While vehicle PCBs have traditionally been hardly more than a supporting character on the automotive technology stage, as vehicles884547-3787_400/0/m-update_header/1bii02125/2023-012014.bhtml#6767:" data-ecid="2023-012014.bhtml#6767">,887646-482001:2023-012014.bhtml#6767">_01">,884384-15451:2023-012014.bhtml#6767">87745:2023-012014.bhtml#6767">76,665—a costly evolutionary process—have evolved into sophisticated electronic platforms, the role of PCBs has become critical in terms of performance as well as safety, and reliability. Today's cars include scores of Electronic Control Units (ECUs) that control everything from engine timing and braking to infotainment and advanced driver-assistance systems (ADAS). Hence, the design of the PCBs that make the backbone of these ECUs is really important. Automotive PCB design solutions are designed for the severe requirements of the automotive world to ensure that these complex electronic systems operate reliably for the lifetime of the vehicle.
The challenges are significant. Unlike consumer electronics, automotive PCBs are subjected to extreme temperature variations, continuous vibration, humidity and chemicals, and possible electromagnetic interference, all while needing to maintain signal integrity and power delivery for high-performance computing. Reliability is a must; a hiccup on a single ECU can be catastrophic. In this article, we will explore the fundamental automotive PCB design factors, the materials, techniques, and considerations that go into creating reliable automotive PCBs, which help power the best automotive electronic control units to build the smart, connected, and electric vehicles of the next generation.
Material Selection for Harsh Environments
Matters — Automotive PCB — Strong automotive PCB starts with right materials. For applications tucked up under the hood where temperatures can reach the boiling point of the materials, standard FR-4 laminates found in many consumer electronics just aren't enough. Automotive grade PCBs often incorporate high-performance materials such as polyimide or specialty epoxy resins at a higher glass transition temperature (Tg). These ensure structural and electrical integrity at high temperatures without the delamination and allow a stable operation.
Which means that, in addition to the traces needing to be wider to withstand high currents (in particular for electronic control units in power electronics within electric vehicles (EVs) or engine control modules). Thermal conductivity of the substrate is another significant property. The use of improved thermal dissipation materials helps to pull heat away from higher performance processors and power components which is key to avoiding thermal runaway and providing reliable long-term operation. Solder mask and Surface Finish selection is also unique, needing resistance to humidity, salt and various other automotive fluids, providing a shield on the delicate circuitry.
Thermal Management Strategies
The most important feature of automotive PCB design is excellent thermal management. Typically, ECUs are in spaces with unrestricted airflow, which causes a great deal of heat generation. Overheating has the potential to ruin components, decrease performance, reduce lifespan, and cause catastrophic failure. To combat this, designers have many tricks in their toolkit. One of the main ways is using thermal vias, which are plated through-holes that go directly under heat-generating components such as microcontrollers and power MOSFETs. These vias serve as environments: one side, from component side of the board, to inner ground planes or even dedicated metal core layers, where they can more effectively disperse heat.
Many designers choose metal-core PCBs (MCPCBs) or insulated metal substrates (IMS) for the high power applications like control units for EV inverters. These substrates consist of a bottom layer made of aluminum or copper, which provides great thermal conductivity. The metal core, which is what we refer to as the heat spreader heat sink, spreads the heat very quickly across its whole cross-section so as to prevent hot spots. Along with this, the PCB layout itself is designed for optimal thermal performance, with high heat components carefully positioned to connect to copper pour areas and to large ground planes, acting as heat sinks. A lot of simulation tools are used in the design stage to simulate its thermal response to avoid prototyping again.
Signal Integrity and EMC (crosstalk)
Signal integrity is also one of the main priorities, because modern ECUs need to process high-speed digital signals, as well as very sensitive analog readings. Consequences like crosstalk, reflections and attenuation, can cause data corruption and failure of the system. Impedance is another important parameter for clean signal integrity, and designers should be aware to calculate proper trace width, spacing, and dielectric material properties. High-speed interfaces also be implemented with differential pair routing which provides noise immunity and can implemented through technologies such as CAN FD, Ethernet, and LVDS. Continuous ground planes are designed for stable grounding and provide a low-impedance return path for the signal.
Electromagnetic Compatibility (EMC) is a signal integrity accomplice. An ECU also should not be susceptible to any external interference and clearly must not emit excessive electromagnetic radiation itself, as a vehicle is an electrically noisy environment. The first line of defense for EMC is by means of PCB layout. Examples of this type of approach are partitioning the board into analog, digital, and power supply sections to minimize noise coupling, placing guard traces and/or ground shields around sensitive circuits, and the use of filtered connectors to filter noise entering or leaving the board. Antenna PCB design and layout are essential foundations to pass hot automotive EMC standards such as CISPR 25.
Robustness and Reliability Considerations
Automotive is truly another beast, with a design lifecycle commonly exceeding 15 years and the requirement for unprecedented reliability. PCBs need to be mechanically robust to withstand the conditions in which they will be used, mainly vibration that can lead to fatigue in solder joints and fractures of traces. For mechanical robustness, designers refrain from placing big or heavy components like transformers and capacitors near the center of the board where the flexing motion is the greatest. Rather, they're located beside supporting studs. Connectors also exhibit strain relief, and conformal coating — a thin polymer layer — protects the entire assembly against moisture, dust, and chemical contaminants.
Principles of Design for Reliability (DfR) are embedded into the process. This includes conducting complete DFM and DFT walks. Design for manufacturing (DFM) makes sure that the board can be manufactured easily and without defects, while design for test (DFT) places test points and structures to enable automated testing after assembly and quickly catch faults. In safety-critical systems, redundancy can be built into the circuitry. These are all part of the "zero-defect" quality target that the automotive industry aims for, so that each ECU will reliably perform over the full service life of a vehicle.
Becoming EV-ready By Electric and autonomous vehicles
PCB design and testing in the context of electric and autonomous vehicles have undergone significant changes. Electric Vehicle electronic control units (ECUs)—such as those controlling the battery management system (BMS) and traction inverters—must process extremely high voltages and current. That requires more creepage and clearance—the distance between high voltage traces and any other circuit—to avoid arcing events and improve safety. Such PCBs would typically need the use of advanced materials with higher CTI values in order to withstand electrical breakdown.
The computational requirements for autonomous driving systems are massive. Scrambling through this data from LiDAR, radar, and cameras means the ECUs are little above high-performance servers on wheels. This necessitates HDI PCBs with both microvias and fine-pitch ball grid array (BGA) components, as well as multiple rounds of sequential lamination. The great challenge that defines managing signal integrity at the multi-gigabit data rates for sensor fusion, and thermal management of powerful system-on-chips (SoCs). On the other hand, the PCB provides a split second driving decision and thus becomes a key enabler of low-latency high-bandwidth communication which is required to do this safely!