Materials and approaches to solving thermal, grounding and shielding applications in vehicles have evolved at pace in recent years. Eoin O'Riordan reports.
The 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.
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.
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.
With vehicles now containing very large amounts of sensitive electronics, the issue of protecting modules from damage or malfunction due to spurious electrical signals has gained importance.
A wide choice of shielding materials is available to design engineers; the main two categories relevant to automotive are moulded and extruded elastomers and form-in-place (FIP) gaskets.
Moulded and extruded elastomers can provide an environmental seal with shielding properties. They typically combine an elastomer - usually silicone - with a conductive filler.
By using these materials almost any finished shape can be achieved. Overmoulding the material onto plastic or metal components such as ECU housings is also possible. This provides a one-piece shielded housing that can simplify and speed assembly.
Dispensed, electrically conductive FIP gaskets provide the lowest total cost of ownership for small cross-section and complex pattern applications. Chomerics FIP materials can reduce the installed cost of an EMI gasket by up to 60 per cent. The corrosion resistant nature of FIP materials provides protection against galvanic activity and in many applications eliminates the need for plating or secondary environmental gaskets.
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Eoin O'Riordan is Marketing & Technology Manager wth Chomerics Europe (a division of Parker Hannifin), High Wycombe, Bucks, UK. www.chomerics.com