20 AUTOMOTIVE DESIGN
Increasing electronics content sets new thermal challenges
Materials and approaches to solving thermal, grounding and shielding applications in vehicles have evolved at pace in recent years. Eoin O’Riordan reports.
T
he quantity value and complexity of electronics content in passenger vehicles continues to rise. This brings a corresponding increase in shielding, grounding and thermal management
challenges for the automotive design engineer. Vehicle electronics can loosely be split into ‘in
cab’ and ‘out of cab’ applications. Equipment inside the passenger compartment includes well established modules such as heating, ventilation and air conditioning (HVAC), instrument panels and radios. There is also a host of new and emerging functionality such as infotainment, satellite navigation and head-up displays. Similarly ‘out of cab’ applications comprise established applications such as engine management ECUs plus newer developments such as braking ECUs and a diverse array of sensors units to feed information to the ECUs. The general trend in automotive is for newer concepts
and technology to be introduced in high-end vehicles and to then filter down to smaller high volume vehicles. Materials and approaches to solving thermal,
grounding and shielding applications in vehicles have evolved at pace in recent years. This is in response to the new applications, and also due to companies, such as Chomerics, developing innovative new compounds,
materials and processes. When combined with many years of problem solving experience, this enables a solutions approach that gives lowest total cost of ownership to be adopted.
Thermal management
Electronics modules in passenger vehicles, particularly those mounted out-of-cab, are often sealed to prevent moisture ingress. This increases thermal problems due to the fact that air is prevented from circulating effectively and so convection cooling is inhibited. Under bonnet modules are often exposed to extreme temperatures, this coupled with the smaller physical size of the latest generations of electronic components results in a minimal footprint through which to dissipate heat giving the automotive designer considerable thermal problems. Early generations of ECUs used through-hole power components; the accepted approach in this situation was to couple the metal tab of vertically mounted power devices to the metal enclosure or other heatspreader using thin electrically isolating thermal pads. However, as PCB designs moved to 100 per cent surface mount designs this approach was no longer appropriate. Insulated metals substrate (IMS) PCBs provided an option, but these can be costly and often still need coupling to a larger heatspreader. The emergence and evolution of thick, soft thermal
gap fillers in either die-cut sheet or form-in-place formats such as Chomerics THERM-A-FORM (T630 and T652 types) range has enabled engineers to effectively couple surface mount devices to a chassis or enclosure. At the same time this approach can often simplify and speed module assembly by removing the need for some mechanical fixings.
Grounding
Electrically conductive elastomers can offer a beneficial alternative to mechanical fixings for the purpose of grounding PCBs in automotive ECUs. By sandwiching materials such as Chomerics CHO-SEAL 6371,1285 and 6305 elastomers between the PCB and chassis, a low resistance path to ground can be created that saves on assembly time and cost versus mechanical approaches. The use of electrically conductive elastomers can
allow low closure force assembly methods to be employed and may also simplify the tooling requirements reducing the cost of the ECU housing further still.
Shielding Fig. 1. Form-in-place gaskets can provide a low cost shielding solution.
With vehicles now containing very large amounts of sensitive electronics, the issue of protecting modules from
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