Tuesday, May 11, 2010 | Pylin Sarobol, Aaron E. Pedigo, Peng Su (Cisco Systems), John E. Blendell and Carol A. Handwerker, Purdue University
Tin whiskers spontaneously grow from electrodeposited Sn-containing films, but the growth mechanism is not well understood. The current tin whisker testing outlined in JEDEC22A121 fails to provide information on film properties necessary to predict the propensity of a film to whisker. Evaluating local grain structures, grain boundary mobility and grain orientations around whiskers and correlating these film properties to whisker growth provide valuable information regarding the propensity of a film to whisker.
Defects-whiskers and hillocks-originate from surface grains at the top of the films.
Cross sections reveal largely columnar structures. All defects appear to originate from "surface grains."
Surface grains may arise from recrystallization of the film or be present in the as electrodeposited films. The physics-based mechanisms by which these happen are not known. Defect growth occurs in effort to relax stress within the film  .
Cu6Sn5 intermetallic forms at the interface between Sn film and Cu substrate. Addition of Cu to Sn film results in additional Cu6Sn5 intermetallic formations at the grain boundaries . Addition of Pb to Sn film results in finer grains.
Figure 1: Schematics of whisker and hillock formation on Sn and Sn alloys electrodeposited films on Cu substrate.
Figure 2: FIB cross sections revealing microstructures of the Sn, Sn-Cu, and Sn-Cu-Pb films near defects.