Investigating crack propagation in lead-free solder joints

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Dr. Krammer Olivér
Department of Electronics Technology

In my thesis work cracks were generated in the solder joints of surface mounted devices, the generated failure modes were investigated with non-destructive experimental methods, and then the results were checked by destructive manners.

My first task was to design a printed circuit board which is compatible with the later experimental procedures. Resistors with 1.52×0.76 mm were mounted on the printed circuit board, which were soldered with different temperature profiles. The corresponding cooling rates were measured with K-type thermocouple. Out of the two solder pastes which were used, one (96.5 % tin, 3 % silver, 0.5 % copper) has high silver content, the other one (98.89 % tin, 0.3 % silver, 0.7 % copper, 0.1 % bismuth, 0.01 % antimony) has low silver content and contains micro-additives. Thermal Shock cycles with 140 °C maximum, and -40 °C minimum temperature were managed in order to investigate the crack propagation in the samples after different temperature cycles. The selected sample groups were investigated with two- and three dimensional X-ray machines, and then cross sections were made of them in order to validate the results with optical microscopy. During the crack investigations the main goal was to show the connection between the crack propagation methods and the manufacturing parameters, namely the cooling rate and the solder composition. The other goal was to do inspections on the availabilities of exploring crack propagation, mainly with computed tomography.

During the experiments two different computed tomographic machines were used, and according to my results, which are proved with cross sections, both of them are able to detect cracks. I pointed out the benefits of the tomographic machine against other industrial crack investigating methods, although, the knowledge of the limits of the measuring devices would be grateful in order to find out the shortest crack length and width, which can be detected by them.

In function of the cooling rate relating the crack length could not be shown obvious tendency, thanks to the uncertainty of the measuring and the small specimen number, although direct relationship was found between the crack properties and the solder content. Comparing the samples suffering the same fatigue cycles, the one with lower tin content showed shorter crank length.


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