Cu3Sn

Growth Mechanism

Solid-state diffusion drives the continuous expansion of intermetallic layers after the initial solder solidification. Within the joint assembly, the formation of cu3sn is governed by the migration of copper atoms into the tin-rich matrix and the simultaneous counter-migration of tin. This process accelerates when assemblies are exposed to elevated operating temperatures or prolonged baking cycles.

Over time, the growth consumes the available copper, which can deplete the thin copper traces or plating on the printed circuit board. Submicron voids, called Kirkendall voids, often nucleate at the copper interface during this diffusion process, creating weak points that are highly susceptible to mechanical shock. These tiny cavities coalesce as the thermal stress continues, further detaching the solder from the trace.

Mechanical Reliability

Brittle interfaces present a distinct risk under mechanical stress like drops or vibration. The presence of cu3sn introduces a localized region of high hardness and low ductility compared to the surrounding bulk solder. When the joint experiences mechanical tension, cracks tend to propagate along this intermetallic boundary.

Severe thermal shock or physical impacts can cause sudden, catastrophic fractures that detach the solder ball from the circuit board pad.