Some circuit modules have a thin, flat substrate with semiconductor chips that are attached to metal pads at an array of chip sites on the upper surface of the substrate.  The orientation of the substrate with the chip surface facing up is arbitrary to simplify the terminology. 
The substrate has pins that extend from its lower surface and plug into connectors of a supporting circuit board.  Layers of wiring inside the substrate interconnect the pads and the pins.  Some modules have a structure called a hat that forms part of an enclosure for the chips and also forms part of the module cooling apparatus.  The lower surface of the hat has cylindrical holes opening to face the chip sites and the holes carry cylindrical elements called pistons that each contact the upper surface of a chip.  Conventionally, the pistons are made of a thermal conductor and transfer heat from the chips to the hat by conduction, and the module is commonly called a thermal conduction module or TCM.  The hat is conventionally cooled by chilled water. 
Many improvements in chip performance require the chip to dissipate more power, and it is a general object in this art to provide a TCM-like device with improved heat transfer.  It is also an object in this art to operate the chips at a low temperature, for example at the temperature of liquid nitrogen.