Intermetallic Phase
Intermetallic compounds form at the boundary between copper substrates and tin based solders during electronic assembly. The compound cu3sn represents the epsilon phase of the copper tin binary system and is located adjacent to the copper substrate in solder joints. This thin, hard layer grows during thermal aging and sits beneath the more tin rich eta phase.
Its presence influences both the electrical conductivity and the long term mechanical stability of the subassembly connection.
Growth Mechanism
Solid state diffusion drives the continuous evolution of solder joint microstructures at elevated operating temperatures. In this process, cu3sn develops as copper atoms migrate into the adjacent tin rich phase while tin atoms diffuse in the opposite direction. Elevated temperatures accelerate this atomic transport, leading to a steady increase in the thickness of the epsilon phase layer over time.
The rate of this growth is generally governed by parabolic diffusion kinetics.
Mechanical Vulnerability
Structural vulnerabilities in subassembly joints often arise from the presence of brittle intermetallic layers. Because cu3sn has high hardness and low ductility, the compound is susceptible to cleavage when subjected to shock or vibration. Furthermore, the migration of copper atoms can leave behind microscopic voids known as Kirkendall voids along the boundary of the cu3sn layer.
These microvoids reduce the effective load bearing area and act as stress concentration points that initiate cracks.
Joint Lifespan
Reliability standards for microelectronic connections specify limits on intermetallic thickness to prevent premature failure. Solder joints that exhibit excessive cu3sn growth are prone to sudden fracture during drop tests or thermal cycling. Industry practices dictate that the combined thickness of all copper tin intermetallics should remain below a few micrometers to ensure the physical integrity of the interconnection.
