Tin Whisker Mitigation with Conformal Coatings: Big Hazards Come in Small Packages

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March 12, 2021

As a high-reliability electronics design engineer, you likely consider the big things that can jeopardize your application’s reliability. Something as small as a hair-like protrusion can cause irreparable damage to mission-critical devices. Tin whiskers are electrically conductive, crystalline tin structures that have caused a nuclear power plant shutdown, guided-missile failures, and pacemaker malfunctions.

While tin whiskers have been studied for years, design engineers have not always accounted for them. In 1998, commercial satellites started to experience partial to complete failures, which were later attributed to tin whisker-induced short circuits, and soon thereafter prompted an investigation into tin whisker mitigation strategies. For mission-critical application design teams, the topic has never been more relevant.

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What are Tin Whiskers and Why Are They a Problem?

Since tin whiskers are made of tin, they are conductive and can cause short circuits by bridging closely-spaced circuit elements. Tin whisker-related field failures have caused significant customer dissatisfaction and have cost individual programs many millions of dollars.

The mechanisms by which tin whiskers form are currently unknown, but scientists and engineers have been growing their understanding over decades of sustained research and development. So far, there is no proven way to completely prevent them from forming, but with electronic industry shifts and global regulations barring the use of lead, finding tin whisker mitigation methods is more important than ever.

Why is Tin Whisker Mitigation More Important Now?

The global initiative to reduce potentially hazardous materials like lead (Pb) has prompted regulations like RoHS (Restriction of Hazardous Substances) and WEEE (Waste Electrical and Electronic Equipment). These new restrictions strongly encourage major suppliers to convert their product lines from tin-lead to lead-free alloys. The predominant choice for a lead-free solder alloy and component lead finish is predominantly composed of tin. Although there is no industry-accepted explanation as to why pure tin has a propensity to form tin whiskers, many believe that lead alloyed with tin inhibits tin whisker growth.

Pure tin plating is a risky strategy for high-reliability systems, but unfortunately, the commercial market for sustainable components far outweighs mission-critical application needs. The only dependable prevention method to entirely eliminate tin whiskers is to eliminate pure tin from a system, but this proves difficult to implement with the increasing use of COTS components in high-reliability applications. For these reasons, high-reliability electronics designers must incorporate low-cost, effective mitigation strategies to manage tin whisker growth.

View Our Guide to Conformal Coating Defects & Their Solutions

a tin whisker

What are Tin Whiskers and Why Are They a Problem?

The application of conformal coatings is one of the few processes that OEMs can fully control, unlike other functions, such as the actual tin plating process. There have been several well-known instances where a procurement specification required “No Pure Tin,” yet the product that was supplied was later determined to be pure tin. Alarmingly, in some of these instances, tin whisker growths were also discovered.

After companies experienced commercial satellite failures, NASA and many aerospace-defense companies launched a study that examined conformal coatings as a mitigation strategy. It was postulated that stiff, hard coatings might be able to suppress whisker formation mechanically. If whiskers did grow, the coatings might trap them.

Parylene C was a strong candidate due to its ability to apply the most considerable mechanical resistance. It has the highest modulus, the highest tensile strength, and is relatively hard.

a table explaining the physical properties of conformal coatings

Evaluation of Conformal Coatings as a Tin Whisker Mitigation Strategy, The Boeing Company

After the study, representatives from Boeing concluded that “Parylene C suppressed odd-shaped eruption (OSE) and whisker formation the longest.”

Use HZO Coatings for Tin Whisker Mitigation

Although Boeing identified Parylene C as one of the most effective ways to mitigate tin whiskers, at HZO, we wanted to put our own coatings to the test. We reached out to the Center for Advanced Life Cycle Engineering (CALCE), a research facility that focuses on risk management, assessment, and mitigation for electronic systems. CALCE subjected our coatings on tin-plated brass and copper coupons that were exposed to 500 hours of 85 °C and 85% relative humidity (RH) to attempt to encourage tin whisker growth.

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There is no proven way to completely prevent tin whiskers from forming, making tin whisker mitigation methods more important than ever.

The Results Came in

2 examples of tin whiskers

On one copper coupon, a potential whisker was observed to be trapped under the coating. 

a close up of a tin whisker

No changes were observed to two HZO coated coupons (one brass and one copper) after 500 hrs at 85°C/85%RH. A scanning electron microscope image of the coating on the copper coupon shows an approximate thickness of 10 micrometers. 

CALCE concluded that “HZO finish has been found to be effective at preventing or containing tin whisker formation.”

The Big Picture

Of course, mitigation is not elimination, but an effective strategy that includes the use of conformal coatings drastically reduces the risk of whisker-induced reliability threats. And relative to coating thickness, HZO’s Parylene has proven superior at suppressing tin whisker growth and reducing the risk of tin whiskers that may be present by providing an electrically insulating layer over conductive surfaces and makes it more difficult for tin whiskers to form a short circuit. For more information on Parylene, tin whisker mitigation, or our coatings’ exceptional corrosion resistance, reach out today.

CALCE concluded that “HZO finish has been found to be effective at preventing or containing tin whisker formation.”

Ryan Moore

Ryan is a 9-year veteran to the world of protecting electronics from harsh environments and a lover of all things technology.

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