Investigation of Cadmium Plating Microstructure and Whisker Growth Conditions

Zahra Ghanbari, Donald F. Susan, Jamin Pillars, Sara Dickens, Bonnie McKenzie

Sandia National Laboratories

Metallic whisker growth is an aging mechanism that has been observed for decades in several pure metal systems, in which high aspect ratio single crystal features grow out of polycrystalline substrates, often thin coatings. Motivated by changes in microelectronic soldering requirements, most literature has focused on tin (Sn) whisker growth and its associated mechanisms. In contrast, limited literature exists on cadmium (Cd) whisker growth, its associated mechanisms, and similarities or differences compared to Sn whiskers.
In a recent survey of Cd plated steel fasteners, half of the fasteners analyzed exhibited whiskers. Though all fasteners that exhibited whiskers had been in service for many years, additional details of the environmental conditions that the fasteners experienced, and thus conditions that contributed to whisker growth, were unknown. This work seeks to determine the microstructural and environmental conditions under which Cd whiskers grow, as a first step to better understand the mechanism of Cd whisker growth.
Preliminary efforts have focused on understanding microstructural conditions favorable for Cd whisker growth via characterization of the thickness, grain size, and texture of industrial Cd plating on parts that exhibit whiskers. In parallel, Cd whiskers have been harvested from fasteners, and electron backscatter diffraction (EBSD) has been used to determine the crystallographic growth directions. Finally, laboratory-plated coupons were prepared and thermally treated to investigate the environmental conditions that contribute to whisker growth in a controlled manner. This presentation will discuss the results of these efforts and how they relate to the current understanding of Cd whisker growth.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.