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Knocking Down the Bone Pile: Tin Whisker Mitigation in Aerospace Applications, Part 3

Tin whiskers are slender, hair-like metallic growths that can develop on the surface of tin-plated electronic components. Typically measuring a few micrometers in diameter and growing several millimeters in length, they form through an electrochemical process influenced by environmental factors such as temperature variations, mechanical or compressive stress, and the aging of solder alloys. They can sprout from the surfaces of copper electronic device leads, pads, or substrates finished with solder alloys containing tin (Sn). Low-melting-point solder alloys, such as cadmium (Cd), indium (In), zinc (Zn), or antimony (Sb), can also develop whiskers.

Although tin whiskers have gained increased attention in recent years, they are not a recent phenomenon. In the early 20th century, during the vacuum tube era, high-purity tin solder used in early radio equipment led to the formation of these unintended growths, which frequently caused short circuits. This historical context underscores the importance of understanding their formation mechanisms, associated risks, and potential solutions in modern electronics.

Historically, people addressed tin whiskers by adding lead (Pb) to solder alloys used for component leads, pads, and circuit boards with hot air solder leveled (HASL) finishes. Research indicates that even tin-lead (Sn-Pb) surfaces can develop tin whiskers under certain conditions, but they tend to be shorter and less harmful than those on lead-free surfaces. Because of the classification of lead as a hazardous substance and the introduction of strict regulations, this method is now primarily used in specific industries such as the military, aerospace, and medical sectors.

Mechanisms of Growth
Tin whisker growth is influenced by a combination of plating conditions and environmental factors, including thin tin plating, residual stress during the tin plating process, and insufficient intermetallic compound formation. When plating tin onto copper, copper-tin intermetallic compounds—specifically Cu6Sn5 or Cu3Sn—form at the interface between the tin and the base metal. Thin tin plating is more susceptible to whisker growth because of the greater compressive stress it develops compared to thicker plating. This can distort the tin structure, creating the conditions for whiskers to form and grow.

Environmental factors also play a crucial role in whisker development. Temperature fluctuations and high humidity can induce mechanical stress and promote electrochemical reactions that drive whisker formation. In addition, compressive stress from assembly processes, handling, thermal cycling, and mechanical vibrations during operation can exacerbate the issue. Over time, aging components become more susceptible to whiskers, while surface roughness and defects serve as nucleation sites. Electrochemical migration, driven by voltage or moisture, can further facilitate whisker formation. Understanding these factors is essential for minimizing whisker growth and ensuring the reliability of electronic devices.

In contrast to tin pest, which occurs solely in very low temperature environments, tin whiskers can form at ambient temperatures. Tin whiskers do not need to be airborne to cause damage; they typically grow directly in the environment of the electronic device, where they can create short circuits and disrupt normal function. Studies show that tin whiskers, as illustrated in Figure 2, can reach lengths of 0.025 inches (0.635 mm) on 100% bright tin-plated connector leads stored under typical conditions within just four months.

Risks in Electronic Systems
Tin whiskers present several risks to electronic systems, especially in critical applications such as aerospace and military technologies. According to NASA, three primary types of failures in space-based systems are associated with tin whiskers:

Short circuits: Tin whiskers can create internal shorts between terminals, terminals and headers, cases, and other components, or between whiskers themselves.

Metal vapor arcs: If a tin whisker vaporizes, it can generate a highly conductive path capable of carrying a substantial current. This can cause severe damage to the system, leading to component failure or destruction.

Contamination: Loose tin whiskers can move within the device, potentially reaching areas deemed safe from shorts or arcs, causing unintended electrical paths or other issues.

Another major concern in aerospace applications is the impact of tin whiskers in high-speed digital circuits, especially those with frequencies above 6 GHz. In such environments, whiskers can function as miniature antennas, affecting circuit impedance and causing signal reflections. Moreover, in the vacuum of space, high-power components are particularly vulnerable. Tin whiskers can become ionized into plasma, capable of conducting hundreds of amperes of current, amplifying the destructive potential of the short circuits they cause.

