Comparison of the ageing process of solder alloys using strength and materials structure analyses

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Dr. Illyefalvi-Vitéz Zsolt
Department of Electronics Technology

The aim of this work was to identify the changes in mechanical properties and micro-structure of two lead-free solder alloys caused by thermal treatments. The two lead-free solders were: the first was a SAC (Sn-Ag-Cu) alloy, widely used in electrical engineering, and the other was the highly reliable Innolot alloy, specially developed for the automotive industry. The mechanical properties were measured with fracture-test. The thermal treatments were thermal storage and thermal cycle. Under the thermal storage the fracture-specimens took 1000 hours at 125°C. The thermal cycle took 1000 hours too, in the first half hour of a cycle the temperature was -40°C, in the second half hour it was 125°C. The specimens were pulled with 2mm/s velocity until fracture. Whit the fracture-test, the fracture strength and fracture-work values were established. The fracture strength of the SAC and Innolot specimens were 32-40MPa and 79-95MPa. The fracture-work values were 5-16J/cm3 for the SAC alloy and 23-53J/cm3 for the Innolot alloy. The high dispersion of the fracture-work values can be caused by the large size of the solder grains according to the cross-sectional sizes of the specimens. In this case the number and the orientation of the grains could have a high effect on the counted values.

From two parts of the fractured specimens cross-sections were made, one of these parts was under mechanical stress and the other was not. These cross-sections were analyzed with electron microscope. On the cross-sections of the SAC alloy large Sn-crystals and tin-silver, tin-copper intermetallics were seen. The microstructure of the Innolot alloy contained tin-nickel-copper, tin-silver intermetallics and bismuth precipitates inside the Sn-crystals. The microstructure of the cross-sections from different parts of the specimens did not show any differences, so the mechanical stress did not have any effect on the micro-structure.

Both alloy’s structure changed as the effect of the thermal storage and the thermal cycle, but it did not cause changes in the fracture strength and in the fracture-work values. These values did not change although their decrease was expected. In the SAC alloy the number of the Ag3Sn intermetallics increased, and their morphology became spherical. Near the Sn-crystals Cu6Sn5 intermetallics were generated. In the Innolot alloy, bismuth precipitates disappeared from the specimens, where such precipitates were observed before the thermal treatment. The Ag3Sn intermetallics changed as in the SAC alloy.


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