The paper describes attempts to explain the scaling effect on microstructure and creep of Sn-based solders. It compares creep data that was gained on bulky samples and on small solder joints. The microstructural properties of the bulk specimens and real solder joints were examined using metallographic sectioning, optical microscopy techniques, and SEM-microprobe analysis. The results of the microstructural analysis were related to the investigated mechanical properties of the solders. The solidification behaviour of SnAg- and SnAgCu was investigated experimentally. Microstructural analysis point out that bulk solder has typically a dendritic microstructure. At ultra small solder joints complex forms of solidification behaviour were detected. The different nucleation behaviour is supposed to be the reason for the differences in solidification. Creep experiments on SnAgCu-based solders have been conducted on several types of specimens: flip chip and bulk specimens. The results of the experiments show that the influence of size and composition of SnAgCu-based solders is very complex. Size and composition are no independent factors.