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A Comparison of Immersion Gold and Tin Surface Finishes on Sensing Electrodes for PCB Environmental Saltwater Concentration Sensors
Keywords: Immersion tin, immersion gold, environmental sensors
Printed circuit board (PCB) sensors using low-cost commercial printed circuit board fabrication processes have been demonstrated for environmental sensing applications [1, 2, 3]. One configuration of these sensors uses exposed electrodes to measure saltwater concentration in coastal biosystems, through monitoring the resistance between the electrodes when they are immersed in the saltwater/freshwater solution. Applications for this sensor configuration include detecting saltwater intrusion into freshwater aquifers [4], and drought monitoring in saltwater marshes [5]. The lowest cost commercial PCB processes use an immersion tin surface finish on exposed copper cladding, including the sensing electrodes. This commercial PCB process has been demonstrated to make an effective, low-cost, short-lifetime sensor for saltwater concentration testing. The tin finish, however, is certainly not optimal for this application. Tin oxidizes, which can interfere with sensor performance. Additionally, tin and tin-oxide are potentially reactive with chemical constituents in seawater and seawater/freshwater solutions. An immersion gold surface finish is certainly less reactive with the atmosphere and chemicals likely present in the testing environment. However, an immersion gold finish significantly increases the cost of the sensors. Are the possible benefits of the more expensive gold finish worth the extra expense? To investigate this question, a study was performed where identical PCB sensors were procured from a commercial vendor with their standard low-cost immersion tin finish. The exposed sensing electrodes on half of the sensors were then coated with a thin layer of gold using a gold electroplating process. Both sets of sensors were then evaluated in concentrations of seawater and freshwater, from 0% to 100% seawater concentration, using freshwater samples from two natural freshwater sources near the coast where the seawater was obtained. Testing demonstrated that there was no significant difference in sensor performance between the tin-only and the gold plated sensing electrodes. The results of this investigation indicated that for applications where the sensors will not be used for long periods of time, the added expense of a gold surface finish is not worth the cost. Since biofouling [6] limits the useful lifetimes of these types of low-cost sensors to short periods, the tin surface finish is optimal for this application.
Robert N. Dean, McWane Endowed Professor
Auburn University ECE Dept.
Auburn, AL

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