Tin Whiskers Mitigation by Co-electroplating Antimony

Abstract

Electroplating Sn coatings are used extensively in the electronics industry because of their excellent solderability, ductility, electrical conductivity, and corrosion resistance. However, tin whiskers have been observed to grow spontaneously from the electroplating tin coatings for over 70 years. This filament-type structure can lead to short circuits and device failures by bridging adjacent electronics. Over the last several decades, adding a small amount of lead (Pb) has been the most widely adopted method for mitigating Sn whiskers growth. However, this 70-year-old issue of whisker growth has reemerged due to the ban on the use of Pb from Sn-based plating and solders enforced by the European legislation on Restriction of Hazardous Substances (RoHS). The actual mitigation provided by the co-electroplating lead reamins unclear. Some have attributed the mitigation to the formation of equiaxial grains in a Sn-Pb plating process. Others have attributed it to reducing stress buildup in the film while doping Pb. This study proposes an innovative controllable way to mitigate Sn whiskers growth by electroplating Sn-xSb alloys (x=3 wt.%, 10 w.t%, 15 wt.%). In effect validation, a fixture was designed to apply high wide-plane compressive stress rather than using a hardness indenter to accelerate whisker growth. The results revealed that no whisker growth was observed on any Sn-xSb surface, while pure Sn samples produced whiskers up to 200 um under the same conditions. Adding a minor quantity of Sb could refine the Sn grains, however, the Sn grain size was not further refined with the increase in Sb content. In correspondence, the mitigation effect on whiskers growth was not further improved with the increase in Sb content. As confirmed by a series of EBSD studies, the addition of Sb changed the grain structure from a monolayer columnar structure to a multilayer equiaxed structure, which is favorable in whisker mitigation. The inhibition effect of Sb addition on whisker growth can be ascribed to the assistance of dynamic recrystallization and produces horizontal grain boundaries of Sn grains. This dissertation details the following studies: background of tin whisker problems; development and validation of an effective co-electroplating recipe for tin whisker mitigation using advanced analysis tools; identification of the primary mechanisms of the whisker growth mitigation