A U-shaped magnetic circuit (2) provided with a field winding (3), including respective induction electrodes (5s, 5i) at an outer end of each arm of the magnetic circuit (2) Both electrodes are perpendicularly arranged as regards the multi-layer printed circuit, coaxially to each other and capable of vertical movement in both directions. The end of each induction electrode (5s, 5i) that comes into contact with the multi-layer printed circuit is provided with a thermal barrier, in order to prevent, during the welding, heat transmission by thermal conductivity from the welding zone to the induction electrode (5s, 5i).