Compact size, low thermal resistance and high reliability packaging technology
The market for renewable wind power generation devices continues to increase rapidly. Power electronics for wind power systems should be small, lightweight and highly reliable in order to minimise maintenance and enhance reliability throughout the product lifetime. In order to satisfy these stringent needs Hitachi has developed a new 600A, 1700V direct liquid cooling IGBT module.
By Neil Markham, Hitachi
The key features of the new IGBT module include an integrated copper base and pinfin technology. This allows the implementation of greaseless direct liquid-cooling which enables an extremely low thermal resistance. The module also adopts a Si3N4 insulated substrate that is highly fracture resistant and enhances the long term reliability of the module. RoHS compliant lead free solder and ultrasonic bonding technology are used for the main terminals.
The footprint size of the cooling jacket is the same size as the number of IGBT modules required for the inverter or converter, up to a maximum of six. Depending on the ability of the coolant pump, users can choose a cooling jacket with serial flow channel, 2-parallel flow channels, 3-parallel flow channels or 6- parallel flow channels. Each jacket has internal flow junctions and two connections; one inlet and one outlet. The power unit is 37% lighter and 45% smaller than a conventional power unit of equivalent power capacity consisting of indirect liquid-cooling IGBT modules and heatsink.
The thermal resistance of the new IGBT module is reduced by direct liquid-cooling technology. In an indirect liquid-cooling IGBT module, heat from the IGBT die flows through the solder layer, metal layer, ceramic layer and thermal grease layer to the heatsink. The thermal conductivity of grease mounting compounds is almost similar in value to the thermal conductivity of copper. Therefore the thermal resistance of indirect liquid-cooling IGBT is high. Conversely, the direct liquid-cooling IGBT module does not use thermal grease and the thermal resistance of a direct liquid-cooling IGBT is small. The temperature distributions under switching operation in the IGBT die are shown in Figure 2. The temperature distribution is presented using thermal-liquid simulation.
The pressure drop in the coolant channel is measured with a coolant jacket of a serial flow channel for 2 IGBT modules. The coolant used in the experiment was comprised of 50% ethylene glycol and 50% water. Coolant temperatures are 0°C, 10°C and 50°C. The newly designed pin-fin base plate and channel cover jacket enabled a reduced pressure drop compared to that of a conventional indirect liquid-cooling heatsink.
The reliability of the new IGBT module is highly affected by any coolant leakage. In order to predict the risk of leakage under operating coolant pressure, stress simulation testing was performed (see Figure. 3). The maximum warpage deformation at the O-ring contact surface points under 500kPa coolant pressure, which is regarded as the typical discharge pressure of the coolant pump, is approximately 0.04mm. This value is smaller than the accepted deformation to avoid coolant leakage. Therefore the module can endure coolant pressure under operation and avoid any coolant leakage. A channel jacket for 2 IGBT modules was used in the test.
The thermal fatigue life of the solder layer is improved by using a coefficient of thermal expansion (CTE) matching technology. In a conventional IGBT module a thick AlN (aluminium nitride) substrate with a thin aluminium layer is used to insulate the IGBT circuit. The new Hitachi IGBT module uses a thin Si3N4 (silicon nitride) substrate with a thick copper layer. The high fracture resistance of Si3N4 compared to AlN allows the use of a thick copper layer. The thin Si3N4 substrate and thick copper layer increases the equivalent coefficient of thermal expansion of Si3N4 and copper laminate. The difference between the CTE of the Si3N4 and copper laminate and CTE of copper pin-fin base plate is small. Thus, the thermal stress of substrate/base plate connecting solder layer is reduced and the thermal fatigue life of the solder layer is improved. Hitachi’s bespoke lead-free solder also improves the fatigue life of the solder. Thermal fatigue life diagram of new IGBT module is shown in Figure 4. Power cycling life diagram of new IGBT is shown in Figure 5.
Junction temperatures under several operating conditions are estimated. In the estimation a 500kW electric power conditioning system with six IGBT modules (two IGBT modules per phase) is considered. Coolant flow rate is set to 8 litres per minute/channel. There are 3 flow channels and a serial channel for 2 IGBT modules. Total flow rate is 24 litres per minute. Coolant temperature is assumed as 50°C Tj. Estimated results show that according to its low thermal resistance, the new Hitachi direct liquid-cooling IGBT module can operate over 5 kHz switching frequency with the proposed flow channel diagram.
As a result of extensive research and development a new Hitachi direct liquid-cooling IGBT module has been developed. The new module uses an integrated pin-fin base plate to reduce the thermal resistance from IGBT chip to coolant without the use of thermal grease. The pin-fin layout and channel cover jacket design have been optimised by fluidthermal simulation. Coolant leakage reliability is also optimally designed by stress simulation. The Si3N4 substrate and RoHS bonding technology are developed to ensure high reliability and long lifetime of the IGBT module. Due to its low thermal resistance, approximately 65% of the thermal resistance of conventional indirect liquid-cooling IGBT module, a 500kW power conditioner with 6 IGBT modules can operate at 5 kHz or double typical application frequencies of 2.5 kHz. Additionally to offer support for many wind and solar power applications Hitachi will also be launching 600v and 1200v versions of the new liquid cooled direct cooling IGBT module in the near future.