17.05.17
17.05.17
IGBT’s (Isolated Gate Bipolar Transistors), in common with all power semiconductor devices, are not 100% efficient and most of the “lost” power manifests itself as a dramatic temperature rise of the silicon chip(s). The temperature rise needs to be managed as efficiently as possible by minimising the thermal resistance between the chip(s) and the heatsink on which the device is mounted.
Within a discrete package or a module, the manufacturer will use various packaging techniques that will optimise the transfer of heat away from the chip(s) and into the heatsink. Their methods will include; chip bonding (sintering, soldering), minimising thickness of substrate (whilst maintaining necessary isolation), use of advanced ceramics within Direct Bonded Copper (DBC) substrates (for example Aluminium Nitride), elimination of module base plate etc. But it may surprise you that with a typical IGBT module mounted on a heatsink, the Thermal Interface Material (TIM) used between the module base and heatsink, makes up over 50% of the total thermal resistance from chip to heatsink.
With the TIM being more than half of the total thermal resistance you may well question its benefits. There is a very good reason why it needs to be used; fresh air is a very POOR conductor of heat.
“But my heatsink is very flat and the module base is very flat, where is this air?”
No heatsink is perfectly flat, in fact a degree of “roughness” is desirable and no module base is perfectly flat (modules with a baseplate are deliberately made slightly convex to counter the bi-metallic effects of flexing under heat stress). So inevitably, air pockets occur between module base and heatsink and these need to be filled with a thermally conductive material. Choosing the right material and applying it correctly is therefore extremely important to keep the total thermal resistance to a minimum, avoid hot spots and get the maximum performance and reliability from the power module.
Six pack IGBT modules can have up to twelve holes for fixing bolts. These bolts not only have to be the correct type and diameter as recommended by the module manufacturer, they have to be tightened to the correct torque setting and in the correct sequence to avoid distortion of the module and risk cracking of the substrate. The bolts also have to be the correct length and the tapped heatsink holes the correct depth to avoid “bottoming out” which would fool the assembler into thinking that the correct torque setting had been reached resulting in insufficient pressure of the module onto the heatsink.
It is very important to apply thermal material evenly and to the manufacturer’s recommended thickness. Ideally the grease should be screen printed on to the heatsink or rolled on with a hard rubber roller. A depth gauge or comb should be used to check that the thickness is within tolerance; too much or too little will greatly influence the thermal performance of the module. The heatsink and module surfaces must be absolutely clean and free from debris prior to grease application to again avoid the risk of cracking the substrate.
Power Products have been manufacturing power electronic assemblies for over 30 years. This experience and knowledge of how to correctly mount modules to heatsinks, the right choice of TIM for the application and in-house precision machining of heatsinks, combine to ensure the most reliable and efficient power assembly.
Many of our Semikron IGBT modules can be supplied with pre-applied thermal grease to the correct thickness; this is a huge benefit as it not only saves production time, it ensures optimum thermal performance.
Power Products are also able to choose from a vast range of TIMs from Aavid Kunze for use in assemblies, and offer these materials for sale to customers who use them in their own production processes.
Contact us for assistance with IGBT assemblies or any questions regarding thermal management of power electronics you may have.
