Fretting behaviour of various intermetallic compounds in electrical contacts: Influence on reliability

Many connectors are made of cuprous substrates with electroplated final surfaces such as tin and nickel. The main failure mechanism of such devices has been shown to be fretting. Intermetallic compounds (IMC) formed due to the ageing of tin coatings have been shown to be an aggravating factor. Much work is still dedicated to better understand the failure mechanisms involved in tin fretting. The aim of this work was to elaborate improved coatings for better reliability of electronic devices. The work has been performed on tin layers electrodeposited on cuprous substrates such as those used in the electronic industry. Sine displacement cycles were imposed to ball/plane samples in “controlled displacement” fretting” tests. Experiments were performed for samples with underlayers of different types and thickness both as received and after heat treatment. The number of cycles for the contact resistance to exceed the threshold value of 10 mΩ increased with thin underlayers of 0.1 μm. The heat treated tin layer with a 0.1 μm cobalt underlayer showed a low and stable Rc value for a highly increased number of cycles. Various physicochemical techniques were used to analyse the composition profiles of the samples and to measure the thickness of the intermetallic compounds. Thin underlayers allow copper to diffuse; intermetallic compounds of different composition and with different structures are formed: Cu6Sn5, Ni3Sn4 or unidentified (Cu–Sn–Ni) and (Cu–Sn–Co) compounds. The electrical and tribological behaviours of bare IMC were investigated after stripping the free tin; sticking phenomena could account for the lower values of resistance recorded for the (Cu–Sn–Ni) and (Cu–Sn–Co) compounds. The particular needle shape of the intermetallic compound formed with the thin cobalt underlayer could cause different fretting conditions leading to lower values of contact resistance for a longer time. Despite the complex phenomena involved, indications are given for the elaboration of coatings with improved fretting behaviour in electrical contacts applications.