When it comes to military and aerospace applications, electronics are expected to operate first-time, every time. This means that materials are expected to be reliable under all conditions with no excuses. While most problems under this scope are straightforward, like dealing with extreme pressures and avoiding contaminants, a unique challenge in aerospace and military electronics is the growth of tin whiskers.
Tin whiskers are tiny, hair-like crystalline structures of tin that can spontaneously grow from surfaces where tin is used as a finish, especially bright or electroplated tin. Although the phenomenon was first reported in the 1940s and 1950s, tin whiskers remain a persistent and critical reliability concern in high-reliability sectors. These whiskers are electrically conductive and can grow to lengths exceeding 10 millimeters, capable of bridging adjacent circuit elements and ultimately causing short circuits and catastrophic device failures.
What are Tin Whiskers and How are they Formed
Tin whiskers form due to compressive stress within the tin plating layer of electronic assemblies. These stresses can arise from: intermetallic formation beneath tin layer, thermal expansion mismatches, or ever stress from corrosion. The exact mechanisms triggering tin whisker growth are still unknown, but it is found that whiskers can grow spontaneously without any external stimuli over time.
What we know for sure is that whisker formation involves atoms migrating to relieve stress, resulting in the extrusion of crystalline filaments from the tin surface. These tiny whiskers are very small, typically 3-5 microns in diameter. This especially makes them difficult to detect by simple visual inspection, making them (ironically) a bigger challenge for engineers.
Why are Tin Whiskers Especially Harmful in Aerospace and Military Electronics?
It is no question that military and aerospace sectors have stringent reliability demands. Tin whiskers pose a severe risk because:
- They can cause short circuits by bridging fine-pitch leads or adjacent conductors in high-density electronic assemblies.
- In the vacuum of space, vaporized tin whiskers can create plasma arcs, which are highly destructive and can lead to catastrophic component failure.
- Whiskers can also act as miniature antennas in high-frequency circuits above 6 GHz, disturbing signal integrity and causing impedance mismatch.
The transition to lead-free soldering materials and finishes in compliance with environmental regulations has worsened the problem, as lead traditionally suppressed tin whisker growth. Learn more about the shift to lead-free solders in our previous blog, “Tin-Based SAC Solders as the Standard for Lead-Free Electronics.” Military electronics, while often exempt from these bans, face supply chain risks due to limited availability of leaded components.
Given these severe risks, the true impact of tin whiskers are not just theoretical. History has shown us a proven track record of inconveniences these tin whiskers have delivered. For example, in the early 2000s, a relay used in military hardware, for Northrop Grumman, despite being MIL-spec, was found to have an internal finish of pure tin instead of the required tin-lead plating. This pure tin plating developed whiskers that caused internal shorts inside the relay, which led to relay failures. Around 2005 at Millstone Unit 3, a tin whisker short caused a false low-pressure reading, which led to an automatic reactor shutdown, a safety-critical event.
With that, there’s no question how much inconvenience tin whiskers bring. How do we make sure that we don’t encounter this problem?
Mitigating Tin Whisker Growth
Tin whisker growth is usually linked to the manufacturing process. Ultimately, to completely stop the growth, the intuitive answer would be to completely eliminate pure tin from electronics manufacturing, which can be quite impossible to implement.
Another suggestion would be to use components with no bent leads, such as thin dual flat no-lead (TDFN), thin quad flat no-lead (TQFN), and quad flat no-lead (QFN) packages, since they are less likely to promote whisker growth.
Another way to prevent whisker growth is by eliminating the generation and relaxation of stresses. Stress generation can be controlled by blocking the diffusion of copper into tin through the use of diffusion barrier, like nickel. However, there is currently no known method of eliminate stress relaxation. Nonetheless, there is still no confirmed method that prevents the growth of tin whiskers entirely. This is why the next line of defense would be to mitigate it.
The Role of Conformal Coatings
Conformal coatings DO NOT stop the growth of tin whiskers, instead, they serve as a universally recognized as an effective risk mitigation strategy to reduce the electrical shorts and damage caused by these whiskers.
They act as a physical barrier that prevents whiskers from growing outward and bridging the gap between adjacent conductors. Even if whiskers form beneath the coating, the layer reduces the likelihood that they will extend into open air and create a short circuit.
In addition to blocking whisker growth, conformal coatings also help control the risk from broken whiskers. When whiskers snap off due to stress or vibration, there is a possibility that they may fall onto uncoated areas and cause unintended electrical connections. The coating prevents this by trapping loose whiskers, stopping them from bridging across conductors.
