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.
Atomic Interdiffusion
Meaning
Solid-state transport phenomena at the contact boundaries of joined materials govern the migration of individual atoms across the interface. In high-temperature manufacturing processes, atomic interdiffusion can lead to the formation of brittle intermetallic compounds that weaken the mechanical strength of joint assemblies. It governs the structural integrity of welded or brazed joints in microelectronics.
The process ceases to occur at a measurable rate when temperatures drop below the activation energy threshold of the constituent elements.
Diffusion Mechanism
Thermal activation drives the movement of atoms through crystal lattices or along grain boundaries. Atoms migrate from regions of high concentration to areas of low concentration to achieve thermodynamic equilibrium. This process depends on the temperature and the available defect density in the material.
Joint Degradation
Mechanical reliability suffers when uncontrolled atom migration creates void networks at the joint interface. These voids, known as Kirkendall porosity, arise from unequal diffusion rates between different elements. Under mechanical stress, these void structures initiate cracks that lead to catastrophic failure.
Thermal Boundary
Diffusion rates drop exponentially as temperature decreases. Engineers utilize diffusion barriers to prevent atom migration during long-term storage or under operational heat cycles. These thin layers of refractory metals or nitrides stop the movement of atoms even under continuous thermal load.