Thermal Design and Properties of IGBTs – More Heat Than Light
Rapidly increasing power densities in electronics make efficient heat removal a crucial issue for progress in information, communication and energy storage technologies, especially in the transportation sector. Development of next-generation integrated circuits (ICs), three-dimensional (3D) integration, and ultra-fast high-power density communication devices means the thermal management requirements are extremely demanding. Efficient heat removal became a critical issue for the performance and reliability of modern electronic, optoelectronic, photonic devices and systems. Thermal interface materials (TIMs), applied between heat sources and heat sinks, are essential ingredients of thermal management.
Thanks to the recent improvements in handling large currents at high voltage and at high switching frequencies, insulated gate bipolar transistors (IGBT) have almost completely replaced bipolar power transistors (BPT) and they are challenging the position of gate turn-off thyristors (GTO) in their traditional fields of application. In recent years, the need to increase reliability of high-power IGBT multichip modules has been one of the most powerful drivers that forced engineers to design new products, especially targeted for traction, power transmission, and power distribution applications.
Nowadays, rapid developments in markets served by the electronic industry, including renewable energy, automobiles, industrial equipment and infrastructure projects, have placed great challenges on the thermal management capability and requirements of high-power semiconductor devices, which have been widely used in harsh environments with elevated temperatures beyond 200 °C. The IGBT module characteristics needed differ according to the particular market, but higher efficiency and size reduction are common requirements. There is increasing demand in the automotive industry for insulated-gate bipolar transistor (IGBT) modules, as they are used in electric vehicles (EVs), where static thermal flux is up to 300 W/cm2., An effective approach to reducing junction temperature is the application of good thermal conductive materials.
Thermal Grease and Phase Change Material - A Comparision of the Most Common TIMs
The connection between the power module and the heat sink is both centre piece and bottleneck at the same time. Here materials are often used that cannot cope with the demanding environment found in power electronic applications. The considerable amount of heat generated by the power modules is cooled by connecting the module to a heat sink or cooling plate – however the heat cannot be transferred to the heat sink effectively due to the voids caused by distortion and surface roughness of the module base plate. To fill the voids to transfer the heat effectively, the most common thermal interface materials used are thermal grease and phase change material (PCM):
The advantages of thermal grease are its ability to fill the interstices and eliminate interstitial air. Thermal grease is often made of silicone or hydrocarbon oils and is commonly found in desktop PC between the processor chip and the heat sink. In general, the grease itself has very low thermal conductivity but it is enhanced by loading the grease with highly conductive particles (often metals and/or ceramics). Its low viscosity makes it possible to use a thin layer which gives low thermal impedance. However, thermal grease is difficult to apply during manufacture and requires care and attention into correct dosage preparation and cleansing. For volume manufacturing this raises questions of workability and reliability. Thermal greases do not contain any chemical curing properties, so they will not cross link to form a gel or a hard adhesive type material. Their uncured state, which gives them the ability to provide a low interfacial thermal resistance, also makes them susceptible to a variety of failure mechanisms during their service life – and ipso facto causes additional maintenance costs. The two main causes of an increase in the thermal resistance of a grease layer are grease pump-out and grease dry-out. The powering up or powering down of the device causes a movement between the die and the heat spreader (in-plane and out-of-plane), which tends to squeeze the thermal grease out of the interface gap. This phenomenon is referred to as “pump-out” and results in increased thermal resistance due to loss of grease material from the interface. Grease “dry-out” occurs due to the separation of the filler from the polymer matrix at elevated temperatures. The polymer matrix tends to flow out of the interface preferentially and results in ‘drying-out’ of the thermal grease. This results in increased in-situ thermal resistance of the material. Exposure to high humidity levels has also been shown to induce changes in the thermal resistance of a grease layer, primarily an effect of the filler and resin system employed and their response to high levels of moisture.