Finally, conformal coatings function as an insulating medium against conductive debris. The coating’s dielectric properties ensure that even if such debris comes into contact with the board surface, it cannot easily form an electrical path.
There are two major factors that affect the performance of conformal coatings on tin-whisker mitigation:
- Coating thickness: Thin coatings are less effective due to higher compressive stresses and limited ability to resist whisker protrusion. In contrast, thicker coatings tend to trap whiskers and force them to bend or buckle (a phenomenon known as Euler buckling)
- Material Selection: Each conformal coating chemistry has its own set of performance tradeoffs. Some coatings are more rigid than others, while others are more flexible. It also matters how these coatings are applied, depending on the application.
Best Conformal Coating Chemistries for Tin Whisker Mitigation
The most commonly used conformal chemistries are generally polymer-based such as acrylic, urethane, UV-curable (acrylates) , silicone, and parylene conformal coatings. As with all applications, selecting the right conformal coating that best fits the specific performance requirements and environmental conditions of the device is important to ensure long-term reliability and protection against failure mechanisms like tin whisker growth. Among these options, urethane and acrylic coatings stand out as the most effective and practical choices for tin whisker mitigation.
Urethane Conformal Coatings
Urethane Coatings are highly regarded for their durability and chemical resistance. They form a hard, resilient barrier that limits tin whisker growth and penetration. Extensive research by NASA shows that urethane is capable of reducing tin-whisker protrusion when applied at proper thicknesses even in long-term applications.
Acrylic Conformal Coatings
Acrylic Coatings are popular due to their ease of application and removal. They offer good moisture resistance and dry quickly– best for large scale manufacturing processes and parts. While they might not be as robust as urethanes, they offer the best reworkability for applications that require coating removals.
HumiSeal Conformal Coatings: A Trusted Solution for Tin Whisker Risk Reduction
For military and aerospace electronics manufacturers seeking reliable, cost-effective conformal coating solutions, HumiSeal conformal coatings provide proven performance. The HumiSeal conformal coatings are an industry favorite, offering excellent environmental protection, fast drying, and compatibility with a variety of PCB assemblies.
| Acrylics | Urethanes | ||||||||
| Property | 1B31 | 1B31 LOC | 1B73 | 1B73 LOC | 1A20 | 1A20R | 1A33 | 1A34 | 2A64 |
| MIL-I-46058C | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes |
| IPC CC-830B | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes |
| Continuous Use Range (°C) | -65 to 125 | -65 to 125 | -65 to 125 | -65 to 125 | -65 to 125 | -65 to 125 | -65 to 125 | -65 to 125 | -65 to 125 |
| Thermal Shock (°C) | -65 to 125 | -65 to 125 | -65 to 125 | -65 to 125 | -65 to 125 | -65 to 125 | -65 to 125 | -65 to 125 | -65 to 125 |
| Tg (°C) | 14 | 14 | 42 | 42 | 71 | 71 | 18 | 12 | 12 |
| CTE (ppm/°C) | 340 | 340 | 193 | 193 | 338 | 338 | 532 | 225 | 225 |
| Dielectric Constant (1 MHz) | 2.5 | 2.5 | 2.5 | 2.5 | 3.5 | 3.5 | 3.5 | 3.5 | 3.5 |
| Insulation Resistance (Ω) | >1×10¹⁴ | >1×10¹⁴ | 5.5×10¹⁴ | 5.5×10¹⁴ | 4.8×10¹⁰ | 4.8×10¹⁰ | 1.6×10¹⁰ | 6.3×10¹⁰ | 4.8×10¹⁰ |
| Resistance to Chemicals | Poor | Poor | Poor | Poor | Excellent | Excellent | Very Good | Very Good | Excellent |
In aerospace and military electronics, the cost of failure is simply too high. Conformal coatings are not just a protective layer. They are a mission-critical safeguard against extremes with service lives that often exceed 20 years. Accompanied other assembly materials like adhesives, solders, underfills, epoxy molding compounds, wire bonds, die attach, and thermal interface materials (TIMs), each material plays a calculated role in making sure that aerospace and military grade devices run for the first time every time.
At CAPLINQ, we help engineers and procurement teams specify the exact coating that fits their mission profile: balancing protection, reworkability, and compliance. Talk to us to learn more today.
