A combination of self-assembled monolayer and hydrophobic conformal coating for anti-corrosion of Cu/NiP/Au 3D circuitry in artificial sweat solution

3D circuitry on moulded interconnect device (MID) is usually made of multi-layered metals, for example, electroless copper (Cu), electroless nickel (EN or NiP) and immersion gold (IG or Au), i.e. so-called Cu/ENIG. Anti-corrosion of MIDs in harsh environments, such as in contact with human sweat, is a challenge because the three metals have large differences in electrode potential and part of the circuitry is positively polarized during operation. Furthermore, any coating or surface modification applied onto the metal traces must meet other requirements such as good compatibility with soldering and less effect on the surface conductivity. In this study, failure analysis of 3D MID devices working in sweat environment was carried out to understand the corrosion mechanisms. It is shown that EN coating showed better corrosion resistance than Cu substrate, which is attributed to the passivation effect of phosphorus in NiP alloy material. The reasons causing the metal trace corrosion include existence of coating defects in both of Au and EN coatings, poor corrosion resistance of Cu in sweat, and large difference in electrode potential among the three metals. Based on the failure analysis results, a synergetic effect of surface passivation by using self-assembled-monolayer (SAM) of 2-mercaptobenzothiazole (MBT) and hydrophobic conformal coating was studied. The sweat tests show that a combination of SAM and hydrophobic conformal coating can effectively retard the corrosion progress of Cu substrate of the metal traces on 3D MIDs.