CTI Mag: What ECU Concept is the right one for your Project?
Ing. Philipp Neumann MSc, MELECS EWS GmbH
What options are possible?
When you design an ECU, the most important thing is to never lose the clear vision of what the customer requires and what options apply to those requirements. Will a standard ECU be enough, or do you need to go all the way and opt for a smart ECU system with an integrated motor?
Issue: #December 2017
Step 1 – Standard ECU
Choosing a standard ECU concept is the most common way of building control units in a car. This concept has many advantages but also some serious disadvantages. On the pro-side, the development of the ECU is very fast and the motor is connected with a cable, which means that the motor does not heat up the ECU. There is also no need to define an internal interface between ECU and motor, which speeds up development time and minimizes the costs. Normally this ECU consists of five major parts that need to be designed:
The housing can be made of different materials. During development, it is important to keep an eye on what the purpose of the ECU will be, and where it will be installed in the car. Some questions must be asked: what is the ambient temperature range, what is the water tightness level and does the ECU need to survive stone chips. Usually, the answers to these questions are -40° to 140 °C ambient temperature with IP6K9K and yes, stone chips will occur on the ECU in combination with high g vibration. Two options are available to meet the stone chip requirements: The first one is to design casualty fins that stop the stones and may break away when an impact occurs. The second and preferred option is choosing a high-quality material that can survive the stone chip test, for example PA66GF30.
Figure 1 ECU options
2) Sealing / Vibration Damping / Thermal Springs
With the IP6K9K requirement, the ECU needs to withstand a highpressure cleaner, which requires a good sealing concept. Normally you also have a vibration requirement on the ECU that comes from the combustion engine and/or the gearbox(es), which is up to 30 g. Without additional vibration damping features, the PCBA will not win the battle against vibration. To solve those two points, the fastest way is to put an O-ring inside the ECU for sealing and to pot the vibration sensitive parts to fixate them. This is ok if a small number of ECUs are required, but at higher production lots, you need a more stable and higher-quality process as well as a cheaper and faster assembly of the two features. A good, proven option is to include these features inside the housing and create a 2K molded part. The shell of the housing is made of PA66GF30 and the sealing, thermal springs and vibration damping features consist of a second softer material. A good start is to use red liquid silicon rubber (LSR) that is molded inside the PA66GF30 housing in a second shot. This creates a high production quality, removes the manual assembly process of an O-ring and eliminates the potting process. To thermally connect the PCBA to the back cover, you can either apply thermal glue between PCBA and cover or include some springs in the LSR that press the PCBA to the cover with a defined force. This eliminates the need for a thermal glue, because force is the best heat conductor between two materials. The trick here is that you also need electrical isolation which could be easily integrated, with no cost added, into the PCBA by using a layer of prepreg FR4. That would result in a layerstack and a concept as shown in Figure 3 for a four-layer PCBA, but this could be scaled to any layer count.
Figure 2 Housing with special features Figure 3 Layerstack example
Since the ECU is completely sealed, another problem occurs. During thermal cycles, there will be temperature differences between the outside and the inside of the ECU. This will result in suction and condensation, which could damage the PCBA. To prevent this it is common to include a vent inside the housing. This vent is made of polytetrafluoroethylene (ePTFE) material, which is ultrasonically welded to the inside of the ECU. This vent allows vapor to escape from the ECU and pressure constantly remains alike inside and outside the ECU while leaving it completely IP6K9K watertight.