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.
Vacancy Coalescence
Structural Failure
Aggregation of atomic scale vacancies into larger, detectable voids within the crystal lattice of a metal. Vacancy coalescence occurs in copper traces and solder joints when a high concentration of lattice defects migrate and group together under the influence of heat or electrical current. These voids act as precursors to larger cracks and can eventually lead to the complete failure of the electrical connection.
The process is driven by the reduction of total surface energy as many small voids combine into a single larger one.
Void Formation
The presence of these internal cavities reduces the effective cross sectional area of a conductor and increases the local current density. This acceleration of electromigration can create a feedback loop where more vacancies are generated and then join the existing voids. Over time, the coalescence of these defects at grain boundaries or interfaces creates a path for crack propagation.
In solder joints, this phenomenon is often observed at the interface between the bulk solder and the intermetallic compound layer. The resulting voids weaken the mechanical strength of the joint and make it susceptible to failure under vibration or thermal cycling.
Failure Detection
Scanning electron microscopy allows for the visualization of these voids in cross sectioned samples. An engineer identifies vacancy coalescence by the presence of small spherical or faceted pits along the grain boundaries.