Phase Structure
Solid state diffusion forms an inner epsilon phase intermetallic compound at the interface between copper substrate metallization and bulk tin solder. Formation of the cu3sn sub-layer occurs adjacent to the copper base metal beneath the thicker eta phase cu6sn5 layer. High temperature exposure accelerates atomic transport between copper and tin atoms.
Microstructural examination reveals a dense crystal lattice that alters electrical conductivity and mechanical compliance at the pad boundary.
Interfacial Degradation
Prolonged thermal exposure causes progressive growth of the inner intermetallic film at the expense of adjacent solder material. Accelerated copper migration through the intermetallic compound creates micro-cavities along the copper interface through Kirkendall voiding. These sub-micron voids reduce mechanical shear strength across the solder joint.
Cyclic thermal stress concentrates along the brittle intermetallic interface, promoting micro-crack initiation under mechanical shock. Extended aging at temperatures above one hundred degrees Celsius increases void density near the copper boundary. Brittle fracture paths propagate preferentially through this degraded interfacial region during drop impact testing.
Microscopic void coalescence eventually causes complete mechanical separation of the solder interconnect under low impact energy levels.
Thickness Limit
Industry acceptance standards evaluate intermetallic layer thickness via cross-sectional metallography and scanning electron microscopy. Thickness values exceeding two micrometers indicate excessive thermal degradation and heightened risk of mechanical joint separation.
