Automotive Circuit Design Essentials

Key Takeaways

• Essential requirements for developing automotive circuits.
• Design tips to minimize RF interference in automotive circuits. 
• Manufacturing and reliability tips for automotive circuit operation.

Modern automobiles are now more digitized than ever, requiring advanced circuit design skill sets

Nowadays, automobiles are becoming more and more digitized, with everything from airbag sensors to safety and convenience features like backup cameras and lane assist. We are currently in an “automobile renaissance” with the incorporation of all these new electronic technologies in our vehicles.

However, in incorporating so many electronic features, there is always a risk of failure, making automotive circuit design that much more of a critical process. Electronic circuit simulation for functionality and reliability based on component tolerances, temperature variations, and other parameters becomes increasingly more important. Performing advanced yield and reliability analyses through simulations can mean the difference between a field failure and a successful rollout in automotive circuit design.

Automotive Requirements for Circuit Design

Modern vehicles have hundreds of ECUs, enabling virtually every functional element of the vehicle

There are typically hundreds of electronic control units (ECUs) found within newly developed vehicles. Assorted ECU examples include airbags, all-wheel drive, GPS navigation systems, advanced driver assistance systems (ADAS), entertainment systems, cameras, LIDARs, and other sensors.

A large amount of electronics creates an array of challenges in developing them, from communication issues to power distribution difficulties. Here is a selection of common problems encountered during automotive circuit design:

• Wiring connectivity issues that necessitate the use of “drive-by-wire” where mechanical linkages are replaced with arrangements of sensors and actuators.
• Power distribution and hybrid drives allow for more miniaturized high-density power electronics.
• Embedded system modeling that includes hardware architecture, software requirements, and functional requirements.
• Mixed-signal simulation due to the large number of analog sensors (oxygen, tire pressure, etc.) that require digital control loops. 
• Virtual prototyping that tests hardware and software compatibility before it is built. 
• Software validation that ensures complete system functionality and reduces costs from building hardware, subsequently addressing any issues that come up.

When designing for automotive circuits, it’s necessary to account for multiple combined factors you might not experience elsewhere. Specifically, automotive electronics are both cost and weight driven but must withstand harsh environments, retain relatively lower layer counts, and maximize reliability with a 10-year-plus life expectancy.

It is important that your PCB manufacturer is capable of producing especially small circuit boards, as most vehicles utilizing PCBs have limited space. Despite the small size, boards must be programmed to handle everything that is digitally displayed.

Strategies for Automotive Mixed-Signal PCB Layout

Due to the large number of sensors, you’ll have to be careful with your mixed-signal routing. Follow good mixed-signal practices with your return paths, such as isolating analog and digital components and routing return paths on separate conductors. Additionally, keep your signal loop areas as small as possible and avoid locating circuitry between connectors. Ensure your digital signal transition times are controlled and there are no gaps in solid return planes.

Automotive Circuit Standards

There are all sorts of automotive standards for circuits. In the 1990s, QS9000 was established for ensuring quality management systems. In the 21st century, a new quality administration system was created by world automotive manufacturers, defined as ISO/TS16949. This standard integrates requirements for the automotive industry and covers defect prevention, quality fluctuation, and waste reduction within the automotive supply chain. Many PCB manufacturers must achieve the ISO/TS16949 certificate prior to entering production in the automobile market.

RF Interference Circuit Design

Automobile circuit boards are especially sensitive to RFI, necessitating multiple precautions for smooth operation

In automotive circuit design, it is especially critical to be conscious of electromagnetic interference (EMI), also known as radio frequency interference (RFI). Multiple parts of the vehicle are vulnerable to RFI – even non-metallic body components such as bumpers or panels. Therefore, it is crucial to take measures to ensure the smooth operation of electronic components. Proper shielding and grounding will help decrease the risk of unwanted EMI and unintentional antennas.

Automobile Communication

The automotive high-speed communication bus (controller area network or CAN bus) is made to operate in harsh electromagnetic environments, making it a useful part of vehicle design. Regardless of the automotive networking protocol used, it is important to take measures to limit on-board ECU’s RFI exposure to ensure safe vehicle operation and function.

Crosstalk is also a major issue in automobile circuit design. Crosstalk can occur when energy from one line couples to communication buses in automobiles, thus degrading the signal integrity of the automotive network. Bundling cables and having multiple pairs in the wiring harness can result in crosstalk interference in automotive communication networks. Twisting pair cables is a good solution for limiting some of these effects. Additionally, compared to other automotive communication protocols, automotive Ethernet provides strong crosstalk immunity, signal integrity, and better performance.

