Experimental Investigation of Microstructural Effects in Sn-Pb Solder Accumulated During Ten Years of Service Life

Aim: Solder joints of microelectronic devices are subjected to a wide range of loadings. They affect the microstructural evolution of the alloy. Long term investigations are commonly performed under thermally accelerated conditions, but are not available for real-life environmental conditions in literature yet. Therefore, the solder bumps of ten-year-old graphic cards are inspected here. Objectives: The primary objective of this study was the investigation of long-term accumulated effects in solder bumps. We classified the solder concerning its prior electrical functionality and analyzed theproperties of the microstructure. Methods: The image reconstruction is based on component identification, scratch elimination, and image stitching. Additionally, environmental scanning electron micrographs are employed to investigate the microstructure of the lead-rich phases. Results: Power supplying solder has the widest circularity distribution as a result of anisotropic diffusion and phase decomposition. On average, a bump with a cross-section of 0.12 mm2 contains 800 Pbrich phase islands. In the central region of the solder, broad Pb-rich platelets are present. Such platelets are typically perpendicular to the electric current flow and affected by the mechanical deformation of the bump. Additional electron-microscopy shows several micro-porous Pb-rich phase islands, which are induced by an uncalibrated diffusion of tin. Conclusion: We found the phase islands’ circularity to be the best indicator for the bulk dynamics. We conclude that device operation at normal working conditions leads to no hints for functional limitations at the end of the designed life span.