An apparatus comprising: a substrate having a recess in a top surface of the substrate; a first thermal insulator submerged in the recess of the substrate; a cover comprising a sidewall and a top portion, wherein the sidewall and the top portion comprise a second thermal insulator that, together with the first thermal insulator, defines a thermally-insulated volume; one or more components to be cooled located over the first thermal insulator and within the thermally-insulated volume; one or more electrical conductors located over the first thermal insulator, the one or more electrical conductors extending laterally over the first thermal insulator and under the sidewall of the cover such that a first portion of each electrical conductor is located within the thermally-insulated volume and a second portion of each electrical conductor is located outside the thermally-insulated volume; wherein at least one of the one or more components is electrically connected to the one or more electrical conductors; wherein the one or more electrical conductors are thermally-insulative at cryogenic temperatures; one or more conductive traces disposed on the top surface of the substrate outside the thermally-insulated volume, the one or more conductive traces electrically connected to the one or more electrical conductors; and a dielectric layer over the second portion of each electrical conductor that is outside the thermally-insulated volume and a portion of the one or more conductive traces that is located within the recess. a substrate having a recess in a top surface of the substrate; a first thermal insulator submerged in the recess of the substrate; a cover comprising a sidewall and a top portion, wherein the sidewall and the top portion comprise a second thermal insulator that, together with the first thermal insulator, defines a thermally-insulated volume; one or more components to be cooled located over the first thermal insulator and within the thermally-insulated volume; one or more electrical conductors located over the first thermal insulator, the one or more electrical conductors extending laterally over the first thermal insulator and under the sidewall of the cover such that a first portion of each electrical conductor is located within the thermally-insulated volume and a second portion of each electrical conductor is located outside the thermally-insulated volume; wherein at least one of the one or more components is electrically connected to the one or more electrical conductors; wherein the one or more electrical conductors are thermally-insulative at cryogenic temperatures; wherein at least one of the one or more components is electrically connected to the one or more electrical conductors; wherein the one or more electrical conductors are thermally-insulative at cryogenic temperatures; one or more conductive traces disposed on the top surface of the substrate outside the thermally-insulated volume, the one or more conductive traces electrically connected to the one or more electrical conductors; and a dielectric layer over the second portion of each electrical conductor that is outside the thermally-insulated volume and a portion of the one or more conductive traces that is located within the recess.