Investigation of the mechanical properties of lead-free solders with electrochemical methods

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Supervisor:
Dr. Hurtony Tamás József
Department of Electronics Technology

During my thesis work I analyzed the most commonly used lead-free solder alloy, SAC305 with a unique measurement method based on electrochemical impedance spectroscopy. During this process the selected solder alloy sample is inserted into an electrochemical cell to measure its impedance spectrum. The second step involves selective electrochemical etching using a sulfuric acid solution. The selective electrochemical etching process reveals the microstructure of the solder alloy for further investigation. After the etching the impedance spectrum of the sample is also measured, but now with its structure exposed. Then we fit different model circuits to the curves obtained from our experiments. The basis of comparison is the change of the fitting parameters of the equivalent network circuit before and after the electrochemical etching. A series of experiments were designed in which samples that had different cooling gradients in their reflow profiles were produced in order to compare their microstructure based on this characterization method. I have developed a rapid and easily adjustable process for producing my own specimens. Therefore, based on the investigation of several different heating and cooling solutions, I have designed a special crucible for preparing appropriate solder samples from SAC305 solder wire. After producing the required samples, systematic measurements were performed on each type of samples. While evaluating the measurement results an unexpected deviation in the results were recognized. The standard deviation of the results was too dominant to draw any conclusions, so I have carried out additional analysis in order to find the possible root cause of the measurement error. Or to prove that the deviation of microstructures of the samples are responsible for the experienced results. During these experiments, I managed to restrict the possible explanations, but I only partially discovered the true reason behind the deviation of the results. In a separate experiment I compared electrochemical impedance spectroscopy results between samples prepared using OP-S polishing suspension versus samples prepared using the same method, but without applying OP-S.

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