During failure analysis, Kirkendall voids and Champagne voids can appear similar because both are microscopic void defects. However, they differ fundamentally in formation mechanism, location, morphology, timing, and controlling factors. Kirkendall voids are associated with unequal atomic diffusion across a metal interface, whereas Champagne voids are generally caused by trapped gases or volatile residues during coating and soldering processes.
The primary differences are summarized below.
| Characteristic | Kirkendall Voids | Champagne Voids |
|---|---|---|
| Formation mechanism | Caused by different interdiffusion rates of atoms; this is a thermodynamic process. | Caused by flux residues, process gases, moisture, or other volatile substances becoming trapped during heating; this is a physical entrapment process. |
| Primary cause | Different diffusion coefficients of metals on both sides of an interface, such as Cu and Sn, lead to vacancy accumulation. | Gases, flux volatiles, moisture, or contaminants cannot escape from the coating or solder interface during soldering or reflow. |
| Typical location | Inside or near the interface of an intermetallic compound (IMC) layer. | At the interface between a solder ball and the PCB pad, typically above the IMC layer. |
| Void morphology | Usually small and numerous, with a relatively uniform distribution. The structure may appear porous or sponge-like. | Variable in size, usually round or nearly round, with a random distribution. Voids may cluster in specific regions. |
| Time of occurrence | Gradually forms and grows during long-term high-temperature aging or product operation. | Already exists when the soldering process ends and is generated during manufacturing. |
| Key influencing factors | Temperature, which accelerates diffusion, and aging time. | Flux chemistry, the soldering thermal profile, heating and cooling rates, solder paste volume, and the cleanliness of the PCB and component surfaces. |
Definitions
Kirkendall Voids
Kirkendall voids are voids generated between two dissimilar materials because their atoms diffuse at different rates. They commonly occur at metal interfaces, particularly in dissimilar-metal solder joints or alloy interfaces. In electronics packaging, the phenomenon is often associated with the growth of Cu-Sn intermetallic compounds and the resulting accumulation of vacancies.
Champagne Voids
Champagne voids, also called planar microvoids, are interface microvoids with a diameter of less than 50 μm. They typically occur above the IMC layer between a PCB copper pad and the solder. The term “Champagne void” refers to the bubble-like appearance of the defects when the interface is observed after a BGA solder joint is pulled apart.
Formation Mechanisms
How Kirkendall Voids Form
Kirkendall void formation begins when metal atoms diffuse across the interface between two different metals or alloys. When dissimilar metals are joined, their atoms generally do not migrate at the same rate. This unequal atomic flux causes an imbalance in material transport across the interface.
As one species diffuses more rapidly than the other, vacancies can accumulate in the region where the material supply is not balanced. These vacancies may combine and form microscopic voids. The resulting defect is known as a Kirkendall void. The process is particularly significant at elevated temperature, where atomic diffusion is accelerated, and at interfaces where intermetallic compounds are growing.
One example involves a BGA solder joint using a Cu substrate and a Sn-37Pb solder alloy. After solid-state aging at 125 °C for 20 days, the different diffusion rates of Sn and Cu during the formation of the Cu3Sn phase can cause an imbalance in material transport. This imbalance promotes Kirkendall void formation at the relevant interface.
How Champagne Voids Form
Champagne voids are associated with gases or volatile residues that become trapped during coating deposition, soldering, or reflow. During processes such as electroplating, bubbles may result from the plating solution, impurities, or excessive coating thickness. If the gas is not removed or dissolved completely, it can become trapped as the coating continues to deposit.
In soldering, flux residues, moisture, organic contaminants, or other volatile substances may decompose or vaporize as the assembly is heated. If the resulting gas cannot escape before the solder solidifies, it can remain at the solder-to-pad interface and form a planar microvoid. This is a physical entrapment process, not a defect caused by unequal atomic diffusion.
For example, an immersion silver coating may contain approximately 30% organic impurities. When the coating is very thin, such as 0.2 μm, the silver may dissolve into the solder within a fraction of a second during assembly, leaving little organic residue in the solder joint. If the coating is excessively thick, however, it may not dissolve completely. Organic material retained in the silver coating can decompose during reflow and release gas, producing a dense population of interface voids. Their appearance resembles bubbles in Champagne, which is the origin of the term.
Location and Appearance
Kirkendall Voids
Kirkendall voids generally occur in the interfacial region between dissimilar metals, especially in dissimilar-metal solder joints or alloy interfaces. They may be located within an IMC layer or near the boundary between the IMC and the adjacent metal.
