As deep earth oil exploration transits into deeper earth, it faces challenges in producing reliable microelectronics devices and sensors that will not degrade at high temperature, especially those related to packaging materials and assembly of the devices. It has been reported that the mechanical performance of a high temperature Au-Ge eutectic solder is able to fulfill the minimum interconnection properties specified by the oil and gas exploration industry. However, the impact of thermal aging to Au-Ge eutectic solder's electrical property is not clear. In this work, it is observed that the bulk Au-Ge electrical resistivity decreased initially with thermal aging and then saturated. This effect is mainly contributed by the grain growth of Au, which results in a reduction in grain boundaries, as well as coarsening of the Ge phases. On the other hand, study on the microstructure and electrical resistance of Au-Ge solder bump joint after aging shows that the electrical resistance increased. The increase in resistance is contributed mainly by the NiGe and Ni5Ge3 intermetallic compounds growth, which also led to a degradation in the Ni(P) barrier layer.