SnAgCu

SnAgCu, commonly abbreviated SAC, denotes a family of lead-free solder alloys based on tin with small additions of silver and copper. The most widely used variant is SAC305, consisting of approximately 96.5% tin, 3.0% silver, and 0.5% copper by weight. Other common grades include SAC387 (3.8% Ag) and SAC405 (4.0% Ag) with similar Cu contents. These alloys are designed to replace traditional tin–lead solders in electronics.

SAC alloys have a melting temperature around 217°C and a narrow melting range, typically around 217–223°C for SAC305. They are near-eutectic, which helps minimize the amount of time the alloy spends in a mushy state during reflow. They exhibit good solderability and wetting on copper and nickel surfaces, and they provide higher creep resistance and mechanical strength at elevated temperatures than Sn-Pb solders.

The microstructure consists of a tin-rich matrix with dispersed intermetallic compounds. Intermetallics such as Ag3Sn form in the bulk, while Cu6Sn5 forms at the copper interface, yielding an intermetallic layer at pads during joint aging. Over time and with thermal exposure, the IMC layer thickens, which can influence joint brittleness and long-term reliability.

Applications and processing: SAC solders are widely used in surface-mount assembly and reflow soldering of printed circuit boards in consumer electronics, computers, and automotive applications. Processing typically uses fluxes compatible with RoHS-compliant solders, and reflow profiles commonly reach peak temperatures around 235–260°C, depending on board materials and flux.

Regulatory context: SAC alloys are lead-free and comply with RoHS and related environmental regulations, contributing to the global phase-out of tin–lead soldering in electronics.