PCB design and layout process
an overview, tutorial and information about the PCB design process or PCB
layout process used for designing printed circuit boards for use in electronics
equipment and the associated PCB CAD systems and PCB software.
Printed circuit boards, PCBs, form an essential part of any
electronics equipment these days. The PCB design and layout forms an integral
part of the design of the whole product, and it can be the key to the success of
the product meeting its performance requirements in many instances.
PCB technology has progressed significantly in recent years.
The design technology has improved with PCB CAD systems and PCB software to
layout the boards better, and also analyse the performance under conditions such
as the operation at high frequencies. In addition to this the technology for the
manufacture of PCBs has improved enabling far smaller tracks to be used as well
as incorporating features such as multilayer boards with blind vias, etc..
PCB design equipment
In the early days of PCBs, the design and layout was
undertaken manually using tape that was placed onto a translucent film. These
PCB designs were normally four times the required size and they were
photographically reduced onto a 1:1 film for the PCB production process.
Nowadays the PCB design process has been computerised and
there are many PCB CAD systems and PCB software packages that enable the PCB
layout and design to be undertaken more efficiently than before. The PCB
software varies considerably in price. Budget, or even free software provides
the basic functions, whereas the top end packages enable many more facilities to
be incorporated into the design. Simulations, complex routing, and many more
facilities are available.
Circuit schematic capture
The first stage in the development of a PCB design is to
capture the schematic for the circuit. This may be achieved in a variety of
ways. Circuits may be entered into a schematic capture tool. This may form part
of the PCB design suite, or it may be an external package whose output can be
exported in a suitable format.
In addition to purely performing the schematic capture,
simulations of the circuit may be undertaken at this stage. Some packages may be
able to interface to simulation packages. For applications such as RF circuit
design simulation of the circuit will enable the final circuit to be optimised
more without building a prototype.
With the schematic capture complete the electronic design of
the circuit is contained within the file and can be converted to what is termed
a "netlist". The netlist is the interconnectivity information and it essentially
the component pins and the circuit nodes, or nets, to which each pin connects.
PCB component placement
Before proceeding with the detailed PCB design and layout, it
is necessary to gain a rough idea of where components will be located and
whether there is sufficient space on the board to contain all the required
circuitry. This will enable decisions about the number of layers needed in the
board, and also whether there is sufficient space to contain all the circuitry
may need to be made.
Once a rough estimate has been made of the space and
approximate locations of the components, a more detailed component layout can be
made for the PCB design. This can take into account aspects such as the
proximity of devices that may need to communicate with each other, and other
information pertaining to any RF considerations for example.
In order that components can be incorporated into the PCB
design they must have all the relevant information associated with them. This
will include the footprint for the printed circuit board pads, any drilling
information, keep out areas and the like. Typically several devices may share
the same footprint, so this information does not have to be entered for each
component part number. However a library for all the devices used will be built
up within the PCB layout design system. In this way components that have been
used previously can be called up easily.
Routing
Once the basic placement has been completed, the next stage
of the PCB design is to route the connections between all the components. The
PCB software then routes the physical connections on the board according to the
netlist from the schematic. To achieve this it will use the number of layers
that are available for connections, creating via holes as required. Often one
layer will be allocated for use as a ground plane, and another as a power plane.
Not only does this reduce the level of noise, but it enables low source
resistance connections to be made for the power.
The routing can use a significant amount of computing power.
This is particularly true for larger designs where there may be upwards of three
or four thousand components. Where routing is difficult as a result of high
component density, this can result in the routing taking a significant amount of
time.
PCB files
The information for the photo plots of the PCB layout are
outputted in the form of Gerber files. This format is the standard for PCB files
and they are a form of numerical control file that is used by a photo plotter.
In addition to the Gerber files, drill information is also generated along with
the screen print and photo-resist information.
One major element in the cost of a printed circuit board is
the drilling. In any design some holes are required for fixing as well as those
required for any conventional components needed. However to reduce costs it is
wise to use as few hole sizes as possible. In this way the drill will need
changing less and time can be reduced.
PCB overview
Once complete the information for the PCB will be used in
many areas of the manufacturing process. Not only will it be used for the
manufacture of the actual PCB itself, but the files will also be used in other
areas of the manufacturing process. They may be used to develop a pick and place
programme, and in addition to this the files may be used in the manufacture of a
PCB solder mask for adding solder paste to the board prior to component
placement. The files may also be used for developing various forms of test
programme such as an "In-Circuit Test" (ICT), and particularly in developing any
bed of nails test fixture. In this way, the PCB design is a crucial element of
the whole manufacturing process for any product. The PCB design is more than
just a design for the basic board.
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