Comparative Evaluation of Corrosion Effects between Nanocrystalline Silver Alloy and Tradition Gold Stacks Using Spring Probe Contacts
Authors: Budhraja Vinay , Stirn Jarvis , Hallstrom Eric , Lutz Brian , Brennan Michael , Deford Kevin 
Published: February 2020

Although silver is evaluated as an alternative of gold, it is more prone to tarnishing, oxidization and corrosion. Recent advancements in nano materials research resulted in a nanocrystalline silver alloy which has several advantages like high hardness, thermal stability, non-coarsen with age, hence a viable solution for connector applications. In this work, the performance of nanocrystalline silver alloy against corrosion was studied. Two different set of spring probes were electroplated on inner and outer regions and tested using force deflection resistance (FDR), life cycle test (LCT), SEM, EDS and X-Ray CT scan. First set of spring probes were electroplated with nanocrystalline silver alloy, nanocrystalline Ni-W, Ni sulfamate and Ni strike with thicknesses of 2.4 µm, 1.8 µm, 2.0 µm and 0.5 µm respectively. Second set of spring probes were electroplated with Gold, Ni sulfamate and Ni strike with thicknesses of 0.9 µm 1.9 µm and 0.5 µm respectively. The body of spring probes was made up of copper alloy. This work reports the changes in electrical, mechanical and material properties of nanocrystalline silver alloy and gold stacks after reliability operations like humidity test for 24 hours and 96 hours, porosity test for 60 mins and salt-fog test for 24 hours.
Although hard gold is preferred as a final finish for contacts, the demand of finding its alternative has increased because of increase in market price. Further, practices to reduce gold thickness increase the porosity of gold layer which inversely effects the performance of connectors. Nanocrystalline silver alloy is a potential candidate as an alternative of gold in high speed applications [1]. It behaves differently than pure silver because its thermally stable nanostructure improves hardness and eliminates surface sulfidation [2]. All tests in this work were performed on spring probes which is one of the products of Smiths Interconnect used for contacts and connector applications [3].
In FDR, the spring compressed from normal position to 0.14 inch and extended back to normal position. By this way, it completes a hysteresis and measures coefficient of friction between barrel and plunger of spring probes. After reliability operations, the highest compression force in the case of gold was reached up to 12 oz and in the case of nanocrystalline silver alloy, it reached up to 9.5 oz. The difference in the magnitude of compression and extension force at 0.14 inch measures friction. Higher difference between these two forces measures lower Q values which result in higher coefficient of friction. After reliability operations, the range of this difference was from 6 oz to 7 oz in the case of gold but in the case of nanocrystalline silver alloy this range was from 3.5 oz to 4 oz. Moreover the difference in resistance during compression and extension was 2 mΩ in the case of gold and 1 mΩ in the case of nanocrystalline silver alloy. Meaning the alloy of nanocrystalline silver did not allow silver sulfidation.
In life cycle test (LCT), 32 spring probes were installed into receptacles that have been press fitted into a matrix of holes drilled into fiber block and the resistance was measured in 10,000 life cycles. The maximum resistance was reached up to 11 mΩ in the case of gold stack but in the case of nanocrystalline silver alloy it was reached up to 8 mΩ only. In the reliability tests, many probes failed to measure contact resistance in the case of gold stack, but in the case of nanocrystalline silver stack it maintained below 6 mΩ. This means that the failure of probes was limited by the wear performance of the gold in the mating pair between barrel and plunger.
The failed probes in reliability operations were tested using front and cross section SEM and the elemental composition was observed using point and line scan of EDS. The mechanical marks, which is a function of wear rate, were observed in SEM on the inner regions of the probes at the mating of barrels and plungers. The failure of the probes was further tested from X-Ray CT scanning. In the final version of the manuscript additional results and discussions will be presented.

References:

Goodrich, K. Bui, M. Osborne and P. Stokoe, “Performance Testing and Evaluation of a Ag-W nano-crystalline silver alloy as a gold replacement in electrical connectors,” 60th IEEE Holm Conference on Electrical Contacts, 2014.

Bui, T. Hasanali and T. Goodrich, “New Final Finish Stack Including a Custom Nano-crystalline Silver Alloy fo Mobile Connector Systems,” 65th IEEE Electronic Components and Technology Conference, 2015.

http://www.smithsconnectors.com/us/markets/board-test-ate

Figure 1