Power electronics device, such as a frequency converter, which includes liquid-cooled power units (REC5, REC6, AFE5, AFE6, INU4, INU5, DC/DC, DC/AC) and also a substrate arrangement. The power units include at least one power semiconductor component, e.g. an IGBT or a diode, connected to at least one power circuit connecting them. The power units are mechanically separate and they are disposed on at least one substrate provided with liquid cooling ducts. The substrate is common to more than one power unit, wherein the substrate includes liquid cooling interfaces and power circuit interfaces for each power unit that can be connected to it. In addition, the substrate includes one liquid cooling interface and one power circuit interface (DC+, DC−) to outside the substrate.