PCB design guidelines

- guidelines for use in the design and layout of PCBs, printed circuit boards.

PCB layout process tutorial includes:

    •  PCB design process overview
    •  Schematic capture
    •  PCB layout using CAD
    •  PCB design guidelines
    •  Signal integrity on PCBs

The design of a printed circuit board, PCB, is one of the most important design elements within the design of an electronics product. In most instances an electronics hardware design engineer will design the circuit, and then a PCB layout specialist will undertake the PCB layout and design from a schematic provided by the electronics design engineer.

The PCB layout and design is a specialist skill requiring knowledge of not only of the PCB design software, but also a variety of standards and techniques used to ensure that the basic circuit design is successfully transferred to an overall printed circuit board that can be manufactured in an electronics circuit manufacturing environment.

In order that a PCB can be designed satisfactorily, it often helps to have some guidelines that can be followed, although there is no substitute for experience.

PCB guidelines

There are many ideas and guidelines that can be drawn up for the design and layout of a PCB. The list below covers a number of them. Obviously there are more, and the design guidelines list below should not be thought of as a complete list.

In order that the PCB design guidelines can be followed more easily, the guidelines are split into sections :

• Board constraint design guidelines - those coving the initial constraints on the board
• Overall layout design guidelines
• Guidelines associated with the planes or layers
• Track design guidelines
• Thermal issues
• Signal integrity and RF considerations

These form some of the main areas for consideration for designing a PCB. For some designs, some of the guidelines will be more important than others, and judgements will often need to be made to balance one requirement against another.

Board constraint PCB design guidelines

These PCB design guidelines are associated with the constraints of the overall board:- size, shape, and some of the factors that affect the overall design or concept of the PCB. These should be some of the first factors to be addressed.

• Choose reference points that suit the manufacturing process. It is normally necessary to have reference holes or points on the board. These are used for pick and place machines and test fixtures. They should be chosen to suit the manufacturing process. Often they may be holes for fixtures, but they may also be crossed marks for optical sensors. They must be clear of components, and not obscured.
• Allow adequate board area for the circuit Often the dimensions of the board will be defined by the overall product size, but before the PCB design starts, estimates should be made regarding the size of the board and whether it can accommodate the components and their tracks.
• Determine the number of layers required It is wise to determine the number of layers needed at the beginning of the design. Additional layers increase the production costs, but may mean that the tracks can be accommodated. Complex designs may have many tracks, and it may not be possible to route them unless sufficient layers are available.
• Consider the board mounting method It is necessary to consider how the board will be mounted at the beginning of the design. Different methods of mounting may require different areas of the board to be kept free, or they may take up different areas on the board.

Overall layout PCB design guidelines

These PCB guidelines should be addressed before the main design of the circuit starts. They should effectively be some of the first elements of the component placement.

• Draw and overview plan of where the different circuit areas will be located One of the first parts of the circuit layout is to draw a rough plan of where the major components and component areas will be located. In this way critical track runs can be assessed along with judging the most convenient design

PCB guidelines associated with the planes or layers used

It is common practice to use a complete layer or plane for earth or power rails. The most effective ay these can be used must be considered early in the design.

• Consider whether complete planes will be used for power, earth, etc It is common practice to use a complete plane for earth and some major power rails. This has advantages in terms of noise, and current capability.

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