A Study on Corrosion Resistance of Electroplated Gold over Nanocrystalline Ni-W for Contact Applications
Authors: Budhraja Vinay , Stirn Jarvis , Hallstrom Eric , Lutz Brian , Brennan Michael , Deford Kevin 
Published: February 2020

Highly conductive metal with least corrosion resistance is the most prominent requirement of contact industries. Although deposition of gold over Ni sulfamate is a standard interface for contact and connector applications, the performance of gold over Ni sulfamate has faced several problems such as wear durability, porosity of gold layers and biased corrosion resistance. The deposition of nanocrystalline Ni-W (nc Ni-W) between gold and Ni sulfamate improves the mechanical and surface properties of interfaces. In this work two different types of samples were processed and compared in terms of corrosion. The first sample (Au/NiSO4) was prepared using electroplating of gold, Ni sulfamate and Ni strike over base material of copper alloy with thicknesses of 0.9 µm, 1.9 µm and 0.5 µm respectively. In the second sample (Au/nc Ni-W/NiSO4), all layers of electroplating were same except the addition of 1.8 µm of nanocrystalline Ni-W before deposition of gold. Both samples were passed through the environments of (a) 85% relative humidity at 85oC for 24 hours, (b) 85% relative humidity at 85oC for 96 hours and (c) nitric acid vapors for 60 minutes. The effect on electrical, mechanical and materials properties of samples were investigated before and after passing through these environments using Force Deflection Resistance (FDR), Life-cycle test (LCT), Scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS) and X-Ray CT scanning.
Although Au/NiSO4 is a well-known interface for connector applications [1], the deposition of nc Ni-W between gold and NiSO4 reduces the rate of corrosion because nc Ni-W has a weak tendency for grain boundary segregation and building broader range of accessible grain sizes [2]. There is formation of grain colonies in Ni-W deposit which controls crack propagation and corrosion [3]. The performance of two different interfaces of Au/NiSO4 and Au/nc Ni-W/ NiSO4 against corrosion was tested after electroplating on spring probes. Spring probe is one of the product of Smiths Interconnect which is used for different applications like defense, medical, rail, semiconductor, space etc. [4].
In FDR, the spring force, coefficient of friction and contact resistance was measured under a hysteresis of compression and extension for a travel distance of 0.14 inch. In the case of Au/NiSO4, the difference in compression and extension force at 0.14 inch was 6.1 oz. In the case of Au/nc Ni-W/ NiSO4, this difference was 4.6 oz which means that there is less friction with the deposition of nanocrystalline Ni-W underneath gold. Moreover, the difference in contact resistances was lower in second sample which means that the addition of nc Ni-W enhances the interface of gold and NiSO4. After treatment in humidity from 24 hrs to 96 hrs the difference in compression and extension force at 0.14 inch was increased by 7 oz in first sample but in second sample this difference was only increased by 3.6 oz which means that there is less increase in friction due to the presence of nc Ni-W.
In LCT the resistance was measured at a test current of 0.025 amps under a stroke length of 0.09 inch for 10,000 life cycles with a cycle rate of 4 cycles/sec. The rate of increase in resistance with number of cycles was higher in the case of Au/NiSO4 than Au/nc Ni-W/ NiSO4. In first sample, the sticky coefficient was increased from 0 to 25 % after treatment in humidity for 24 hrs and it further increased to 37.5% after keeping in humidity for 96 hrs. The wear off rate and friction reduction of Au/nc Ni-W/ NiSO4 helped in maintaining the sticky coefficient to be constant even after kept in humidity for 96 hrs.
The failure of the probes were tested from front and cross section SEM, point and line scan EDS and X-Ray CT scanning. SEM was used to observe mechanical marks and microstructure analysis which explained the reasoning of low wear rate in the case of Au/NiSO4. The failure of the probes was also confirmed from X-Ray CT Scanning in the probes during compression of spring. The complete set of experimental results will be added in the full version of the paper.

References:

Jon Diller, “Spring Contact Probe Technology in Product Connector Applications,” 3rd Edition, Chapter 4, pp. 61-64.

Detor, J. and C.A. Schuh, “Tailoring and Patterning the Grain Size of Nanocrystalline Alloys”, Acta Materialia, vol. 55 , pp. 371-379, 2007.

Ruan, S. and C.A. Schuh, “Meso-scale Structure and Segregation in Electrodeposited Nanocrystalline Alloys”, Scripta Materialia, vol. 59, pp. 1218-1221, 2008.

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

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