No device represents the next big thing better than the self-driving car for consumer technology. It was conceived as an end to the monotony of the work commute and sold as a revolutionary product that is quickly making gains with advances in computer vision. Computer vision is driven by harnessing data from onboard sensors. While devices have become more sophisticated, the core to solving the problem (and many similar issues across a bevy of industries) relies on enhanced communication.
Automobiles are still a bit special compared to some consumer electronics as the physical design must be highly robust considering the wear that installed components will undergo. Automotive printed circuit boards must be highly rugged above and beyond standard industry expectations. For the present and future challenges of automobile electronics, designers should be mindful of the technology and topology that makes a car go.
The Controller Area Network Enables Modern Safety Features
Modern automobiles contain a wealth of sensors and controllers to regulate feedback with external stimuli. However, these stimuli alone do notpossess the flexibility and responsiveness when the sensor information has a mode of intercommunication. To facilitate, automobile designers created and later adopted the controller area network (CAN) bus as a standard. There are numerous benefits to this method of implementation.
First, by utilizing components that are already on the board to cover features and devices, the complexity and cost of the design fall. Though the benefit may seem marginal, modern logistics currently have to deal with the supply chain issues arising from the plant shutdowns and similar pandemic restrictions. Second, the CAN bus architecture allows for additional programmability levels between sensors and microcontrollers that would otherwise be papered over by adding additional components. While there’s no reason, a CAN bus couldn’t have additional sensors, designs that include the requisite features can devise a more adaptable software solution that can save time and money over board-level revisions.
CAN bus architecture can be either low- or high-speed, though high-speed is a bit of a misnomer relative to some modern data transfer protocols. A low-speed CAN bus network operates at speeds less than 125Kb/s, while high-speed can run anywhere from 1-5 Mb/s. Perhaps the most interesting thing about the protocol is that it functions asynchronously. Therefore, no clock signal is involved in handling conflicting calls/sends. Instead, nodes (control units starting from simple logic gates up to microcontrollers and beyond) sample every bit on the network simultaneously: the smallest (by most significant bit) value message wins out on priority, and higher value messages are resent and continue to be read in order from least to greatest value.
Because of the relatively low speeds by today’s standards, laying out CAN bus architecture should not be quite as intensive as protocols functioning with bit transfer rate well into the Gb/s range. Still, it’s good to review some best practices:
- Be aware of aggressor/victim signals when routing. Fast-rising/falling signals and switching nodes should be routed and placed at a comfortable distance away from data lines to avoid inductive coupling.
- Length matching will not be a concern in an asynchronous hierarchy, but it’s good to remember that best performance arises from the shortest and most direct traces between nodes.
The Challenges of Automotive Printed Circuit Boards
While automotive printed circuit boards are subject to the general industry guidelines that govern board operation and evaluation, additional concerns come into play due to the high-stress environment in which the boards must operate. To account for this, multiple changes must be made to the standard operation of board design, especially regarding the assembly process.
One of the biggest adjustments is the introduction of press-fit pins into a design. Press-fit pins are a mechanical method of joining a part or part interface to a bare board without introducing heat to the weld joint. Strange as it sounds, a weld occurs at the junction between press pin and plated through hole — cold welding. Metals placed in contact with one another can form a weld joint simply by the presence of direct contact and pressure propelling the joint formation. There are significant benefits to bypassing a molten metal fusion process. The primary one is that a union driven by the adherence between the pin and the PTH has no welding metal to weaken in high-temperature environments. As the pin is simply held tight in the PTH by a significant normal force, it is much less susceptible to the effects of heat and vibration. There is a danger of the high pressure generated by a press pin insertion resulting in stress regions around the joint; one solution is press-fit pins that enter elastic deformation during the insertion process. Press-fit pins usage in automobiles, as well as other high-reliability industries where heat and vibration are likely to promote failure at the solder joint between board and component, is overseen by IPC-9797.
Additionally, IPC-6012D provides some further quality control and tolerancing for design aspects heavily influenced by an automobile environment installation:
- Pad lifting: Caused by the adhesion of the copper foil decreasing at the surface when heated up. A lifted pad can cause connectivity issues up to and including opens.
- Solder wicking: The heat cycles and elevated temperature holds can lead to issues of solder escape from its intended joint position if the proper material concerns are not heeded.
Your Contract Manager Can Ensure Your Board is Built To Last
While automotive printed circuit board design carries some additional challenges and concerns, an expert team of professionals who take pride in designing electronics for customers can handle and exceed modern industry demands. Our team at VSE and our valued partners will ensure your board is built to spec with the same vision and care you placed in your design.
