Kirkendall Void Formation Mechanisms at Lead Free Copper Interface Boundaries
Suppressing Kirkendall voiding demands controlling substrate copper electroplating additives and microalloying solders to balance diffusion rates.
Interface Fracture
Meaning
Structural separation along the boundary between two dissimilar materials occurs when the cohesive force of the bond is exceeded by applied stress. An interface fracture represents a common failure mechanism in composite materials and electronic packages. This type of failure results in the detachment of thin films or solder joints from their substrates.
It limits the mechanical strength and thermal reliability of layered systems.
Mechanical Behavior
Stress concentrations at the boundary of mismatched materials cause crack initiation and propagation. An interface fracture propagates when the strain energy release rate exceeds the critical interfacial toughness. This behavior depends on the difference in elasticity, thermal expansion, and Poisson ratio between the two layers.
Testing Methodology
Measuring the resistance of a bonded joint to separation involves specific double cantilever beam or peel tests. Evaluating the risk of interface fracture requires applying a controlled tensile or shear load to the sample until failure occurs. Technicians analyze the exposed surface with electron microscopes to determine if the failure was adhesive or cohesive.
This categorization determines whether the adhesive recipe or the surface treatment of the substrate needs modification.
Prevention Strategy
Improving the surface energy of the substrate before bonding helps prevent premature joint separation. Modifying the deposition process to reduce interface fracture can involve chemical etching, sandblasting, or plasma activation. These treatments increase mechanical interlocking at the boundary.