Cool, Calm And Collected

Making the most of natural heatsinks in automotive applications—we look at a thermal material that can help.

There is no doubt that power semiconductors in automotive applications help car designers to reduce vehicle weight and system cost while simultaneously improving the efficiency and capability of the vehicle's systems. However, it falls to subsystem designers to ensure effective thermal management of power semiconductor solutions.

A recent example of this is a custom fan speed controller developed by AB Automotive Electronics using thermal materials from The Bergquist Company. The fact that MOSFETs make a good replacement for heavy relays or switched resistor packs is already well recognised. Power MOSFETs can efficiently support a wider range of control modes for HVAC (Heating, Ventilation, Air Conditioning) and other subsystems. However, because power switches may be mounted in confined spaces or close to heat sources within the car, special consideration must be given to thermal management.

One example can be seen in the growing demand for automatic climate control systems that are increasingly expected by European car buyers. The trend is driving the development of advanced fan speed controls to enable automatic control of multiple operating modes and an increased range of fan speeds. AB Automotive Electronics (ABAE) has developed an advanced fan speed controller (FSC) to support these next-generation functions. During its design and development, engineers at the company sought help to overcome the predicted thermal dissipation issues. The power dissipated within the MOSFET switching element of the FSC needs to be removed swiftly to prevent the die temperature exceeding 144°C. At this temperature the MOSFET's internal thermal protection mechanism will shut down the device. This prevents damage to the device, but would also prevent the driver from using the climate control system.

To assist heat removal, ABAE's engineers decided to mount the FSC assembly on the evaporator of the vehicle's air conditioning system. This would act as a heatsink, assuming heat could be dissipated effectively from the PCB. However, because the poor thermal conductivity of standard FR4 PCB materials would restrict heat transfer to the metal body of the evaporator, a more thermally conductive substrate was needed. So, the team looked at insulated metal substrate (IMS) technology.

IMS technology is relatively easy to use as a direct replacement for FR4 PCBs as it has minimal impact on PCB layout and requires only minor adjustments to the reflow profile. In addition, there is little impact on the process parameters either for solder paste printing or component placement. Thermal Clad IMS from Bergquist comprises an aluminium or copper base layer, which is thermally coupled to the circuit layer by an electrically isolating, thermally enhanced dielectric. The dielectric layer also bonds the base metal and circuit foil together. Heat dissipated by top-side mounted components is absorbed by the circuit layer and transferred efficiently across the dielectric to the aluminium substrate, which has a high thermal capacity. As a result, designers can raise the watt-density of an assembly, reduce printed-circuit board size, and eliminate or reduce the need for heatsinks and other mounting hardware, simply by specifying Thermal Clad PCB material instead of FR4.

By using Thermal Clad IMS as the PCB substrate, ABAE were able to maximise heat flow from the switching element of the FSC to the metal body of the evaporator. Using the evaporator as a heatsink for the FSC presented an efficient and convenient way to deal with the heat generated by switching and eliminated the need for any additional heatsinking. In addition to Thermal Clad IMS technology, ABAE also used Hi-Flow 225F-AC phase change thermal interface materials from Bergquist to maximise thermal coupling between the FSC assembly and the evaporator. Hi-Flow material is easy to handle at room temperature, and can be placed by automatic pick and place machines. However, when Hi-Flow reaches its phase-change temperature, it changes from solid and flows to ensure a total wet-out of the interface without overflowing. As such, it is much easier to control and use than thermally conductive grease, but behaves in much the same way at elevated temperatures. As the material flows, it closes any air gaps that may exist between the two assemblies. The phase-change temperature is adjusted by controlling the material composition.

Automatically placing a thin Hi-Flow layer on the surface of the FSC before attaching it to the evaporator allowed ABAE to eliminate the effects of unevenness in the surface of the evaporator housing. Left unfilled, these would have impaired heat transfer from the FSC to the evaporator. Surface-mount components can be placed on Thermal Clad in the same way as for standard FR4. However, some extra consideration of soldering issues help to enhance reliability in the field. In Thermal Clad assemblies, the solder fulfills two important extra functions; it must act as a good heat transfer medium and it must withstand thermal cycling. In order to transfer heat effectively, the solder alloy must display high thermal conductivity as well as good wetting, with no voids forming.

When assembling the FSC onto Thermal Clad IMS, ABAE made changes to the pattern of solder paste depositions beneath the FET, as well as fine-tuning the reflow profile to suit the revised thermal characteristics of the assembly. Bergquist had already analysed reflow requirements and ABAE was able to use a database of reflow profiles for use with Thermal Clad.

For power system designers, the high temperatures of the automotive environment will always be a challenge; many try to take advantage of naturally occurring heatsinks such as nearby metal housings or surrounds. Thermal materials like Thermal Clad IMS and Hi-Flow phase change material can deliver an effective solution.

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