Effect of colling rate on the structure of solder alloys

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Dr. Krammer Olivér
Department of Electronics Technology

The solder alloys play an important role in electronic technology, because they primarily define the electrical and mechanical properties of the solder joint. With the presence of the lead-free solder alloys new researches were considered to be necessary. The purpose of the researches is to get more information about the physical and chemical changes in the alloys in different compositions during the soldering process. It is known that many intermetallic compounds are born in the solder joint during the consolidation. The mechanical and electrical properties of these compounds are different from the components, thus they can influence the quality of the solder joint. It is also known that the size of the intermetallic compounds depends on the cooling rate during the soldering process. Former researchers mostly made qualitative analysis on the solder alloys, therefore I consider it important to do researches due to which quantitative analysis will also be available.

In my research, I made experiments on Sn3,5Ag solder alloy. I dealt with the quantitative analysis of the size of the Ag3Sn intermetallic compounds in samples cooled under different conditions. I worked out a sample preparation process which the heating and the cooling of the solder alloy is possible. During this process, the temperature fixing is also possible with the slightest error. I took photographs with optical microscope of the cross section of the samples and I analysed them in MATLAB with self-made image processing algorithms. I got information about the average area, extension, and specific distribution of the intermetallic compounds. I could get the latter property from the fitted exponential curve on the histogram of the area distribution.

The examination manifested in accordance with former researches proved that the size of the intermetallic compounds decreases with the increase of the cooling rate, and the microstructure becomes grainy. With the lowest cooling, (measured from 250 °C to 230 °C is about 0,4 °C/s) the mean area of the intermetallic compounds is about 1,43 µm2, but with the highest cooling rate (~4,5 °C/s) this parameter is about 0,84 µm2. The mean intercept length also decreased from 0,97 µm to 0,8 µm. Because of the graining, the amount of the small shapes increased in case of high cooling rate, and just a little amount of large, needle like shapes were formed. This can be seen on the parameters of the fitted exponential curve in the histogram made by area distribution: In case of low cooling rate the amplitude and the falloff were 470 and 0,17 respectively, but by high cooling rate they were 1036 and 2,6.


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