Cu3Sn

Cu3Sn is an intermetallic compound of copper and tin with the chemical formula Cu3Sn. It is one of the intermetallic phases in the copper–tin system and is commonly referred to as the eta phase. It forms under diffusion in solids and is often found at the interface between copper and tin-containing solders or finish layers.

Formation and structure: Cu3Sn forms when tin diffuses into copper or copper diffuses into tin at elevated temperatures. It has a fixed stoichiometry and crystalline structure characteristic of the eta phase in the Cu–Sn system. In solder joints, it typically appears as a layered, adherent intermetallic sheath at the copper interface, sometimes within the solder near the interface.

Properties: As an intermetallic compound, Cu3Sn is generally hard and brittle, with a melting temperature above that of tin-based solders. Its electrical conductivity is lower than that of copper, and thick intermetallic layers can reduce joint ductility and reliability. Growth is diffusion-controlled and accelerates with temperature and time.

Relevance: Cu3Sn formation is a key consideration in electronics solder joint reliability. Excessive growth can lead to cracks or voids at the interface, especially under thermal cycling. Mitigation involves controlling soldering processes, material interfaces, and aging conditions to limit excessive intermetallic growth.