Imidazolium ionic liquids as neat lubricant for electroplating Sn on Cu substrate under sliding electrical contact at high temperatures up to 80 °C

Abstract

Three kinds of imidazolium ionic liquids (IILs) with different fluorine-containing anions, i.e. imidazolium tetrafluoroborate (LB108), imidazolium hexafluorophosphate (LP108), imidazolium methyltributylammomiumbis ((trifluoromethyl)sulfonyl)imide ([HEMIM]NTf2), are selected as neat lubricant for electroplating Sn on Cu substrate under sliding electrical contact at room temperature (25 °C) and elevated temperatures (40 °C and 80 °C). All the IILs effectively reduce friction coefficient (0.28 for LB108 and LP108, 0.4 for [HEMIM]NTf2) and prevent Sn film from rapidly worn out and severe adhesive wear at 25 °C. However, at high temperatures, only LB108 well lubricate Sn-on-Sn contact up to 80 °C. It is found that, at 80 °C, the microstructure and chemical composition of the worn surface and the subsurface of the electroplating Sn are markedly modified due to two aspects: (1) temperature related phenomena (i.e. thermal diffusion of Cu to Sn film and even to SnO2 layer, as well as formation of intermetallic phase Cu6Sn5) and (2) interaction of the worn surface and IILs (i.e. adsorption, tribochemical interaction of the worn surface and the anion species, and diffusion of N species in Sn film). The interaction of the worn surface and the anion species of IILs plays an important role in wear reduction of electroplating Sn at 80 °C.