Meaning
Microscopic zones of intermediate phases formed by chemical reactions between different metals during soldering or welding provide the essential metallurgical bond. The intermetallic layer grows through a diffusion controlled process at the junction where the liquid solder meets the solid substrate pad. Although this boundary layer is necessary to establish a strong mechanical connection, it consists of compounds that are inherently hard and brittle.
Excessive growth of these compounds during subsequent thermal processing or product operation compromises the shock resistance of the joint.
Growth Dynamics
Solid state diffusion continues to drive the expansion of the boundary phases even after the solder solidifies. This process depends on time and temperature, running much faster when the assembly operates in high temperature environments. The continuous migration of metal atoms across the interface thickens the brittle compound zone.
Mechanical Fracture
Thicker boundary layers act as failure sites because their low fracture toughness makes them highly sensitive to mechanical shocks or vibration. When the thickness of this zone exceeds a few micrometers, the joint loses its ability to absorb impact energy through plastic deformation. Crack propagation occurs directly through the interface rather than through the ductile bulk solder.
Surface Treatment
Applying diffusion barriers such as nickel or palladium on the copper pad slows the growth rate of the compound layer. These barrier metals change the chemical reaction paths and reduce the diffusion coefficients of the participating atoms. This treatment preserves the mechanical toughness of the electrical contacts over long service lives.
