Examination of correlations between the microstructure and the mechanical properties of solder joints

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Dr. Hurtony Tamás József
Department of Electronics Technology

Conclusion In my thesis, a literature study was conducted in which I studied the formation of intermetallic compounds and their effects on soldered joints. Then, the characteristic mechanical and structural studies of soldered joints were reviewed followed by the examination of the specimen preparation process based on selective electrochemical etching. To continue with, a series of experiments were designed and carried out, which provided the specimen needed for further examination.

Three solders were examined with different cooling gradients. At first, the specimens underwent microhardness testing. No obvious difference was shown with different gradients with the SAC305 and eutectic tin-silver solders as they all fell in the dispersion domain of the measurement. As far as the bismuth tin specimens are concerned, differences were detected with the appropriate number of measurements. The microhardness rate was 18.86 with the 6,89°C/s cooling gradient specimen and 20.23 with the 2,81°C/s one.

As there was no suitable method for measuring the microhardness of the first two specimens, I examined them in a different way. The specimens were then examined with AFM, which revealed obvious differences in the intermetallic structure of the specimens both in terms of size and quantity. However, this examination refers only to a given flatness and not to the spatial structure. As a result of this, I discovered the spatial structure of the specimens with the help of the previously mentioned specimen preparation process.

In order to analyze the structure of the specimens, I used EIS measurements. During this process, the impedance spectrum of the cell was recorded both before and after the etching. Then, the relative impedance change was calculated. This rate and thus the porosity of the specimen declined due to the decline of the cooling gradients. This rate was 0.1079 and 0.1066 with the SAC305 solder’s 6.59°C/s and 4.49°C/s cooling gradient specimen, while it was 0.2481 and 0.0036 with the eutectic tin-silver solder’s 10.52°C/s and 3.49°C/s cooling gradient specimen. Thus it can be concluded that this rate and thus the porosity of the specimen declined due to the decline of the cooling gradients.

All in all, hardness measurement can be an acceptable method for measuring mechanical properties, in case a suitable number of measurements can be performed. This cannot be achieved with the help of microhardness measurement because of the significant dispersion and its geometrical need. EIS measurement can be an applicable method to determine relative impedance change, which can be used for characterizing the structure of the specimen.


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