Impact of thermal cycling and background gas environment on tin whiskering

(Part A) Work by Hoffman (Savannah River National Laboratory) reported Sn whiskers which grew longer in nitrogen (sealed jar) environments compared to air environments. This is of concern in electronics systems which operate in nitrogen-backfilled environments, such as hybrid circuits and sensor systems. In this work we compare Sn whiskers grown in two different environments-pure N2 and ultrahigh vacuum (UHV). After 140 days of incubation in N2, the compressive films had the highest whisker density (8,320 cm⁻²), of those in the sample set. X-ray photoelectron spectroscopy (XPS) analysis of the sputtered Sn surface after the incubation period indicate a more complex surface structure on the samples exposed to the N2 compared to UHV, suggesting that the details of the Sn oxide influences the whisker growth. (Part B) Under thermal cycling, Sn whiskers on electrodeposited films have been observed to display submicron striations that form perpendicular to the whisker's growth axis, hypothesized to originate from periodic film stress variations during cycling. In this work we have investigated whether striations occur in whiskers grown from sputter deposited films. The samples were exposed to two different thermal profiles cycling between −40°C and 125°C in a conventional cycling furnace. Very few whiskers were grown with any observable striations.