Their size and shape are often irregular. They may appear as small, dispersed voids or as local metal separation and cracking along the interface. Unlike Champagne voids, they normally do not produce an obvious color change or a characteristic reflective appearance.
Champagne Voids
Champagne voids are usually found above the IMC layer at the solder-to-pad interface. They are often circular or nearly circular because they originate from trapped gas pockets. Their distribution may be random, although multiple voids can become concentrated in a particular region of the interface.
When exposed to light, these voids can appear as bright white spots or reflective areas that resemble bubbles in Champagne. This visual characteristic can help distinguish them from the irregular, diffusion-related morphology of Kirkendall voids, although cross-sectional analysis is normally required for a reliable determination.
Reliability Impact
Impact of Kirkendall Voids
Kirkendall voids primarily affect the reliability of metallic joints. They reduce the effective load-bearing area and may weaken the interface, leading to lower joint strength, cracking, or electrical contact failure. In precision solder joints and electronic packages, they can degrade both mechanical and electrical performance.
In one BGA example using SAC solder balls on a Cu substrate, samples were isothermally aged at 100 °C, 125 °C, 150 °C, and 175 °C for 3, 10, 20, and 40 days. Drop and shear tests were then performed. Kirkendall voids were observed at the Cu/Cu3Sn interface. After aging at 125 °C for 3 days, the voids occupied approximately 25% of the solder-pad interface. The voided area increased with both aging time and temperature. After 10 days of aging at 125 °C, drop performance was reported to be 80% lower than that of the unaged samples.
Impact of Champagne Voids
Champagne voids can form a large, planar “air pocket” along the soldering plane. During thermal cycling, mechanical shock, and drop testing, this planar defect can accelerate crack initiation and propagation through the solder joint. As a result, it can significantly reduce joint strength and long-term reliability.
The risk becomes more serious as the package pitch decreases. In fine-pitch BGA packages, a Champagne void can occupy a meaningful portion of the available bonding interface and increase the probability of interfacial fracture. Even when an assembly passes initial production testing, the defect may reduce its performance under thermal and mechanical stress during service.
Experimental results reported in an Intel Arrigotti study indicate that, during rapid thermal cycling, the rate of fracture growth increases as the number of Champagne voids increases.
Improvement Measures
Reducing Kirkendall Voids
- Material selection and interface design: A nickel barrier layer can be introduced on the Cu pad, such as the Ni layer in an ENIG surface finish. The phosphorus content of the electroless nickel layer must be controlled appropriately. A suitable barrier layer can slow interdiffusion between Cu and Sn.
- Optimize the soldering process: Control temperature, process time, and mechanical pressure to avoid excessive thermal stress. Appropriate combinations of base metals and solder materials can also reduce excessive interdiffusion at the joint interface.
- Control alloy composition: Metals with more similar diffusion characteristics can help reduce unstable atomic transport across the soldering interface. Material compatibility should therefore be considered together with the soldering process.
Reducing Champagne Voids
- Optimize the plating process: Control the plating current density, temperature, and plating-bath composition. These factors influence gas release, bubble formation, and the likelihood of gas entrapment in the coating.
- Select suitable materials: Low-void solder paste can reduce trapped-gas defects. The thickness of an immersion silver coating should be controlled, and surface finishes with better solder wettability should be considered.
- Maintain surface cleanliness: The PCB pad and component surfaces should be free of oil, contamination, moisture, and other residues that could impair coating adhesion or release gas during reflow.
- Control the thermal profile: The heating and cooling rates, peak temperature, and time above liquidus should be managed so that volatiles have an opportunity to escape before solder solidification.
Conclusion
Kirkendall voids and Champagne voids are both interfacial void defects, but their mechanisms and reliability consequences are different. Kirkendall voids result from unequal atomic diffusion between dissimilar metals and are typically associated with IMC growth, vacancy accumulation, and long-term thermal aging. They mainly reduce the strength and reliability of the metallic connection over time.
Champagne voids are caused by trapped gases, flux residues, moisture, organic contaminants, or plating-related process conditions. They are generated during manufacturing and often appear as circular, reflective microvoids at the solder-pad interface. Although an assembly may pass initial production testing, these planar voids can substantially reduce reliability during thermal cycling, mechanical shock, and drop testing.
Failure analysis should therefore consider defect location, morphology, formation time, and material interfaces rather than classifying every solder void in the same way. The appropriate corrective action depends on the mechanism: diffusion barriers and material compatibility are central to controlling Kirkendall voids, while plating control, surface cleanliness, solder-paste selection, and thermal-process optimization are central to reducing Champagne voids.
