Over the past few years, the operating environment for automotive electronics has become increasingly harsh, which has pushed the ambient temperature requirement for the automotive electronic components above 150°C. Since most electronic modules perform better when mounted as closely as possible to the motor or transmission (that is to say, closer to the heat source), the physical connection to the electronic modules must now be capable of withstanding elevated temperatures up to a maximum of 200°C. Also, the connections to the power electronics in hybrid vehicles likewise require reliable functionality up to 200°C. Silver (Ag) coated on a nickel (Ni) barrier above bulk copper (Cu) substrates creates an ideal electrical connector that has excellent electrical conductivity, visual appearance, solderability and corrosion resistance. However, when such Ag-coated parts are routinely exposed to temperatures >150°C, oxidation of Ni underneath the Ag begins to occur for reasons not yet fully understood.
There seem to be inherent lifetime temperature limits on Ag plating on Cu substrates with a Ni diffusion barrier: After just 200 hours @180°C delamination between the Ag coating and Ni underlayer becomes inevitable due to Ag-accelerated oxidation of Ni, while new automotive specifications require coatings capable of sustaining 200°C for 1,000 hours with no adhesion issues. Also, for coating electrical conductors where high-current-carrying capability is specified by the application, pure Ag with minimal contact resistance must be used instead of Ag alloys. With these constraints in mind, Dow began an extensive set of methodical experiments to find a way to best integrate an oxidation barrier into the metal stack.
After experimenting with very thin barriers of other metals without positive results, we found that a thin tin (Sn) layer can work as an adhesion promoter. The deposition of a thin film of Sn adjacent to the Ni barrier layer prior to depositing Ag inhibits oxidation of the Ni, and so prevents adhesion failure in the metal stack during high-temperature applications. Figure 1 shows the high-temperature-resistant silver stack after 1,000 hours at 200°C, where the deposited <1 micron-thick Sn “strike” layer has reacted to form intermetallic compounds with both the Ag above and the Ni below, while preventing oxidation of the Ni.
Fig 1. Cross-section image of metal stacks after 1,000 hours @ 200°C:
Standard Ni/Ag-strike/Ag (left) and new Ni/Sn-strike/Ag (right)
Our experiments have determined that there are two critical parameters when applying the Sn strike to ensure high-temperature resistance for Ag on Cu with a Ni diffusion barrier layer:
- Too-thin Sn may not be sufficient to ensure good adhesion; and
- The ratio of the thicknesses of the Ag to the Sn layers must be greater than 2, otherwise Sn can diffuse to the surface of the Ag and reduce the surface conductivity.
Product prototyping is being done in various industries by leading customers. An automotive-connector product manufactured using this metal stack has already passed acceptance and reliability tests at a leading connector maker. This breakthrough technology could also benefit other high-temperature electronic connector applications such as electric vehicles and high-power LEDs. We are proud that European and U.S. patents have been granted for this Ni-Sn-Ag layer structure and process, and that the patent application is pending in other countries such as China and Japan.
This patented technology requires Ni, Sn and Ag plating in a fixed sequence. Dow supplies plating chemistry for Ni, Sn and Ag baths, and can help with integration of the Sn strike layer into manufacturing lines with various plating techniques including reel-to-reel continuous plating tools.