Meaning
The physical height of the copper-rich intermetallic compound phase formed at the interface between copper and tin-based solder determines the mechanical integrity of the solder joint. In electronic manufacturing, controlling the cu3sn layer thickness is necessary to prevent brittle failures under shock and vibration. This phase grows adjacent to the copper substrate, below the tin-rich Cu6Sn5 phase.
It ceases to expand when the copper supply is depleted or when the operational temperature remains well below the diffusion threshold.
Interfacial Growth
Solid-state diffusion drives the continuous expansion of the intermetallic compound layer during thermal aging. Over time, the cu3sn layer thickness increases at the expense of the neighboring Cu6Sn5 phase and the copper pad. This growth is accelerated by elevated operating temperatures and prolonged exposure to solder reflow.
Joint Integrity
Mechanical vulnerability increases when the interfacial layer becomes excessively thick. A high cu3sn layer thickness is associated with the formation of Kirkendall voids at the copper boundary, which act as microcracks. This voiding reduces the shear strength of the electrical interconnection.
Measurement Method
Laboratory technicians employ scanning electron microscopy to analyze the microstructural cross-sections of the joint. They measure the cu3sn layer thickness across multiple points of the interface to obtain a statistically valid average. This measurement assists in predicting the long-term reliability of the assembly.
