Kirkendall Voiding

Atomic Migration

The atomic migration that leads to the formation of these cavities is a fundamental consequence of the different diffusion coefficients of the metals involved. In a standard solder joint, the tin reacts with the copper pad to form a layer of intermetallic compound. According to the principles of solid state physics, the copper atoms move into the intermetallic layer faster than the tin atoms move in the opposite direction.

This imbalance creates a net flow of vacancies toward the interface where the copper concentration is highest. As the device operates and remains at an elevated temperature, these individual vacancies begin to cluster together to form larger, observable voids. This kirkendall voiding typically occurs at the boundary between the bulk copper and the cu3sn intermetallic phase.

The rate of this migration is highly dependent on the temperature, the grain structure of the copper and the presence of impurities in the plating. High purity copper with large grains tends to show lower rates of void formation than thin, electrolytically deposited copper with small grains.

Structural Impact

The structural impact of the voids manifests as a dramatic reduction in the mechanical toughness of the solder joint. As the kirkendall voiding progresses, the individual cavities eventually merge into a continuous or semi continuous line of empty space at the interface. This line of voids acts as a pre existing crack, making the joint highly susceptible to brittle failure when subjected to mechanical shock, bending or vibration.

The total area of the metallic bond is reduced, which also leads to a gradual increase in electrical resistance. While this resistance change is often too small to measure in the early stages, it can cause signal integrity issues in high frequency circuits as the voiding becomes extensive. In extreme cases, the voids can cover more than fifty percent of the interface area, leaving the joint held together by a thin and brittle intermetallic layer.

This leads to the sudden and total separation of the solder ball from the pad, an event that can disable the entire electronic device. The structural integrity is further compromised if the voids grow in size during the thermal cycles the device experiences in the field.

Prevention Strategy

The prevention strategy for this defect involves the use of specialized surface finishes and the careful selection of the copper substrate. One of the most effective ways to stop kirkendall voiding is to use a nickel barrier layer between the copper and the solder, as nickel has a much slower diffusion rate with tin. In the electroless nickel immersion gold process, the nickel acts as a shield that prevents the copper atoms from migrating toward the solder.

Another approach is to control the quality of the copper plating, ensuring it is free of organic additives like brighteners that have been shown to accelerate the formation of voids. Manufacturers also optimize the reflow process to minimize the time the joint spends at high temperatures, which limits the initial diffusion. In high reliability applications, samples are subjected to long term thermal aging followed by cross sectional examination to verify that the voiding remains within the acceptable limits.

This proactive testing is essential for products used in aerospace, automotive and medical systems where failure is not an option. The kirkendall voiding remains a primary challenge for the long term reliability of lead free electronics.