A power module assembly with low thermal resistance and enhanced heat
dissipation to a cooling medium. The assembly includes a heat sink or
spreader plate with passageways or openings for coolant that extend
through the plate from a lower surface to an upper surface. A circuit
substrate is provided and positioned on the spreader plate to cover the
coolant passageways. The circuit substrate includes a bonding layer
configured to extend about the periphery of each of the coolant
passageways and is made up of a substantially nonporous material. The
bonding layer may be solder material which bonds to the upper surface of
the plate to provide a continuous seal around the upper edge of each
opening in the plate. The assembly includes power modules mounted on the
circuit substrate on a surface opposite the bonding layer. The power
modules are positioned over or proximal to the coolant passageways.