Many designers start out with the perception that circuit functionality and reliability are the key factors to be considered in printed circuit board (PCB) design. In practice, PCB layout is a much more critical element in creating successful end products than many engineers realize in the initial design cycle.
Layout is so critical to electronic products that in some PCB design teams, designs are passed to specialized layout technicians for implementing best practices and to avoid known placement considerations. In most cases, sophisticated computer aided design (CAD) software is utilized to maximize efficiency, detect potential design issues, and warn of obvious errors.
Some of the key factors in successful PCB layout are:
- Experience – design experience contributes greatly to quality results, for both simple and complex PCB designs.
- Tools – robust toolsets such as CAD and Design for Manufacturing (DFM) computer applications have become a must for designing today’s complex PCBs.
- Adherence to guidelines and best practices – there are many generally accepted guidelines and best practices that can be beneficial in developing PCB layout, all the way through the manufacturing and testing processes.
Guidelines for PCB Layout
Some guidelines will be more relevant toward specific types of PCBs than others, such as those pertaining to specific use like flexible PCBs, multilayer boards, and specialized technology including high-density interconnect boards. But there are also a number of guidelines that lend themselves to many disciplines:
Start with a rough plan – a general design that illustrates proposed components and layout will generate a good working plan for proceeding with detailed circuit engineering and PCB layout.
Physical board constraints – determine any factors that influence design
- Size of the board – predetermined dimension requirements
- Materials – need for use of specific board materials will drive component use
- Connectivity – how the board will be mounted or installed will strongly influence layout, as connection points and methods could impact component placement
Estimates – once board components and dimensions are determined, evaluate whether or not the combination of devices and the necessary traces will accommodate the board size.
Layers – determine the need for multiple layers, how connections will be routed among layers, and whether or not the resulting design can be fabricated by the manufacturer.
Planes – along with layer design, consider how power and ground planes will be implemented, with a best practice being to separate the two to minimize electromagnetic interference (EMI) and manage power efficiently. Avoid using partial planes to avoid warping, laminating issues, soldering, and other manufacturing problems. Such construction problems can lead to board reliability issues or total failure.
Traces need to be considered early in the design to avoid a number of PCB issues.
Trace width – traces that are too close and too narrow can result in shorting conditions. Conversely, traces positioned too widely can increase the real estate needed on the board, increasing size, number of layers required, and/or cost.
Trace power capacity – depending on the amount of current to be handled by an individual trace and the board construction, trace width may need to be increased.
Pad and hole size and ratio – by determining the pad/hole ratio and size during initial design, tolerances can be calculated for hole drilling and other considerations. This may vary by manufacturer, making consulting with the intended fabricator early in the process very beneficial. This is even more critical as PCBs continue to shrink in size, and via holes are accordingly critical, as well.
Pad shapes – pad size and shape can vary depending on the components to be incorporated and the manufacturing processes to be utilized. This will impact the PCB layout. Here again, consult with the PCB manufacturer early in the process to ensure the board design adheres to manufacturing standards.
Thermal issues – if board functions will incorporate components that are significant heat generators, allow for dissipation tolerance or heat sink requirements.
PCBs are subject to such common problems as electromagnetic interference, electromagnetic compatibility, and other undesirable elements. Avoiding such issues and the resulting board defects requires attention to detail in ground placement and trace angles that tend to increase EMI.
Parallel traces in close proximity can generate crosstalk that generates board performance or failure issues. Where traces actually must cross, a best practice for minimizing capacitance and induction problems is to have them cross at right angles.
Methodology for Solving PCB Layout Problems
Many PCB layout challenges can be avoided by the adherence to basic guidelines, but experience is still an attribute that cannot be over-emphasized. An experienced PCB layout technician, armed with sophisticated and up-to-date design software is the best possible scenario for development of PCBs that will function as designed, and will also be translatable to efficient manufacturing.
Software programs are available today – some at little or no cost – that provide PCB engineers with drag-and-drop simplicity in design and layout functions. Computer aided design and manufacturing (CAD/CAM) systems are today considered essential components in the development of more complex and technical circuit design and fabrication.
Design for manufacturing (DFM) tools are the next step before actual PCB production, validating the design and detecting areas that could potentially generate problems in the manufacturing process.