between package 20 and circuit board 24 is accomplished by means of a set of arcuately shaped leads 28 which extend out from base 4 and soldered to electrical pads 15 that are built into circuit board 24 referring now to fig3 and 4 a preferred embodiment of the present invention is shown in the form of a hermetic package 20 which is adapted for sealing off a multichip module from environmental influences and thus protecting the module and the chips mounted thereon from the adverse effects of exposure to moisture and the like package 20 includes a planar ceramic base 30 constructed of a material such as aluminum nitride having thermal expansion characteristics similar to the silicon used in a multichip module and having good thermal conductivity for promoting transfer of heat away from the module during operation a multichip module may be attached to base 30 by a thermally conductive adhesive ceramic base 30 is specially constructed to include a ledge 32 which extends around the perimeter of the base and which is elevated from the surface of the base 30 a multiplicity of metalized contacts 34 operative as electrical feed throughs extend through and under ledge 32 from the interior to the exterior of base 30 ledge 32 includes a very thin metalized layer 36 all along its top surface ledge 32 contacts 34 and metalized layer 36 are integral to base 30 being part of a single co fired construction which provides a particularly durable structure package 20 also includes an endless sidewall 50 which is adapted at its lower edge 56 for engagement with ledge 32 sidewall 50 is most advantageously constructed of a metal alloy which in the preferred embodiment is.