In order to study the relation between whisker growth and the structures of deposits with respect to matt Sn deposits plated on Cu substrates, the dimensions of the grains of the deposits and those of the whiskers and/or nodules (whiskers/nodules) were determined as a function of the thickness of the deposits. Furthermore, the extruded mass per unit area attributed to the whisker/nodule growth has been estimated from the above data and the number density of the whiskers/nodules.
It has been found that (1): the matt Sn deposits are formed during electrodeposition in accordance with the mechanism known as “geometrical selection”: the average grain size is proportional to the root of the thickness of the deposits, (2): the whisker growth originates from the grains situated on the surface of the deposit ; the whiskers are considered to be the outward protrusions of the grains, and (3): the whisker growth can take place when the radius of the grain, consequently of the whisker, is larger than a critical radius, r
*, and which was found to be 0.34<r
It has also been found that the extruded mass per unit area was constant regardless of the thickness when the thickness of the deposits was more than 2 μm. On the other hand, it has been shown that, in thick deposits, the grain growth, which is expected to reduce the grain-boundary free energy and the strain energy included in the deposits, can easily take place. These suggest that the nature of the excessive energy that drives the whisker/nodule growth is different from the grain-boundary free energy or the strain energy.