  Here, we study the role of stress state and stress gradient in whisker growth
in Sn coatings electrodeposited on brass. The bulk stress in Sn coatings was
measured using a laser-optics based curvature setup, whereas glancing angle
X-ray diffraction was employed to quantify the surface stress; this also
allowed studying role of out-of-plane stress gradient in whisker growth. Both
bulk stress and surface stress in Sn coating evolved with time, wherein both
were compressive immediately after the deposition, and thereafter while the
bulk stress monotonically became more compressive and subsequently saturated
with aging at room temperature, the stress near the surface of the Sn coating
continually became more tensile with aging. These opposing evolutionary
behaviors of bulk and surface stresses readily established a negative
out-of-plane stress gradient, required for spontaneous growth of whiskers. The
importance of out-of-plane stress gradient was also validated by externally
imposing widely different stress states and stress gradients in Sn coatings
using a 3-point bending apparatus. It was consistently observed that whisker
growth was more in the coatings under external tensile stress, however, with
higher negative out-of-plane stress gradient. The results conclusively indicate
the critical role of negative out-of-plane stress gradient on whisker growth,
as compared to only the nature (i.e., sign and magnitude) of stress.
