To ensure the efficiency of high-frequency (HF) heating during sealing by soldering the microblocks packages, the penetration of the electromagnetic field into the body by choosing the required frequency range of HF currents should be excluded. The thermal profile of HF heating has been optimized through the use of a semi-closed magnetic core made of ferrite, covering body along the perimeter. In this case, the eddy current densities in the package and the lid are aligned, and the maximum current density on the lid surface and in the solder is 1.7 times higher than that without the magnetic circuit.