PCB Design Considerations and Tools
Original Title：PCB Design Considerations and Tools
Proper printed circuit board (PCB) design is critical to the ability to produce electronics prototypes that are both operationally and commercially efficient. This is particularly true for embedded applications. Embedded circuits vary in size and type based on the microprocessor, components, and operating system, but above all on the complexity of the software, which can vary from a few hundred bytes to several megabytes of code.
From the circuit diagram developed, it is possible to perform simulations and design the PCB by exporting Gerber/drill files. No matter what the design, engineers need to know precisely how the electrical circuits (and electronic components) should be arranged and how they will work. For EEs, finding the right software tools for PCB design can be a daunting task. A software tool that’s ideal for one PCB project might be a less appropriate fit for others. EEs want board design tools that are intuitive, include useful functionality, are stable enough to limit risks, and have a robust library that makes them applicable to multiple projects.
For projects targeting the internet of things, in which integration is central to performance and reliability, the integration of conductive and non-conductive materials within a PCB requires IoT designers to study the interactions between the various electrical and mechanical aspects of the design. In particular, electrical heating on a PCB becomes an increasingly critical factor as component size continues to shrink. At the same time, functional requirements are on the rise. In order to achieve merit-based performance as designed, temperature response, electrical-component behavior on the board, and overall thermal management are critical to the functionality and reliability of the system.
A PCB must be isolated to ensure protection. Short-circuits are prevented by protecting the copper traces placed on the board to create the electronic system. FR-4 is preferred as a substrate material over lower-cost alternatives such as synthetic resin bonded paper (SRBP, FR-1, FR-2) because of its physical/mechanical characteristics, especially its ability to retain data at high frequencies, its high resistance to heat, and its ability to absorb less water than other materials. FR-4 is widely used for high-end construction and for industrial and military equipment. It is compatible with ultra-high insulation (ultra-high vacuum, or UHV).
But FR-4 faces a number of limitations as a PCB substrate that stem from the chemical processing used in production. In particular, the material is susceptible to the formation of inclusions (air bubbles) and streaks (longitudinal air bubbles), as well as deformation of the glass fabric. These imperfections lead to inconsistencies in dielectric strength and impair PCB trace performance. New epoxy glass materials solve these problems.
Other commonly used materials are polymide/fiberglass, which supports higher temperatures and is more rigid, and KAPTON, which is flexible, lightweight, and suitable for applications such as displays and keyboards. Factors to consider when selecting a dielectric material (substrate) include the coefficient of thermal expansion (CTE), glass transition temperature (Tg), thermal conductivity, and mechanical rigidity.
Military/aerospace PCBs require special design considerations, based on layout specs and 100% design-for-test (DFT) coverage. The MIL-STD-883 standard establishes methods and procedures for testing microelectronic devices suitable for use in military and aerospace systems, including mechanical and electrical testing, manufacturing and training procedures, and other controls, to ensure a uniform level of quality and reliability across the various applications for such devices.
The design of an electronic device for an automotive system must follow a series of rules in addition to meeting various standards, such as AEC-Q100 mechanical and electronics testing for packaged integrated circuits. Cross-talk effects can impede vehicle safety. To minimize those effects, PCB designers must impose a minimum distance between the signal and power lines. Design and standardization are facilitated by software tools that automatically highlight design aspects that need further modification to meet interference limits and heat dissipation conditions in order to avoid compromising system operation.
Figure 1: Altium Designer (Image: Altium)
Interference from the circuit itself is not the only threat to signal quality. PCBs in cars are bombarded with noise that interacts in complicated ways with the car body, inducing unwanted current in the circuits. And peaks and fluctuations in voltage caused by the car’s ignition system can push components well outside their machining tolerances.
Today’s PCB layout tools must have a combination of features to meet the designer’s requirements. Choosing the right layout tool should be at the forefront of PCB design considerations and should never be ignored. Offerings from Mentor Graphics, OrCAD Systems, and Altium are among today’s most popular PCB layout tools.
Altium Designer is one of the most popular high-end PCB design software packages on the market today. With auto-routing functions, it supports trace-length tuning and 3D modeling. Altium Designer includes tools for all circuit design tasks from schematic capture to HDL and circuit simulation, signal analysis, PCB design, and FPGA embedded development (Figure 1).
Mentor Graphics’ PCB layout platform addresses the key challenges faced by today’s system designers: accurate, performance- and reuse-oriented nesting planning; efficient routing of dense and complex topologies; and electromechanical optimization. A key functionality of the platform, and a key innovation for the industry, is the Sketch Router, which provides the designer with complete and interactive control over the automatic/assisted uncoiling process, generating the same quality results as manual uncoiling but in much less time.
Figure 2: OrCAD PCB Editor (Image: Cadence)
OrCAD PCB Editor is an interactive environment developed for board design at any technological level, from the simplest to the most complex. Thanks to its real scalability toward Cadence Allegro PCB Designer’s PCB solutions, OrCAD PCB Editor supports the technological growth of design teams, enabling the management of the most advanced constraints (high speed, signal integrity, etc.) while maintaining the same graphical interface and file format (Figure 2).
The industry-standard Gerber file format is used to communicate design information for PCB production. In many ways, Gerber is analogous to PDF for electronics; it is simply a small file format written in the hybrid machine control language. These files are generated by circuit breaker software and sent to PCB manufacturers for upload to CAM software.
Integrating electronic systems securely into vehicles and other sophisticated systems poses significant considerations for both hardware and software. The goals for engineers are to minimize the number of design iterations and the development time, with significant advantages for designers implementing the workflow. ■
This article was published in the March edition of EE Times Europe Magazine
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