Manual Factors Governing Tin Whisker Growth

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Just as the name implies, conformal coating is a coating with an inert material that can protect electronic circuit boards from the problems related to tin whisker growth: shorts, plasma arc, and debris. Four points should be considered while using conformal coating as a tin whisker mitigation technique:. We acknowledge that tin whiskers cannot be stopped until we understand how they form in the first place. No conformal coating meets all these criteria.

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However, the Arathane coating seems promising when applied sufficiently thick, and the conformal coating does prevent shorts from debris. Thermal effects need to be considered if a conformal coating is used on parts that will need to dissipate heat when operating. If necessary, the device may need to be derated. If the conformal coating fails to contain whisker formation, the effectiveness of a conformal coat in providing protection against electrical leakage and corrosion will be compromised. A puncture site may provide an increased opportunity for excessive leakage currents that can produce transient or permanent failures.

Another concern related to whisker formation is the potential for whiskers to produce minor de-lamination of the conformal coating from the circuit board. The resulting capillary space could provide a void for condensation of the water vapor molecules that slowly diffuse through the coating material. This creates the slim possibility of galvanic corrosion, though it may not be of consequence.

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Further, emerged whiskers that break loose may end up in other areas of your hardware that are vulnerable to conductive debris. However, this risk is substantially reduced compared to the scenario of not using a conformal coat. The formation of intermetallics in the base metal below the pure tin plating may create stresses that promote the growth of tin whiskers.

Based on this assumption, a barrier layer such as nickel between the base metal and the pure tin finish may reduce the likelihood of tin whisker growth. One study indicates that tin whiskers can grow on parts with nickel under-plating. In another study, a nickel barrier layer of 1. Unfortunately, the thin plating may reduce the ability of the finish to serve other necessary functions such as resisting corrosion. Annealing after plating has become a widely accepted whisker mitigation technique.

Annealing has been proved to reduce the maximum whisker length and the growth rate by relieving plating stresses, causing grain growth and increasing grain sizes while also forming a uniform inter-metallic layer of Cu6Sn5 over Cu3Sn, which slows further inter-metallic growth. For semiconductor parts, surface finishes are subjected to various forms of annealing, such as cure bakes and burn-ins.

These processes may have mitigated whisker formation on existing SnPb finishes.

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While evidence of removal of compressive residual stress has been shown to significantly reduce the onset of tin whisker formation, it is not clear that whiskers will not be an issue in 10 to 30 years. Mechanical damage and other factors may induce compressive stresses sufficient to initiate whisker growth. If alternatives to tin plated parts cannot be obtained, it may be possible to remove the tin finish.

This is normally risky on electronic components.

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The ability to remove the tin plating from the affected surfaces and refinish these surfaces must be made after a risk and cost analysis. Such processes should be reviewed to determine the potential for affecting the reliability of the original product e. This method is not approved and should be carefully reviewed prior to any use, as it is perceived to be very risky.

In terms of application-specific considerations, the proximately of adjacent conductors is a particular concern.

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For example, a 0. To remove the threat of shorts due to tin whiskers, engineers should review the criticality of the system or subsystem as well as its desired life expectancy. These factors include circuit geometries that are sufficiently large to preclude the risk of a tin whisker short, mission criticality, mission duration, collateral risk of rework, schedule, and cost.

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At present, any pure tin finished surface can be considered as having the potential to grow tin whiskers. Tin whisker mitigation strategies are recommended due to the whisker growth phenomenon associated with tin during the soldering process. Cypress Semiconductor has acknowledged the problem and minimized it by adding lead to the soldering process. Even with many studies and experiments performed globally on tin whisker growth, there is still no definite conclusion on how or why tin whiskers form.

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Ashworth G. Wilcox Rebecca L. Higginson Richard J.

Heath Changqing Liu. Tin whiskers are filamentary growths that are formed on the surface of electrodeposited tin, which is used extensively in the electronics industry. The presence of whiskers on electroplated finishes has been observed for more than 60 years, but, despite a huge amount of work in this area, a definite mechanism by which whiskers grow remains unidentified.

Whiskers pose a significant problem for manufacturers of electrical and electronic equipment, since they are able to grow across and bridge the gap between adjacent electrical components, resulting in short-circuits and other associated failures. For many years, whisker growth was effectively mitigated by the addition of lead to tin electrodeposits.