The effect of Na2SO4 contamination on different surface finishes during Thermal-Humidity Bias tests

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Supervisor:
Dr. Medgyes Bálint Károly
Department of Electronics Technology

The electrochemical migration (ECM) is a failure phenomenon, wich occours when two opposedly biased electrodes are conducted by a continuous moisture film wich acts as an electrolyte. By the effect of the applied voltage at the anode metallic ions dissolve into the electrolyte and migrate towards the cathode driven by the electric field, where a metallic reduction occurs leading to metallic dendrite growth and ultimately short failure.

The tests were carried out in an envinronmetal chamber which allowed me to simulate the dew condensation mechanism, so I could observe the ECM phenomenon in realistic conditions. It allowed me to measure the required time the electrochemical cells and short failures to form.

I examined two surface finishes which are used widely in electronic devices. These surface finishes were the electroless nickel (EN) and the electroless nickel immersion gold (ENIG). The ECM behaviour of these finishes were examined in non-contaminated and sulphate ionic contaminated cases.

The examination of the sulphate ionic contamination is relevant because the topic of the sodium ionic contamination is not deeply addressed in the literature. The measurements were carried out on uncontaminated reference samples and samples contaminated by 0,1 mM, 1 mM, 10 mM, 500 mM concentrations and on saturated sodium-sulphate solution as well.

After the measurements, the samples were inspected by optical microscopy and SEM-EDS (scanning electron microscope - energy dispersive X-ray spectroscopy) analysis. By the optical microscopy examination I was able to determine the form and the orientation of the dendrites and as well as the colour of the residues. By the EDS analysis I could measure the elemental composition of the dendrites and residues.

In order to ensure the reproductibility of the experiments 4 measurement samples were carried on each solution concentrations using both type of the samples and 2-2 measurements were carried out on non-contaminated samples as reference. It means alltogether 44 measurements were done.

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