| PCB design and layout guidelines |
PCB design and layout guidelines
an overview or tutorial about the basics of PCB design guidleines and the
points to watch during PCB design and printed circuit board layout.
Printed circuit board, PCB design, 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 using a PCB CAD system.
The PCB layout and design is a specialist skill requiring
knowledge of not only of the PCB design software and PCB CAD system, 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 printed circuit board can be designed
satisfactorily, it often helps to have some guidelines that can be followed,
although there is no substitute for experience.
PCB design 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 PCB 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 covering 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 PCB design 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 PCB 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 track layers that are needed within the printed
circuit board 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 printed circuit 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 design 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 design 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 PCB 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.
- Avoid partial planes It is wise to avoid leaving large
gaps in earth planes or power planes, or having partial planes in a certain
area of the board. These can set up stresses in the board which can lead to
warping during manufacture of the bare board, or later when the board is
heating during the soldering process. Warping after surface mount components
have been added can lead to component fractures and hence a high rate of
functional failures.
Track design guidelines
Consideration about the aspects of the tracks on the printed
circuit board themselves needs to be given at an early stage as there are
trade-offs that may need to be made.
- Determine the standard track width to be used It is
necessary to balance the standard track size to be used within the design.
If the tracks are too narrow and too close there is a greater possibility of
short occurring. Additionally if they are too wide and too far apart then it
can restrict the number of tracks in a given area and this may force the use
of additional planes in the boards to ensure the PCB design can be routed.
- Consider track size for lines carrying current The thin
tracks used in today's printed circuit boards can only carry a limited
current. Consideration needs to be given to the size of track for any that
carry power rails rather than low level signals. The table below gives some
track widths or a 10degree C temperature rise for different thickness copper
boards.
Current
(Amps) |
Width for 1 oz board
(Thous) |
Width for 2 oz board
(Thous) |
| 1 |
10 |
5 |
| 2 |
20 |
15 |
| 3 |
50 |
25 |
- Fix the printed circuit board pad to hole ratio and size
At the beginning of the PCB design it will be necessary to determine the pad
and hole dimensions. Typically a ratio of about 1.8: 1 (pad : hole) is used,
although sometimes a pad 0..5 mm larger than the hole is used as the
measure. This allows for hole drilling tolerances, etc. The manufacturer of
the bare PCB will be able to advise on the standards that are required for
their process. The ratio becomes more important as the size of the pads and
holes reduces, and it is particularly important for via holes.
- Determine PCB pad shapes Component libraries associated
with PCB CAD systems will have libraries for the schematic and PCB
footprints for the different components. However these may vary according to
the manufacturing process. Typically they need to be large for wave
soldering than for infra-red reflow soldering. Thus the manufacturing
process needs to be determined before the design starts so that the optimum
pad sizes can be chosen and used on the PCB CAD system and hence on the
printed circuit board itself.
Thermal issues
Although for many smaller printed circuit boards thermal
issues do not present a problem, with higher processing speeds and higher
component densities for modern PCBs, thermal issues can often start to become a
significant hurdle.
- Allow sufficient space for cooling around hot components
Components that dissipate large amounts of heat may require additional space
around them. Allow sufficient space for heatsinks that may be required.
Signal integrity and RF considerations
There are many issues with PCB design associated with Signal
integrity, RF and EMC considerations. Many of the ways to avoid problems are
associated with the way tracks are routed.
- Avoid running tracks in parallel Tracks that run in
parallel for any length will have a higher level of crosstalk with signals
on one track appearing on another. Crosstalk can lead to a variety of
problems in the circuit and it can be very difficult to eliminate once the
printed circuit board has been designed and built.
- When tracks need to cross have them cross at right angles
To reduce the level of crosstalk generated, when two signal lines need to
cross, they should cross at right angles to reduce the level of capacitance
and mutual inductance between the two lines.
Summary
There are a host of PCB design guidelines that can be
documented. The PCB design guidelines here are just a few of the many that could
be devised, but they can form the basis of a set of guidelines that could be
used general PCB design.
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