Effect of Cu additives on Sn whisker formation of Sn(Cu) finishes

Abstract

Sn whisker formation on Sn(Cu) finishes has been studied. (1) With respect to the thickness effect, we found that Sn whisker density for pure Sn and Sn0.7Cu finishes has a linear relationship with the finish thickness. The safety thickness for Sn and Sn0.7Cu finishes is about 10 mm and 20 mm, respectively. (2) With respect to the alloying effect, we found that Sn whisker formation could be retarded by increasing Cu content in the Sn(Cu) finishes. We conclude that the Cu additives could reduce the two major driving forces of the Sn whisker formation, i.e., metal underlayer dissolution and thermal stress. The Cu additives self-formed a Cu-Sn compound barrier layer, which effectively prevents the reaction and dissolution with the metal underlayer. On the other hand, the Cu additives precipitated out as Cu-Sn compound in the Sn(Cu) finish layer, which is believed to be the reason for smaller values of the coefficient of thermal expansion (CTE) for Sn(Cu) alloys. The smaller CTE values results in a lower thermal stress level in the Sn(Cu) finishes.

Thermally driven redistribution of phases and components in Cu/Sn thin films

“…It is known from the previous studies that the intermetallic phase Cu 6 Sn 5 has specific structural morphology and usually appears in the shape of scallops in images obtained by electron microscopy [7,17]. Tin also tends to have a rough surface in comparison with copper.…”

Evolution of micro solder joints under electromigration and elastic field

“…The compressive stress is generally induced by the diffusion of Cu into Sn and the formation of intermetallic compounds (IMC) [16,17]. To prevent the growth of Sn whiskers, various methods have been investigated, such as annealing the Sn deposits, using a thick layer of Sn with larger grains, incorporating additives in the layer of Sn, and introducing a diffu- sion barrier [16][17][18][19]. Recently, it has been reported that a layer of Ni is an effective barrier to restrain the growth of Sn whiskers [20,21].…”

“…The stress field in Eq. (18) and (19) can be numerically solved by following the same procedure with the numerical implementation of the diffusion equations.…”

Risk of whiskers formation on the surface of commercially available tin-rich alloys under thermal shocks

“…As a result of this, the growth rate of the new strain-free Sn grains is faster than depleting rate of strained Sn grains. High pressure, created around the nucleation site causes breaking of finish surface and pushing up the material as an Sn whisker [4].…”