Detection Challenges
Given that their median thickness is in the 3–5 micron range (much smaller than a human hair, which is about 75 microns), tin whiskers can be difficult to observe with standard visual inspection methods. To improve the chances of detection, inspectors need to use enhanced lighting techniques, such as right-angle lighting and high magnification. Despite these efforts, the small size and unpredictable growth patterns of tin whiskers mean detection remains a significant challenge in ensuring the reliability of electronic components.

Mitigation Strategies for Tin Whiskers
There are several strategies to mitigate the growth of tin whiskers and their associated risks. A common question is whether conformal coatings can prevent tin whiskers. While there is no known conformal coating that can entirely stop a tin whisker from forming, a properly applied coating can reduce the chances of electrical shorts caused by whisker growth. Among the most effective coatings are Parylene C and silicone, which have been shown to suppress whisker growth. In contrast, acrylic coatings are generally less effective because of their lower hardness. Harder coatings perform better because they require greater force for a whisker to penetrate, making it more difficult for whiskers to spread and create shorts.

The International Electronics Manufacturing Initiative (INEMI) recommends specific board finishes to reduce the risk of whisker formation from the copper surfaces of the circuit board. These include nickel palladium gold (NiPdAu), nickel palladium (NiPd), electroless nickel immersion gold (ENIG), and nickel gold (NiAu). Alternatively, manufacturers can use matte tin finishes, provided they apply a plating thickness of at least 6 microns (µm), as thinner plating is more prone to whisker formation.

There are also several manufacturing techniques to mitigate whisker growth, including modifications to the annealing process, which involve repeated heating and cooling, as well as the addition of elements such as copper and nickel to solder alloys. Historically, lead was used to slow whisker growth in tin-based solders. However, with the implementation of lead-free regulations, alternative approaches have become more prominent.

The GEIA-STD-0005-2—a standard for mitigating the effects of tin whiskers in aerospace and high-performance electronic systems—provides a framework for developing a tin whisker risk management plan. The standard defines three control levels, each offering varying degrees of mitigation:

• Control Level 2A: For low critical applications, where tin whiskers are considered infrequent and acceptable
• Control Level 2B: For non-critical applications, where mitigation protocols are in place, including the avoidance of Pb-free tin finishes
• Control Level 2C: For critical applications with limited system redundancy, and completely prohibits the use of Pb-free tin finishes

For most military and aerospace applications, Control Levels 2B and 2C offer the most comprehensive mitigation strategies to manage tin whisker risks. A more effective and reliable solution, particularly for aerospace applications, is robotic hot solder dip (RHSD) processing. This involves removing 100% of the pure tin plating from component leads and terminations, replacing it with a tin-lead alloy, eliminating the risk of tin whisker formation. Robotic hot solder dip processing can be applied to a wide range of components, including through-hole and surface-mount devices, axial and radial components, connectors, and discrete electronic devices.

The RHSD process has several advantages over manual dipping methods for electronic components, particularly in reducing the effects of tin whisker development. It offers greater precision and control over solder applications, ensuring a uniform coating that minimizes imperfections conducive to whisker growth. It also allows for improved temperature management during soldering, resulting in a consistent intermetallic layer that mitigates conditions favorable to tin whiskers. By automating the process, human error is significantly reduced, leading to enhanced quality assurance and lower contamination risks. Furthermore, the RHSD process enables higher throughput while maintaining quality across a vast array of components, making it an optimal choice for applications requiring reliable performance.

Conclusion
The mitigation of tin whiskers is crucial for ensuring the reliability and longevity of electronic assemblies, especially in critical applications like aerospace. While traditional manual dipping methods may appear cost-effective, they often introduce challenges not present when performed robotically. For optimal results, manufacturers should utilize an RHSD process because it effectively mitigates tin whiskers, minimizes contamination risk, and ensures consistently high-quality intermetallic bond formation.

By employing these advanced solutions, adhering to industry standards, and using the proper equipment, aerospace manufacturers can significantly enhance the solderability and reliability of their components. This leads to more dependable electronic devices and assemblies. As technology develops, continued advancements in mitigation techniques are crucial in addressing the complexities of tin whisker growth.

Future columns will delve further into the robotic hot solder dip process, providing insights and best practices to enhance the quality and performance of electronics assembly in high-reliability sectors.

This column originally appeared in the June 2025 issue of SMT007 Magazine.