A hermetically sealed electronics module includes a core IC installed on a substrate. A collar surrounds the core IC and is sealed to the substrate and to a lid, forming a sealed chamber. A heat spreader bonded to an internal surface of the lid extends downward into proximal thermal contact with the core IC. A thin layer of TIM can be applied between the heat spreader and core IC. The heat spreader does not overlap any tall components that extend above the core IC, and can extend over regions adjacent to the core IC. Tall components can be limited to a periphery of the chamber, and/or the heat spreader can include openings that surround central tall components. The heat spreader can be soldered or welded to the lid over an entire upper surface of the heat spreader. X-ray and/or CSAM scanning can detect heat spreader bonding flaws.