Scaling effect of interfacial reaction on intermetallic compound formation in Sn/Cu pillar down to 1 μm diameter

Yingxia Liu, Ying Ching Chu, King-Ning Tu*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

48 Scopus citations

Abstract

Different diameter Sn/Cu pillars varying from 30 to 1 μm have been made by focus ion beam, and the solid state reaction to form Cu6Sn5 in these pillars has been studied. An increased Cu-Sn reaction rate to form Cu6Sn5 was observed as the pillar diameter decrease from 20 to 1 μm when annealed at 185 and 195 °C. With the shrinking of pillar size, surface diffusion becomes important as compared to grain boundary and lattice diffusion in the Cu-Sn interfacial reaction. Also, surface diffusion induced Kirkendall voids was observed to locate at the peripheral area just above Cu of the Cu-Sn pillars. A simple kinetic model of surface diffusion controlled intermetallic compound growth of Cu6Sn5 is proposed for pillars with diameter below 5 μm. What is essential in the model is that we assume a rapid interstitial diffusion of Cu in Sn, and we are able to calculated surface diffusivity of Cu on Cu6Sn5. At 185 °C, the surface diffusivity is about 3.1 × 10−7 cm2/s and the activation energy is about 0.2 ± 0.1 eV/atom.

Original languageEnglish
Pages (from-to)146-152
Number of pages7
JournalActa Materialia
Volume117
DOIs
StatePublished - 15 Sep 2016

Keywords

  • Diffusion mechanism
  • Intermetallic compounds
  • Solder pillar
  • Surface diffusion

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