‘Wet systems’ for automotive designComments Off on ‘Wet systems’ for automotive design

‘Wet systems’ for automotive design

This article was first published in Components in Electronics, February 2014, Page 16 – you can download a copy here.

Ian Loaders looks at how ‘Wet Systems’ can address the thermal performance and total cost of ownership needs of automotive designs

The landscape has changed significantly in recent years in terms of both the challenges and the available technologies for solving thermal management issues in high volume automotive applications. In this sector it is essential that engineers get the heat management of their new designs right not only from the perspective of performance, but in terms of material cost, assembly cost and time, repeatability and long-term stability. So called ‘wet systems’ are emerging as an economical approach suited to high volume automated assembly. There has been as unrelenting increase in the number of electronics systems found in mass production vehicles with new technologies being used to enable the implementation of a myriad of diverse systems and modules that support comfort and convenience, safety, powertrain and infotainment features. Electronic modules and subsystems are now a key element for improving fuel economy, lowering emissions, improving reliability and enhancing the overall driver experience. Mechanical components are being replaced by electronics. For example, there’s brake by wire (BBW) and electronic power assisted steering (EPAS) and of course the carburettor, though a thing of wonder to automotive purists, has, since the 1980s been replaced by a ‘black box’ of electronics – the engine management unit – as a means of ensuring the optimum mix of fuel and air in response to driver throttle inputs; which, just to reinforce the point, are now communicated from the driver’s foot to the engine via a position sensor not a cable! The electric /electronic share of value-added to a modern vehicle is currently at around 40% and this increases to 75% fora hybrid or pure electric vehicle. Hybrid models like Toyota’s Prius have been around for some time and now 100%electric offerings from volume manufacturers including BMW and Nissan demonstrate that the revolution that will see the internal combustion engine ultimately replaced by battery powered vehicles is gaining significant momentum. So what does the massive and unrelenting increase in electronics modules, systems and sub-systems on vehicles mean to the often considered niche market of thermal management? In short, thermal matters should be considered as early in the design process as possible and as an intrinsic part of the development rather than a ‘problem’ to be solved at the end. This will ultimately help the achievement of a cost and performance effective solution. Today the demand for electrical energy in a car has risen above 2000 W. In the next decade this can be expected to rise by something in the region of 300% with the10,000 W vehicle not an unrealistic possibility. This huge power demand is causing the automotive industry to push towards the move from a 14 V to a 42 V system to satisfy the power needs of the vehicle. Inevitably, more power also means more potential thermal issues to consider and deal with. In the 1980s engineers were offered the wholly unsatisfactory approach of using thermal grease, or mica and grease where the application needed electrical isolation in addition to heat transfer between power device and the heat sink or enclosure wall. This approach was messy, time consuming and completely unsuited to volume application and achieving good levels of repeatability. Next came the thermal pad, a silicon based material loaded with heat conducting particles that could be die-cut to match device footprints and compressed between for example the tab of a vertically mounted TO220 and the heat spreading surface. This approach answered the repeatability problem and was also clean and easy to assemble. However pads do not easily lend themselves to high-speed automated assembly. Thermal pads in their many guises are still the mainstay in many high and medium volume applications in sectors such as audio equipment, motor drive controllers, military and aerospace plus many others, and indeed for some niche automotive applications where they represent an option that merits consideration. For high volume applications, as well as the pre-requisite of adequate thermal performance, the need is for a low solution cost (total cost of ownership or TCO) –both in terms of the component part and the time/money to assemble it into the product. Rapid assembly can be achieved using automated equipment which also delivers a high degree of repeatability. Robustness to survive the rigours of an automotive environment – especially if situated under bonnet, and stable performance over time, are also critical fora good solution. As most automotive modules are ‘sealed’ units, any malfunction sees the whole unit being replaced rather than individual components therefore the ability to be able to rework the thermal interface is most often not a requirement of the design brief.

Getting it right

So called ‘wet systems’ are emerging as an approach for designers tasked with designing automotive systems and needing a thermal management solution. They have a low material cost, lend themselves to fast automated placement and have excellent and stable thermal performance. Wet Systems are usually supplied in a choice of either cartridges for manual low volume or prototyping applications, or bulk pails for high volume production using standard automated dispensing machines. Other material placement approaches such as screen printing or pad printing can be adopted and automated depending on the specific needs and volumes of the application. The silicone based materials are available in a number of formats including single part air, raised temperature or two-part chemical cure where one of the components acts as the catalyst. Certain materials in this category are also designed to provide a tough structural elastomeric bond as they cure which attaches the heat generating component to the heat sink or chassis. This can take the place of mechanical fixings such as a clips resulting in the further speeding of assembly and a reduction in the bill of materials (BoM).Being silicone-based affords wet system thermal interface materials such as UniPutty (no structural bond) and BondPutty (structural bond) offer a very wide operating temperature range. This is often a stipulated requirement in automotive applications where the module is situated close to the engine or other location with a high ambient temperature. Bond Putty 4000 for example has a specification of -55°C to +200°C.The in-application thermal performance of wet system interface materials can be very high; in the case of BondPutty 4000the thermal conductivity of the material itself is 4.0 W/mK, and for UniPutty 2000around 2.0 W/mK. The fact that there is a very thin bond line – just enough to suit the flatness of the device /heat sink assembly – maximises the heat transfer across the interface. In applications that need a degree of electrical isolation between device and heat sink, materials are available with small glass beads dispersed in the silicone that act as a compression stop between the mating parts to guarantee a minimum bond line thickness and prevent metal-to-metal contact. It may appear at a casual glance that thermal solutions have gone full circle as thermal greases were replaced by elastomeric pads and now we have the emergence of paste-like wet system materials. However, these new materials are far removed from greases in that they are available in a wide range of non-slumping thixotropic formats with various degrees of thermal performance. With versions that cure to form a bond, further cost and time savings can be achieved, and ability to apply them using automated equipment adds to economical nature of the technology and the total cost of ownership for the customer.