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

H. J. Kao, W. C. Wu, S. T. Tsai, C. Y. Liu

Research output: Contribution to journalArticlepeer-review

17 Scopus citations


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 μm and 20 μm, 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.

Original languageEnglish
Pages (from-to)1885-1891
Number of pages7
JournalJournal of Electronic Materials
Issue number10
StatePublished - Oct 2006


  • Pb-free solder
  • Sn whisker
  • Surface finishes


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