CAN Bus Layout 

Due to comparatively lower speeds (by today’s standards), the layout for a CAN bus isn’t as intensive as other protocols that require Gb/s transfer rates. Nonetheless, it’s still important to place fast falling/rising signals and other switching nodes at a safe distance from data lines to avoid inductive coupling. Length matching is not too big of an issue in asynchronous hierarchies, but performance will always be better with short and direct nodes.

Manufacturing Features of Automotive PCBs

There are hundreds of automotive circuits found in vehicles, with many of them small, compact, and able to withstand relatively harsher conditions

Automobile anti-collision and predictive emergency brakes require a PCB to communicate with microwave high-frequency signals. For this reason, substrate materials with low dielectric losses, such as PTFE (polytetrafluoroethylene), are recommended. These substrates require distinct manufacturing capabilities such as special via drilling.

Automobiles generate more heat due to the high density of electronics and power. With greater numbers of hybrid and EVs being developed, advanced power transmission systems are required, furthering the necessity for heat dissipation and high-current capabilities. The thickness of copper leads and metals in multi-layer PCBs should be increased for this reason.

Cars now have many of the capabilities of smartphones, computers, entertainment systems, and communication systems, making them also necessitate high-density interconnects (HDIs). Electroplating technologies, microvia drilling, and interconnection technologies will need to be applied to your printed automotive circuit.

Automotive Circuit Substrates

Ceramic PCBs' high thermal conductivity and coefficient of thermal expansion make them especially useful in automotive (and aerospace) applications. This is because standard FR-4-based PCBs often exhibit flaws under extreme conditions such as high temperature and high pressure. Ceramic-based circuit boards, on the other hand, hold up better in these conditions, in addition to high vibrational or corrosive environments. Ceramics are also especially suitable for high power density circuit designs, furthering their potential usefulness for automotive circuits.

Flexible circuit boards, along with their variant rigid-flex boards, are commonly used in the automotive industry. This is because circuits in the automotive sector often experience a lot of vibrations. Specifically, “super flexible”  HA copper in the material-stackup. These circuits can then experience repeated vibrations without getting damaged.

Reliability Testing

It is critical to conduct multiple reliability tests to reduce field failures in automotive vehicles

In developing automotive circuits, reliability testing is incredibly important for success. The environment the PCB must operate in is variable, so there are multiple reliability tests that should be conducted.

• Thermal shock tests are required for the higher heat resistance of automotive PCBs. PCBs are immersed in solder paste with a temperature of  260°C for 10 seconds three times. Then, PCBs are inspected to ensure that there are no lamination issues, bumps, or cracks. 
• As PCBs must operate in extremely rainy and humid environments, temperature-humidity bias (THB) testing is required with “PCB Conduct Anodic Filament (CAF) mobility” being a large aspect of it. This is done between adjacent vias, between vias and wires, and between adjacent wires. 
• Thermal cycling testing (TCT) must be conducted. Different parts of the vehicle–below the chassis, below the engine, at the transmission parts, and within the engine–have their own temperature extremes that circuit boards in the area must withstand. 

Using PCB Tools for Automotive Circuit Design

PSpice can assist in multiple verification and reliability steps, ensuring improved performance and high-quality ECU delivery

PSpice provides a single tool for all ECU-related simulations. Simulation software can help designers find performance issues before they come up, conduct risk assessments, and identify integration issues early in the design process.

With PSpice, you will be able to avoid late-stage design changes with simulation capabilities for evaluating automotive ECU design blocks at any desired level of abstraction. You’ll be able to perform full system-level analyses by incorporating drive-by-wire design with mixed-signal control loops in addition to software control capabilities.

With the power of the PSpice circuit simulator, you’ll be able to minimize design cycles, improve performance, automate design tasks, perform advanced analysis, and deliver ECUs that meet the quality and reliability standards of the automotive industry.

Cadence also provides an array of tools critical for successful automotive circuit design. For example, sensitivity analysis tools can help you decide which components require tighter tolerances. Using the smoke analysis feature can tell you which components are stressed from power dissipation, changes in junction temperature, or other breakdowns that may occur from overvoltage. All these features ultimately help ensure more reliable circuits.