Effects of Surface Roughness and Aging on the Electrical Contact Resistance and Residual Stress in Gold-nickel-copper Connector Materials

Open Access
Author:
Butler, Nan Ellen
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
August 28, 2014
Committee Members:
  • Douglas Edward Wolfe, Thesis Advisor
Keywords:
  • surface roughness
  • aging
  • electrical contact resistance
  • residual stress
  • gold
  • nickel
  • copper
  • connector
Abstract:
The purpose of this research is to investigate the effects of surface roughness and aging environment on the electrical contact resistance and residual stress in nickel and nickel/gold coated copper. In order to accomplish these research objectives, oxygen free copper coupons were prepared to a variety of average surface roughnesses (Ra ranging from 0.05 μm to 2.5 μm) prior to coating application. Three coating systems were investigated: Au/Ni plating, Ni plating, and Ni sputtering. The majority of this research focuses on the Au/Ni plated samples, with Ni plated and Ni sputtered data included for comparison. Nickel, gold, and copper are commonly used in electrical components, such as electrical contacts, in which component reliability is of great importance. As there are a wide variety of environments to which electrical components could be exposed to during service, the coated samples were aged for up to nine weeks in one of three aging environments: mixed flowing gas/atmospheric corrosion (MFG), elevated temperature and relative humidity (TRH), and thermal cycling (TC). The mixed flowing gas/atmospheric corrosion environment was created by using a Battelle Class 2 environment with the following conditions: 30 °C, 70 % relative humidity, 200 ppb NO2, 10 ppb H2S, and 10 ppb Cl2. The elevated temperature and relative humidity environment maintained samples at a constant temperature and relative humidity of 40 °C and 85%, respectively. Samples in the thermal cycling environment were cycled between temperatures of -40 °C and 60 °C with a 15 minute dwell time and an approximately 4.5 °C/minute ramp rate. Characterization using optical profilometry, optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), x-ray diffraction (XRD), and electrical contact resistance (ECR) measurements allowed for the determination of changes in the coupons due to the coating system, surface preparation, and environmental exposure. For the unexposed samples, the surface roughness effects on the residual stress varied depending on the coating material and deposition technique utilized. The residual stress in the gold coating of the Au/Ni plated coupons was tensile and decreased in magnitude with increasing substrate surface roughness. The Ni plated coupons exhibited a compressive stress for the smoothest set of samples, while the rougher samples were nearly stress free. The Ni sputtered samples retained an approximately constant residual stress regardless of the substrate surface roughness. On the other hand, the surface roughness had little to no effect on the electrical contact resistance for the as received samples. All measured electrical contact resistance values for these samples were observed to be within approximately 1 mΩ of each other, with values on the order of 10-12 mΩ. Characterization after environmental exposure revealed aging effects on the residual stress and electrical contact resistance to vary depending on the surface roughness and aging environment. Samples exposed to the atmospheric corrosion/mixed flowing gas (MFG) environment exhibited the largest change in electrical contact resistance. ECR values increased from the as received condition of 10-12 mΩ to over 1000 mΩ. The extent of the sample surface with increased ECR values increased with increasing roughness and amount of corrosion product present. In comparison, little to no changes in the electrical contact resistance were observed in the thermal cycled and elevated temperature and relative humidity aged samples. The tensile residual stresses observed in the Au/Ni plated samples remained approximately constant or decreased in magnitude with aging time (depending on the aging environment), with the largest change resulting from thermal cycle aging. The tensile stresses found in the Au/Ni plated coatings were observed to decrease in magnitude two to three times more for the thermal cycled samples (~60-120 MPa depending on the substrate surface roughness) as compared to the MFG aged samples (~10-50 MPa). Little to no changes were observed as a result of TRH exposure.