed circuit board (PCB) after soldering. It is a critical visual indicator of a good solder joint. The fillet develops as molten solder is drawn by capillary action into the annular ring around the PTH or onto the pad and then wets both the component lead/pad and the PCB pad.

A properly formed solder fillet should exhibit a smooth, shiny surface and be free of voids, excessive solder, or insufficient solder. The ideal fillet shape is concave, indicating that the solder has adequately wetted both surfaces and formed a strong, reliable connection. The absence of a fillet, or a convex or irregular fillet, can signal potential soldering issues such as contamination, improper temperature, or insufficient solder.

The size and shape of a solder fillet are influenced by several factors, including the surface tension of the solder, the wetting characteristics of the materials being joined, the geometry of the joint, and the amount of solder applied. Inspection of solder fillets is a common quality control step in electronics manufacturing to ensure the reliability and integrity of solder connections. Standards and guidelines exist to define acceptable fillet characteristics for different applications.