Reflow Profiling Optimization and Intermetallic Growth Controls in SAC305 Assembly
Optimize SAC305 reflow profiles between 235-248°C with 45-75s TAL and 3-5°C/s cooling to limit interfacial intermetallic growth and prevent brittle joint failure.
Ag3Sn
Phase Formation
Crystalline intermetallic structures form within lead-free solder joints during the cooling phase of the reflow process. The compound ag3sn crystallizes when tin-lead alternatives such as tin-silver-copper alloys undergo solidification. These precipitates appear as dispersed sub-micron particles or as larger needle-like platelets depending on the cooling rate and silver concentration.
Fast cooling minimizes the size of these structures, whereas slow cooling permits unrestricted growth. This difference determines the primary microstructure of the joint, which dictates the long-term reliability of the electrical connection under operating stress.
Mechanical Behavior
Small, evenly distributed particles of the compound increase the shear strength and creep resistance of the solder matrix by pinning grain boundaries. When the concentration of silver exceeds three percent, ag3sn can form long, brittle plates that span the solder gap. These large platelets act as stress concentrators and create easy paths for crack propagation under mechanical shock.
Board designers and assembly engineers typically limit the silver content to avoid this embrittlement.
Joint Degradation
Thermal cycling accelerates the migration of silver atoms toward the interface, where they can coalesce and weaken the connection. The resulting coarsening of ag3sn reduces the fatigue life of the joint under operating conditions. This structural shift leads to failures where vibration is present.