Diffusion Layer
The chemical boundary layer formed between the base metal of a pad and the constituents of the solder alloy determines the structural integrity of a joint. Measuring intermetallic compound thickness is a standard part of failure analysis, as a layer that is too thin indicates a cold solder joint with poor adhesion. Conversely, a layer exceeding five micrometers suggests excessive heat or multiple reflow cycles which can lead to embrittlement.
Growth Mechanism
Thermal energy drives the migration of atoms across the interface during the liquid phase of soldering. For common tin-lead or lead-free solders on copper, the intermetallic compound thickness increases steadily over time as the assembly is stored or operated at elevated temperatures. Copper and tin form specific phases like Cu6Sn5 and Cu3Sn that grow at different rates.
Reliability Consequence
Brittle interfaces are prone to fracture under mechanical shock or vibration. High intermetallic compound thickness reduces the ductility of the solder connection. When a joint is subjected to drop testing, the crack typically propagates through the intermetallic region rather than through the bulk solder.
Proper thermal management during the soldering process ensures that the layer remains within acceptable limits. This prevents failure. Maintaining a thin and uniform interface is required for long term reliability in harsh environments.
