Long-term power cycling of high power diodes for welding application is presented. The devices are packaged in an ultra-slim flat package, which allows very high heat sinking capability and rating current. Tests with different operating conditions were carried out and the results compared with the few data available in literature. Our devices show very good lifetime, especially considering that all tests were terminated without any performance degradation. Finally a thermo-electrical model of the device was built and will be used for both validation of lumped element thermal models and evaluation of new design solutions.