Issue link: https://iconnect007.uberflip.com/i/1389320
78 SMT007 MAGAZINE I JULY 2021 hand, higher relative humidity makes the air less corrosive. At higher humidity, the sulfur con- centration in the chamber decreases because of sulfur vapor absorption by surfaces with higher amounts of adsorbed moisture. Lower sulfur concentration makes the air less corrosive. As already mentioned, we determined the corrosion rates of conformally coated cop- per and silver serpentine thin films. e coat- ings tested were acrylic, silicone and ALD. Silicone coating provided the least protec- tion to the underlying films; ALD coating pro- vided the best protection. Test results of cop- per thin films covered with these two coatings and of bare copper thin film are shown in Fig- ures 6-10. Figure 6 shows the temperature pro- files of the test specimens over the 5.32-day test. Figure 7 shows the uncorrected resis- tance of the thin films measured using 4-point probe and a potentiostat. e data in Figures 6 and 7 provide enough information to deter- mine the temperature coefficient of electri- cal resistivity of the thin films. From these figures, the temperature-compensated (cor- rected) resistance profiles, shown in Figure 8, can be obtained. From Figure 8, we can calcu- late the extent of copper corrosion as a func- tion of time as shown in Figure 9. is copper corrosion refers to the copper thickness lost to corrosion, whereas the corrosion rates of Fig- ure 5 refer to the total thickness of the corro- sion products growing on the copper foils. e total thickness of copper corrosion products growing on a copper foil is twice the thick- ness of the copper thickness lost to corrosion. From the slopes of the plots in Figure 9, we Figure 5: Corrosion rates of a) copper foils and b) silver foils in FoS chamber. Typical time of exposure of foils was one day. Coulometric reduction was used to determine total corrosion product thickness [15] . Figure 6: Temperature of copper thin films as a function of time in FoS chamber at 40°C and 15% relative humidity. a) a) b) c) b)