Crimped connections between aluminum and tin-plated conductors have a stable long-term behavior when installed correctly. In contrast, current-carrying connections between aluminum and tin-plated conductors without an intense deformation at assembly, e.g. bolted joints with busbars, do not exhibit this kind of long-term reliability. Previous research has shown that the resistance of these connections begins to deteriorate immediately after assembly even without an electrical load or an elevated temperature. The reason for this is oxygen that is enclosed in the interface. Upon joining, it starts to react with the aluminum surface to form highly resistive Al2O3. In order to examine the processes in the interface more closely, experiments were conducted on a model geometry consisting of a bare aluminum cylinder and a tincoated copper counter-part with a pointed end. The contact resistance was measured at constant temperature, at constant DC current, and at constant source voltage. The results give new insights into the impact of the growth of the aluminum oxide layer, as well as its electrical and thermal breakdown. Additionally, the Al2O3-film in an aged Al-Sn-contact was examined using transmission electron microscopy. Differences between the stable long-term behavior of crimped connections and the instability of force-fitted connections are discussed.