The invention relates to a process for preparing interconnection substrates of hybrid circuits, comprising effecting on a support the deposition of a thick layer of ink or paste based on a non-noble metal, such as copper or other material having a "copper compatible" formulation, by carrying out in succession a preliminary drying for eliminating solvents at a temperature on the order of 100.degree. C. to 150.degree. C., a firing comprising:a) a temperature rise incorporating a stage for eliminating polymer resins,b) a sintering step at a temperature on the order of 600.degree. C. to 1000.degree. C., andc) a timed cooling,the firing being carried out under a substantially inert gas atmosphere (nitrogen and/or argon and/or helium), the atmosphere of the stage for eliminating the polymer resins having a content of water vapor lower than 15,000 ppm, preferably between 1,000 and 10,000 ppm, while the atmosphere for sintering at high temperature has a content of water vapor in any case lower than 1,000 ppm.