Patent ID: 12255121

DETAILED DESCRIPTION

FIG.1shows a typical switching device arrangement in a power circuit. The electronics are provided as a semiconductor chip or die1. This is connected, e.g. by solder2, to external circuitry (not shown) by means of a conductive busbar3e.g. a copper busbar to which more dies may be connected. The busbar3is mounted onto a base plate4, to one side4a, where the busbar is attached, but electrically isolated via an insulating layer5of dielectric material e.g. ceramic substrate and an additional metallic layer55. A coolant (not shown inFIG.1) flowing inside a heat sink7mounted along the opposite side4bof the base plate4evacuates the heat generated by the semiconductor. The connection of the power module to the circuit can be by means of e.g. bond wires9or sintering a conductive layer. Moreover, in most cases the baseplate is attached to the heat-sink7using a thermal interface material8that contributes with additional thermal resistance.

As can be seen, there are several layers of different materials between the semiconductor chip1and the cooling assembly, and, particularly, the cooling fluid that would flow inside the heat sink7beneath the base plate4. This provides a relatively high thermal impedance thus limiting the ability of the assembly to dissipate heat from the chip1.

Some methods have been proposed to improve cooling, such as shown inFIGS.2and3.FIG.2shows a design in which the semiconductor die is provided with two interfaces to allow thermal flow from each side of the semiconductor chip. Whilst this provides a greater surface area from which heat can dissipate, the use of dielectric and base plate layers is still required and the high thermal impedance is still, therefore, a limiting factor.FIG.3shows an assembly using direct liquid cooling, whereby the flow of coolant is integral with the heat sink6′. This assembly partially reduces the total thermal impedance when compared to the assembly inFIG.1.

Reducing the thermal impedance e.g. by eliminating some or all of the intermediate layers and/or improving the thermal conductivity of remaining layers results in improved cooling of the semiconductor chip.

To achieve improved cooling, in the assembly of this disclosure, rather than the chip being mounted to a busbar and then the busbar being mounted via a dielectric layer to a base plate below with coolant fluid flows, the cooling is provided by the chip and busbar to which it is (directly) mounted being immersed in coolant fluid as shown inFIGS.5and6. The coolant fluid is non-conductive. This thus avoids the additional thermal impedance of the dielectric and base plate layers and provides a direct cooling of the chip over the greatest possible surface area.

FIG.4shows a simple, yet possible, implementation of a direct cooling, through a dielectric coolant, concept as proposed by this disclosure, where the coolant is in direct contact with the heat source, i.e. the semiconductor. In this case the base plate is adapted to directly accommodate the coolant flow. Such baseplate implementation removes one layer of thermal impedance, but still requires the dielectric (ceramic substrate) and the base plate. This embodiment also employs two types of coolants, one of them with dielectric properties, in contact with the semiconductor, while another coolant that without dielectric properties, flowing through or below the baseplate. Although the use of two coolant fluids could provide good performance it increases the system complexity. It is necessary to further reduce the thermal layers while keeping the system simple.

FIGS.5and6show alternative examples, in which the semiconductor chip10is sintered directly to a busbar for connection to the external circuitry. In the example shown, the chip10is sintered to a source bus20and a drain bus30but assemblies can be envisaged with only one bus. The combined switching unit of chip and busbar(s) is immersed in coolant fluid40. In the example shown, the module is provided in a housing50having a coolant inlet60at one end and a coolant outlet70at the opposite end such that coolant can continuously flow through the housing across the surfaces of the busbar(s) opposite the surface to which the chip10is sintered. In this way, only the busbar material and the sinter material provides thermal impedance between the coolant40and the chip10.

The shape of the busbar can be designed to facilitate the transfer of heat from the busbar to the coolant. The busbar can be designed to form different coolant channel patterns such as that shown inFIG.6or alternative patterns e.g. microchannels.

The direct attachment of the busbar(s) to the chip also increases the mechanical strength of the assembly.

As mentioned above, to ensure the necessary electrical isolation, the coolant fluid must be non-electrically conductive. It should also be non-combustible, have a high thermal coefficient and low viscosity.

To further increase the transfer surface from the heat source (the semiconductor die) to the coolant fluid, conductive connections can be used. The high thermal conductivity of the busbars e.g. of copper, is used to spread the heat and thus increase the thermal conductivity in the heat dissipation path.

The immersion of the switching cell in the cooling fluid thus minimises the thermal impedance between the device and the coolant.

The effect of the reduction in thermal impedance can be shown using thermal impedance equivalent circuits to compare various designs.

InFIGS.7A,7B and7C, the impedance elements refer to the thermal impedances as follows:Zch: thermal impedance of the semiconductor chipZcbu: thermal impedance of the copper busbar (including attachment, i.e. solder)Zcs: thermal impedance of the ceramic substrate (including attachment)Zcb: thermal impedance of the copper bonding layerZbp: thermal impedance of the baseplateZtim: thermal impedance of the thermal interface materialZhs: thermal impedance of the heat-sink and to the ambientZCoA: thermal impedance of the coolant and to the ambient

FIG.7Ashows the thermal impedances existing on the assembly for a classically bond-wired system with baseplate and thermal interface material (TIM), such as shown inFIG.1. By using a direct liquid cooling assembly (such as shown inFIG.3), instead, the thermal impedances of the assembly are reduced as shown inFIG.7B.FIG.7Cshows the much reduced thermal impedance for the assembly of the present disclosure.

The described assembly enables power electronic converters and the like to be developed with greatly increased power densities compared to existing designs. This is a critical performance indicator in many fields e.g. in aerospace power converter applications. The design can also provide improved voltage insulation for smaller dimensions. Further, uniform thermal heat can result in reliability improvements of the overall assembly.

While the present disclosure has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this present disclosure, but that the present disclosure will include all embodiments falling within the scope of the claims.