Managing heat in EVs. With materials research and heat-resistant power electronics, how thermal management systems are advancing EV adoption
Electrical vehicles become connected and more and more digitalized and electrified. All creating electric heat waste in PCB housings, in the electric motor and in the batteries. This has increased the need for efficient thermal management.
TIM’s are needed everywhere in automotive applications to remove the created waste heat from electronics – e.g., digital components, electric powertrains, etc. Depending on the situation and overall design, either thermal pads, filler materials, or pastes can be used to ensure good heat transfer by filling out the mechanical tolerances between components and enclosures.
TIM’s are for example needed in the following automotive applications:
- LED lighting
- Power inverters
- Camera modules
- Battery cooling
- ECU modules
- Radar modules
- Infotainment systems
- Electronic instrument clusters
- Connectivity (5G)
Traditional thermal interface material (TIM) can be divided onto a few different subgroups that all have its own advantages and disadvantages.
Thin bond line interface materials will give the lowest thermal resistance and achieve their performance mainly by two mechanisms:
- by wetting out the surfaces in the interface extremely well, thereby efficiently displacing the air in the interface creating a very low contact resistances between surfaces and TIM
- by allowing the interface surfaces to get very close to one another, thereby reducing the distance that heat is conducted through the TIM, thereby reducing the bulk resistance.
The main drawback is that these types of materials are not designed to take up mechanical tolerances and may degrade over time if thickness is more than about 0.5 mm.
Microscopic air voids give high thermal resistance in the contact between two surfaces. TIM is filling up the void and removes all air and heat can flow much more efficient.
Thermal pads are TIMs in form of cured sheets that can fill up gaps in the range of about 0.25-5mm. They come in a vast variety of material types, with different conductivities, pressure/deflection responses, harnesses, tackiness, electrical insulators and so forth, each available in a range of thicknesses and other options. They are typically a cured silicone gel filled with high loading of thermally conductive particles. Higher filling levels will reduce mechanical properties so there is normally a trade-off between high thermal performance and mechanical properties. Pads are less suited to automatic volume production as it normally involves manual handling to attach pads. Efforts to use automatic pick and place systems has been done but the diversity of pad dimension and properties makes this rather hard to execute in an efficient way.
Dispensable TIMs have the advantage that they are volume production friendly and can fill up gaps from 0.2mm and above. This type of material divides into two subcategories: the two-part cure in place filler, and the one-part non curing filler. One of the foremost advantages of either filler type material is that as they are entirely viscous during assembly, they can be deflected to their minimum thickness, irrespective of the original thickness. This makes it possible to optimise the design for much thinner bond lines than with pads, even when tolerance spans are large. Due to high levels of thermal filler particles the 1p fillers can normally have higher thermal performance as the mixing step required in 2p material needs a lover viscosity to handle effective. In automotive applications care must be taken that the non-curing materials is more sensitive to environmental stress. Thus vibrations, mechanical and thermal stress can over time increase thermal contact resistance.
For battery application the main function of TIM is mainly to keep a constant and even temperature. As volumes and gaps are larger it will be more sensitive from outside impact such as temperature variations, on/off cycles and vibrations, thus long-term stability calls for a cured 2p material.
The growth of mass market for EVs as well as the very broad types of applications in EVs create need for optimized thermal interface materials that typically will be somewhat different than what was traditionally used. The reasons for this are discussed below.
As it is high focus on productivity the main materials of interest should be dispensable.
For use in high-capacity batteries there is several cell formats and battery designs but commonly several kilos of TIM can be needed. Traditionally Al2O3 has been used as thermally conductive filler particle in for the most cost-efficient TIM materials, however with the drawback of having a density of 4g/cc giving TIM density of >3g/cc and being very abrasive in high-speed dispensing. To achieve properties such as Lightweight, fast processing, low cost and thermal performance new filler systems may need to be developed for best performance in all aspects.
On the other hand, in electronic devices, miniaturization increases power density. When a device is made smaller, the heat sources in it – conductors, components, all the places where electrons encounter resistance – come closer together. More capable devices inevitably consume more power. Consequently, they also generate more waste heat. Thermal pads have a role here, but ultimately dispensable material will allow smaller gaps and a more efficient production. Traditionally the best 1p dispensing materials hade thermal conductivity in the 1-6W/mk. However recent developed material is now close to 10W/(mK).
One issue with these materials used in new types of applications is the uncertainty of their lifetime performance. TIM’s for EVs are exposed to a variety of environmental conditions and with an expected lifetime of at least 10-15 years. Typically, there will be thermal cycles, power cycles, vibrations, and mechanical shocks as well as environmental aspects with large variations in ambient conditions of temperature and humidity.
Newly designed TIM’s call for proper and relevant testing of long-term stability with a focus on both mechanical and thermal properties. Most important to know, is that there is no global standard for testing and qualifying TIM’s. Most suppliers and users have their own testing schemes, and there are typically big variations in how these tests are performed and evaluated. Normally some if not all of the following methods are considered.
Temperature cycling intended to simulate thermal stress from variations in environmental temperatures as well as variations in temperatures from on/off cycles.
Climate/humidity storage simulates how TIMs are affected by high temperatures and humidity.
High temperature storage that simulates the aging of the material itself.
Vibration and shock testing that simulates the vibrational stress exposed to materials and components when vehicle is in use. To cover both low and high frequencies a random vibration spectrum is most suitable.
Jörgen Gustafsson has more than 15 years’ experience of materials research, process development and product development within photovoltaics, EMC and TIM industry. Since 2012 he has been working at Nolato in Hallsberg Sweden, primarily with development of thermal interface materials.