Meaning
Physical phenomenon involving the creation of microscopic holes at the interface of two different metals due to unequal diffusion rates. In electronics manufacturing, kirkendall void formation occurs primarily between the copper of a circuit board pad and the tin in the solder. These voids weaken the mechanical strength of the joint over time.
Diffusion Process
Copper atoms move into the tin faster than the tin atoms move into the copper. This imbalance leaves behind vacant spaces that eventually coalesce into kirkendall void formation at the boundary. The process is a natural consequence of the metallurgical bond but is accelerated by high temperatures.
Joint Degradation
A high density of these voids creates a plane of weakness that is susceptible to brittle fracture. While a joint may pass electrical testing initially, kirkendall void formation can lead to sudden failure when the device is dropped or vibrated. The voids reduce the effective area of the bond and concentrate mechanical stress.
Storage Condition
Maintaining assemblies at elevated temperatures for long periods increases the rate of void growth. To mitigate kirkendall void formation, manufacturers often use nickel barriers or specific solder additives that slow down the diffusion of copper. Monitoring the thickness of the intermetallic layer helps in predicting the onset of this defect.
