Electrodeposition of Ag-Ni and Pd-Ni Thin Films on Ni-P from a DMSO Solution

The most used electrical contact system in electronics and microelectronics is Cu/Ni(Ni-P)/Au, which offers an interesting combination of high electrical conductivity, corrosion resistance, and wear resistance. However, the top Au thin layer being porous, a galvanic coupling between NiP and Au, and then Cu and Au appears that renders the system unusable. Au being highly expensive, the development of more inexpensive and reliable plating materials such as Ag or Pd can be experimented. Ag is widely considered as an economical alternative to Au. Ag and Au top-coats are traditionally obtained from highly toxic cyanide baths. Therefore, recent studies focused on developing non-cyanide electrolytes, Dimethyl sulfoxide (DMSO) being a promising solution for noble metal deposition. Hard noble coatings, i.e. alloying the noble metal with Ni, Fe, or Co, have a lower content of the noble metal and offer a higher hardness and thus a superior wear resistance. The electrodeposition of Ag-Ni and Pd-Ni layers on Ni-P from a non-cyanide DMSO based electrolyte was investigated, which was especially challenging for Ag deposits given the fact that Ag and Ni are totally immiscible in all the states. The chemical composition and morphology of these systems were highly affected by the applied potential. Therefore, the pulse deposition was used to form films on Ni-P coatings, i.e. a high cathodic potential (-1 V vs. Ag/AgCl) for the preferential deposition of Ni, and a lower value for the preferential deposition of the noble metal (here Ag or Pd). The obtained coatings showed a good adhesion to Ni-P. The Ag-Ni top-coats showed a nanocrystalline structure and presented a higher concentration in noble metal than Pd-Ni. This result was attributed to the formation of stronger DMSO complexes with Pd than with Ag.