There are an increasing number of electronics applications in aerospace, automotive, shale/gas and power management, which are required to operate at or above 200 °C. Organic matrix reinforced substrates such as polyimide, have maximum operating temperatures in the region of 175 °C. Reliable operation of electronics at temperatures higher than this requires a combination of performance improvements in components, interconnects and substrates. Ceramic based substrate options are based on alumina substrates with printed inks fired at ~ 600 °C and can be costly, heavy and prone to mechanical damage. Printed circuit board (PCB) options are restricted to lower working temperatures of the organic resins and degradation of their conductive copper tracks through oxidation. This paper highlights earlier work undertaken by the authors and partners to understand the deficiencies of copper-clad PCB technology and details work to develop a low cost alternative to ceramic substrate based assemblies. The authors have investigated replacing the alumina substrates with high temperature engineering thermoplastics such as PEEK. The high temperature fired inks conventionally used in hybrid circuit manufacture have been replaced with screen-printable silicone based ink systems curing at 250 °C. The specially developed electrically conductive and dielectric inks were utilised to produce a multilayer system demonstrator with high temperature compatible components attached using a high temperature conductive adhesive. Such an assembly system has the potential to benefit from reductions in substrate cost and assembly weight. Energy cost associated with manufacture are significantly reduced. In addition the organic substrate is easier to machine and form into complex shapes and offers the possibility of integrating thermal management solutions. Environmental testing has been undertaken to determine the suitability of the system to operate for extended periods at 250 °C and the results of the electrical and mechanical performance for continuous ageing of test assemblies at 250 °C will be given.