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Tin Whisker Mitigation and Conformal Coating
What are Tin Whiskers?
Tin whiskers are electrically conductive, crystalline structures that sometimes grow from surfaces where tin (especially electroplated tin) is used as a final finish. Tin whiskers typically grow from lengths of 1-2 mm but have been observed to lengths in excess of 10 mm. Electronic system failures have been attributed to short circuits caused by tin whiskers that bridge closely-spaced circuit elements maintained at different electrical potentials.
Are Tin Whiskers an Issue?
There are four main risks with tin whiskers:
- Stable short circuits in low voltage, high impedance circuits
- Transient short circuits
- Metal vapor arc
- Debris/contamination
Of these, a metal vapor arc is often the most destructive. A metal vapor arc occurs when a tin whisker initiates a short in an environment possessing high levels of current and voltage.
How can I Stop Tin Whisker Growth?
Unfortunately, there is no known way to eliminate tin whisker growth, only the use of mitigation strategies to limit their effect on product.
Is Conformal Coating a Viable Tin Whisker Mitigation Strategy?
As a result of an eleven-year NASA study, conformal coating was proven to be a viable tin whisker mitigation strategy. While thinner coats of conformal coating were unsuccessful at preventing tin whisker penetration, Arathane® 5750 (a urethane resin) applied at 2 mm thick was strong enough to prevent tin whiskers from penetrating the coating and causing any potential issues.
Additionally, a study of conformal coatings for metallic whisker mitigation, titled “Strategic Environmental Research and Development Program (SERDP) Nanoparticle Enhanced Conformal Coating For Whisker Mitigation”, found that vacuum deposited Parylene C and Parylene C with AdPro Plus® had the best coverage and no indication of whisker growth or coating penetration. Due to the enhanced adhesion from the AdPro Plus, Parylene C thickness control in the range of 10 to 15 microns is recommended to avoid potential microcracking due to thermal mismatch of assembly materials during accelerated thermal cycling.
