To comply with the Restriction of Hazardous Substances (RoHS) directive, pure tin is replacing lead-tin alloys in commercial electronic devices. Unfortunately, tin can grow whiskers that can lead to electrical short circuits or metal vapor arcing, both of which threaten the long-term reliability of electronic systems, and cause critical systems to fail catastrophically. A current method of whisker mitigation utilizes coatings based on glassy or rubbery unfilled polymers; such coatings are not impenetrable to tin whiskers.
Seeking a more reliable method of whisker mitigation, Sandia developed a composite coating comprised of platelet-shaped particles such as nickel (Ni) or magnesium fluoride-coated (MgF2) Ni/Al dispersed in silicone or epoxy. When the platelets are oriented with their directors normal to the tin-plated surface, they create a multilayer barrier resembling that found in nature (e.g., abalone shell). Platelet orientation is easily controlled using surface modification to prevent the polymer from wetting the platelet surface, extrusion, and applying magnetic or electrical fields.
Platelet-filled composites vastly outperform conventional unfilled polymers. In tests conducted in a thermal cycling environment known to promote whisker growth, commercial capacitors coated with the composite exhibited no signs of whisker penetration, whereas those coated with conventional polymers showed whisker penetration.
In addition to its increased penetration resistance, the new composite enhances thermal conductivity and dielectric properties, and reduces out-of-plane gas permeability.