“© 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.”, This paper presents the thermal design of the electronics unit of the instrument Polarimetric Helioseismic Imager, onboard the European Space Agency mission Solar Orbiter. The thermal design procedure, along with the problems encountered during this design phase, and the solutions found to fix them are described, proving in this way the thermal feasibility and robustness of the unit. Its final thermal behaviour, obtained from thermal analyses correlated with data from thermal tests performed in a vacuum environment, is presented. © 1965-2011 IEEE., This work was supported by the Spanish Ministerio de Economia y Conipetitividad under Grant ESP2014-56169-C6-1-R, Grant ESP2014-56169C6-4-R; Grant ESP2014-56169-C6-6-R, Grant ESP2016-77548-05-1-R, Grant ESP2016-77548-05-3-R and Grant ESP2016-77548-05-5-R. The work of.1. Piqueras was supported by the Grant FPU15/0561S "Formacitin do Profesorado Universitario" from the Spanish Ministerio do Educacion; Cultura y Deporte., Peer reviewed