During use, the PCB undergoes extreme electrical cycling, extreme mechanical environments, and must withstand a large temperature range for years. This thesis studies the mechanical behaviour of the critical Cu layers in PCBs. Detailed knowledge about how the Cu layers mechanically fail under cyclic loading conditions is needed to better understand how PCBs electrically fail. With the in-depth knowledge about the Cu failure mechanisms, both as a foil and in the PCB laminate, future PCB designs can be developed that are more resistant to mechanical, electrical, and thermal failure through advanced materials and mechanical modelling. The goal of the thesis is to provide the necessary material input data for the mechanical simulations that will be performed. The finite element simulation method is a topic of growing interest for PCB companies to digitally design new products without the need for costly and time-consuming prototyping and testing. By including the real material behaviour, the simulations are more accurate, allowing for only the best designs to be further investigated. Getting detailed information about how the material fails, in this case the Cu layers, is essential for reducing costs and time, as well as keeping prototyping to a minimum.