In power electronics, the drive towards miniaturization and ever more compact dimensions while reliably evacuating increasing amounts of dissipated power is a great challenge. CeramTec has developed a double-sided cooling concept that increases the cooling and thereby raises the power density: CeramCool® Sandwich heat sink.
Image: Schematic diagram of the double-sided structure - in this system, the upper heat sink is soldered to the upper side of the chip so that the chip is cooled from both sides.
To meet the growing demands in the power electronics field, it is necessary to reduce the thermal resistance between chip and heat sink, which is made possible by the “Chip-on-Heat sink” technology.
This eliminates thermal interfaces. In comparison with a conventional solution, in which a power module is mounted on a liquid cooled heat sink, the thermal resistance is significantly reduced.
Image: “Chip-on-Heat sink”-technology - Thick (150–300 µm) copper conductor tracks are sintered directly onto a ceramic heat sink.
Double-sided heat sinking with liquid cooling
A further, advanced solution is based on a ceramic, liquid cooled heat sink, the Multi-K. It is directly metalized and the chips are soldered right onto it. This creates an optimum thermal connection between the power semi-conductor and the coolant; electrical insulation is achieved via the ceramic. Traditional cooling concepts only lead power loss away from the bottom of the chip. However, it is also possible to create a cooling channel on the top of the chip. A further ceramic heat sink is added, which conducts additional dissipated heat away from the upper side of the chip. This second cooling path thus operates in parallel with conventional cooling.
There are a number of issues to consider here:
The whole load current flows across the upper surface of an IGBT or MOSFET, requiring large area electrical contacts. Thus the material used must provide good electrical and thermal conductivity. In addition, a minimum clearance is required between the top and bottom heat sink conductor tracks. On the one hand, the gate must be contacted. On the other, a potting compound with high breakdown voltage must be introduced between upper and lower conductor planes to guarantee the insulation voltage. However, because of its adhesion and viscosity values, the dielectric requires a specific gap width. Another aspect to consider is the coefficient of thermal expansion.
The key component – a cuboid of aluminum nitride ceramic
On the upper side of the chip, the same problem arises as on its underside. A solution is required with a coefficient of thermal expansion (CTE) as close as possible to that of semi-conductor material. A metal-ceramic composite similar to using DCB as a circuit board is a solution that meets this challenge. Aluminum nitride was selected for the ceramic cuboid; at about 180 W/mK, its thermal conductivity is very good. In a similar way to a DCB circuit board, a ceramic cuboid can be metal bonded, preferably with copper. The cuboid is perforated with holes, which are then filled with copper paste to create vias. Once the paste is printed across the entire surface of the top and bottom of the cuboid, all of the vias can be connected. The exterior faces of this cuboid can likewise be printed with a film of conductive copper paste.
Due to its physical properties, the thermal expansion coefficient of the cuboid at 4.5 ppm/K is very close to that of silicon (2.6 ppm/K) or Silicon Carbide (4.57 ppm/K). It can easily be soldered onto the top of the chip, thanks to its matching CTE, or, if properly pretreated, it can also be sintered on with silver. The cuboid is highly conductive, both electrically and thermally.
Then the heat-dissipating cuboid is placed onto the copper on this top heat sink and they are sintered together. This results in a body with high thermal and electrical conductivities.