Electrodeposited Copper Recrystallization Mechanics in High Density Circuit Stackups
Thermal cycles in high density stackups drive copper recrystallization, causing grain growth and vacancy coalescence that trigger microvia interface failures.
Copper Self-Diffusion
Kinetic Mechanism
Atomic transport process where copper ions migrate through their own crystal lattice via vacancies or along grain boundaries. Copper self-diffusion dictates the rate of structural changes such as grain growth and void formation during the thermal processing of printed circuit boards. The movement occurs without the influence of an external chemical gradient.
High temperatures provide the activation energy required for atoms to jump from their current positions into adjacent lattice sites.
Diffusion Rate
The speed of this atomic movement depends heavily on the temperature and the density of defects within the copper deposit. At low temperatures, the majority of copper self-diffusion takes place along grain boundaries because the activation energy for boundary diffusion is lower than for bulk diffusion. As the board undergoes reflow or high temperature storage, the atoms redistribute to minimize the internal energy of the metal.
This movement can lead to the formation of kirkendall voids if the flux of atoms is unbalanced at an interface with another metal like nickel or tin. Such voids weaken the mechanical integrity of the solder joint.
Surface Treatment
Organic solderability preservatives or immersion silver layers influence the surface component of this diffusion. Atoms at the surface move more freely than those in the bulk.