PCMs (phase change materials) consist of a mixture of suspended particles of high thermal conductivity, such as fine particles of metal oxide and a base material. Although phase change of the base material is implied, this is not the case. These materials do not actually change phase, but their viscosity diminishes so that they flow. The base material can be a natural material such as fully refined paraffin, a polymer, a co-polymer, or a combination of all of these. The base material is solid at low temperatures but behaves much like grease after reaching the “phase change” temperature (typically 50-90°C). Phase change materials can be supplied in the form of compound only, or in composite form in a specified thickness applied to a substrate. The advantage of this TIM is its high thermal performance at moderate contact pressures. Material flows throughout the thermal joint to fill air gaps and provide minimum thickness by allowing the mating surface to come into contact. When the joint becomes thin, the viscosity “prevents” pump-out from mechanical flexure of the interface surface. It handles easily for installation. Nonetheless, PCM also has a couple of disadvantages. Paraffin-based phase change materials can be very fragile and difficult to handle. They also tend to squeeze out of a gap from the device in which they are applied during thermal cycling, very much like grease. It requires compressive force to bring materials together and cause the TIM to flow. The desired phase change property limits the choice of polymer and filler combinations, limiting the thermal performance of these materials. Further disadvantages include low thermal conductivity, high changes in volume during phase change, flammability and they may possibly generate harmful fumes on combustion. Other problems which can arise in a minority of cases, are a reaction with the products of hydration in concrete, thermal oxidative ageing, odour and volume change.
Panasonic Soft-PGS – Solving Thermal Management and Challenges
Flexible graphite sheet, which is non-viscous, is an alternative to viscous TIMs. Instead of the TIM material flowing into microscopic surface features, graphite sheets rely on compression force to “push” the graphite platelets into the surfaces. Once connected to the metal surfaces under compression the graphite will not flow, shrink, or degrade, since graphite is crystalline carbon, the same element as diamond (see figure 1). The crystal structure backbone is also the thermal highway of the graphite sheet, with very high through plane thermal conductivity. Panasonic Automotive & Industrial Systems Europe has launched a highly-compressible graphite TIM called Soft-PGS (Pyrolytic Graphite Sheet) which enhances the thermal coupling between heat producing devices (heat sources) and heat dissipation devices (heat sinks).
Pyrolytic highly oriented Graphite Sheet is made of graphite with a structure that is close to single crystal. It is produced from polymeric film using a heat de-composition process. The hexagonal structure of graphite is arranged uniformly in a horizontal 2D structure (compare figure 3). Soft-PGS is a 200µm thick graphite sheet designed for use as a thermal interface material for IGBT modules. As Soft-PGS can be compressed by 40% it is an excellent solution for dramatically reducing thermal resistance between a heat sink and an IGBT module. The 200µm thick Soft-PGS sheet is easy to install, and has far lower labour and installation costs than thermal grease or phase change material. Soft-PGS guarantees thermo stability of up to 400°C and high reliability against intense heat cycles (-55°C to +150°C). Its thermal conductivity is guaranteed at 400W/mK for X-Y direction and at 28 W/mK in Z direction. Panasonic offers a wide range of standard sheets for different IGBT modules from various suppliers.
What solves Soft-PGS? Soft-PGS is designed by Panasonic exclusively for thermal interface materials – comparing with former TIMs convinces thanks to:
- high performance
- simple installation
- long-term reliability
Figure 4 shows the comparison of costs between different TIMs and Soft-PGS. Thermal Grease, though the initial costs are cheap demands high maintenance after installing. On the other hand for Soft-PGS, which is maintenance-free, long term use is possible with the initial installation costs only.
Panasonic’s current Soft-PGS line up is corresponding to six power module maker, as: Hitachi Power, Mitsubishi Electric, Infineon, Semikron, Fuji Electric and Littelfuse. For other power module manufactures support can be provided by request. Therefore as soon as costumers’ module information is provided, Panasonic is able to deliver the corresponding Soft-PGS solution.
TU Munich Used PGS For Their E-Racing Car, More on That in the Video Below:
Please compare: Panasonic
Please also compare: RS