Kirkendall Void Density

Vacancy Accumulation

Formation of the voids is a time and temperature dependent process that begins at the atomic level during the solid state aging of the interconnect. The Kirkendall void density increases as more copper atoms migrate toward the solder side of the interface, leaving behind unoccupied lattice sites. These individual vacancies move through the crystal structure and tend to group together at high energy sites, such as grain boundaries or the interface between the Cu3Sn and copper layers.

The presence of impurities in the copper or the plating solution can also accelerate this process by providing more sites for vacancy nucleation. Unlike other types of solder defects, these voids are not the result of poor wetting or gas entrapment during reflow. Instead, they are an inherent consequence of the metallurgical properties of the copper-tin system under thermal stress.

Controlling the quality of the copper substrate and using specialized solder alloys can help to slow down the accumulation of these vacancies.

Stress Concentration

Mechanical impact on the solder joint is greatly magnified when a high number of microscopic holes are present at the interface. The Kirkendall void density directly affects the effective area that supports the load, and each individual void acts as a stress concentrator that can initiate a crack. Under conditions of vibration or mechanical shock, the stress is focused on the thin bridges of metal between the voids, leading to a rapid failure of the bond.

This is particularly dangerous because the failure occurs at the interface between the intermetallic layer and the copper, which is the most brittle part of the joint. A high density of voids can lead to a complete separation of the solder ball from the pad with very little force. This failure mode is often called “brittle interfacial fracture” and is one of the most common causes of field returns for aged electronic products.

Engineers use cross sectional imaging to count the number of voids per unit length to assess the risk of such failures.

Failure Boundary

Measurement of the total void area relative to the interface length allows quality teams to define a safety limit for the integrity of the assembly. A Kirkendall void density that exceeds twenty or thirty percent of the interface length is generally considered a high risk condition that warrants a redesign or a change in materials. Some manufacturers use nickel plating as a barrier layer to prevent the direct contact between copper and tin, which effectively eliminates the formation of these voids.

However, this adds cost and complexity to the manufacturing process, so it is only used where the reliability requirements justify the expense. In applications where copper must be used directly, selecting high purity copper and optimizing the reflow profile can help to minimize the initial vacancy count. Regular monitoring of this parameter during accelerated aging tests is essential for any company producing hardware that must survive for many years in harsh environments.

The final analysis of these voids provides the evidence needed to make informed decisions about material selection and process windows.