Selected microstructural features affecting the shear strength of lead-free solders/substrate couples

The results of shear strength investigations of the selected lead free-solders/substrate couples are presented. A modified shear test of a half of the lead-free solder/substrate couple has been proposed. A comparative assessment of the solder droplet size effect as well as a composition of intermetallic layer and its morphology is presented. The results achieved provide the explanation of the mechanism of solder droplet size influence on the joint quality. If the lead-free solder droplet is bigger, the time of its solidification is longer, and consequently, the varied droplet structure can be observed. In the case of a bigger droplet size, its microstructure close to the surface is composed of the crystals orientated in the same direction as the heat transfer. The layer of directionally solidified crystals weakens the joint. A morphology of the selected intermetallic layers was presented as the second problem of couples quality. In comparison to the locally discontinuous layer, the higher shear strength was obtained in the case of continuous intermetallic layer. The paper contains the results of: wettability, microstructure of soldered joints (lead free solder/Cu substrate) and shear tests investigations. The results make possible to formulate the following remarks and conclusions: the lowest shear strength was observed for the joint of pure Sn with Cu substrate, the highest shear strength was observed for the joint of SnBi solder with Cu substrate; practically, two times higher than for pure Sn with Cu substrate and solder SnIn with Cu substrate or solder SnZn with Cu substrate, in the case of double alloys SnZn the lower contact angle results in higher shear strength, however, for remaining investigated alloys such tendency was not observed, the structure of the intermetallic layer on the border of the solder/substrate plays the essential role in the final shear strength of the received joint, the shear strength results characterize the small scatter what indicates the rightness of applied device design as well as correctness of sample preparation and measurement procedure.