We focus on an assembly line design problem of a fully automated robotic spot-welding line. Different from existing studies, we take the prescheduled unavailability periods, such as lunch and tea breaks, into account in order to reflect a more realistic production environment. This problem includes allocating operations to the stations and satisfying the demand and cycle time within a desired interval for each model to be produced. We also ensure that assignability, precedence, and tool life constraints are met. Furthermore, the existing studies in the literature overlook the limited lives of the tools that are used for production. Tool replacement decisions not only affect the tooling cost, but also the production rate. Therefore, we determine the number of stations and allocate the operations into the stations in such a way that tool change periods coincide with the unavailability periods to eliminate tool change related line stoppages in a mixed model robotic assembly line. We provide a mathematical formulation, propose a two-stage local search algorithm and test the performances of these methods using different problem instances with varying parameters.