I am fond of the boxy car bodies of the 80s and 90s. Call it nostalgia, but seeing one of those beauties rolling down the highway with its lack of aerodynamics puts a smile on my face. Beyond the aesthetic value, I’m terrible at working on cars, and the less complicated it is under the hood, the better I can troubleshoot any issues. Of course, there’s a tradeoff. This relative simplicity means a lack of features people have grown accustomed to and that have made significant strides in safety.
With automation and electric vehicles growing in market share with various regulations and emission targets providing a push for adoption, understanding the best PCB design for automotive practices is more critical than ever. While thermal and EMI concerns are a vital consideration for any board, the logistics of solving these issues in a system that must adhere to extremely high safety and reliability standards can challenge any designer.
PCB Design for Automotive: Start at the Stackup, Then Stay Cool
The PCB can be the most thermally active device in a local system, but not in automotive. These boards must be prepared for a greater operating ambient temperature before factoring in their contributions and system assembly that is likely to inhibit airflow. One of the first areas to consider is the stackup:
- 4-layer: The most common board solution is a comprehensive solution for many design challenges.
- 2-layer: It is possible to design some perfectly capable automotive PCB in two layers. Cost savings would be the major motivation over a 4-layer board, but an increase in circuit complexity/population (more sensors, microcontrollers, etc.) could preclude smaller layer counts.
For high-volume automotive production, the better designers can shrink board layer counts without sacrificing performance, the greater the savings. A standard 4-layer stackup may see the top and bottom layers utilized for placement and routing with power and ground planes on the internal layers. A ground pour on the bottom layer for return paths and signal integrity will be necessary for 2-layer boards and may still be adopted in 4-layer boards for other reliability purposes.
Additionally, because it’s unlikely for even simple circuits to be routed on a single layer without a detrimental impact on the component placement, the bottom layer will almost universally be used as an alternate routing layer. Care must be taken to avoid routing traces in parallel, even across the board’s core, as coupling and crosstalk could arise. Similarly, mixed digital and analog signals should be isolated to prevent a high di/dt digital rise time inducing a current in a victim analog line.
Heat mitigation, like most boards, is accomplished with passive heat transfer features in the design. Excellent power routing is central to any well-performing board but may take on increased importance as analog signals may be relatively depreciated to power. If the top layer shoulders the majority of the electrical and thermal responsibility for the power nets, keep these points in mind:
- Large, unbroken copper pours are essential to prevent hot spots and current chokepoints. Follow the manufacturer’s guidelines for optimal circuit layout. When in doubt, keep pours wide into pins that facilitate the power transfer (input, power, ground, output, etc.) and keep other package traces as short and direct as possible.
- Vias will be used to shuttle heat to the bottom layer. This is an effective heat sink when flooded with a ground copper pour. Vias will offer the greatest benefit at a short distance to the power net copper, where the heat concentration is greatest. Practice caution with via distribution. Many vias spaced too tightly will dilute the in-plane heat transfer.
Best Practices to Reduce EMC at Its Source
The interior of an automobile is an extremely noisy environment from an electrical standpoint. Like thermal performance, electromagnetic compliance (EMC) gets a boost from additional layers (primarily assigning power and ground to adjacent internal layers). Still, it can be accomplished within two layers if cost or production quantity demands it. For electric automobiles, in-noise must be properly filtered out of the power supply lines.
There are two manifestations of conductive noise that can arise in these assemblies:
- Differential mode: The current of the noise travels opposite the signal. Can be alleviated with bypass capacitors or ᴨ (pi) filter.
- Common mode: The current of the noise travels in the same direction as the signal and requires a common mode choke coil.
Preventing both forms of noise will be crucial to optimal performance. These filters will be targeted to a particular frequency range to remove unwanted noise. The best outcome is noise cancellation coming with high resistance filters, which reduce current flow; a pi filter and common mode choke can be connected in series from the battery input to achieve a requisite level of signal conditioning.
Any discussion of EMC in automotive design needs to account for loop areas. Wherever current is conducted, a magnetic field forms in a closed loop centered on the conductor according to Ampere’s Law. A stronger current means a stronger field, which means a greater potential for EMI in the immediate surroundings of the path of conduction. The area of the loop can also be reduced by ensuring that the current’s return path is short and direct; a longer current path also means a larger area over which the magnetic field acts upon. Ensure connections to the ground are facilitated with:
- Short and direct paths to ground traces for low impedance.
- An uninterrupted ground plane. For the sections of the board where the plane needs to be removed, avoid split plane routing (traces that run over multiple planes on reference layers or are only partially routed over reference layers), as this can lead to significant EMI issues.
For Automotive PCBS, Ride With an Experienced Contract Manufacturer
PCB design for automotive requires contributions from multiple engineering disciplines to devise a product of high performance that can withstand a challenging environment. As demand and expectations for autonomous and electric vehicles continue to grow in the coming decades, board design will need to be optimized to keep pace with the market’s will. The first step for design teams looking to produce the cars of the future is pairing them with an experienced manufacturer who can commit to a high level of quality.
We here at VSE are no strangers to these challenges. As a team of engineers committed to building electronics for our customers, we take pride in contributing to numerous life-changing and life-saving devices. Partnered with our professional manufacturing partners, we continue to transform our customers’ visions into reality.
