  Complex cryogenics is still a strong limitation to the spread of quantum
voltage standards and cryogen-free operation is then particularly interesting
for Josephson standards. The main difficulties in He-free refrigeration are
related to chip thermalization. We tested different solutions and interface
materials between the chip and the cooling surface, to improve thermal
conduction. Some junctions were chosen as elements to dissipate electrical
power, while some others were operated as on-chip temperature sensors. Indium
foil between chip and Cu support was demonstrated to provide a good thermal
interface suitable for programmable voltage standard operation. However,
thermal conduction can be further increased by thermal contacting the chip at
the top. Finally, general physical constraints in vacuum thermal contacts are
analyzed in terms of known properties of thermal interfaces at cryogenics
temperatures.
