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A Printed Circuit Board will need to fit into a case or slide into a rack to perform it's function. There may be areas that will
require height restrictions on the board (such as a battery holder molded into the case or rails in a rack the board is supposed to
slide into). Tooling holes and keep-out areas may be required in the board for assembly or manufacturing processes. All these
outside factors need to be defined before the board can be designed, including the maximum dimensions of the board and the
locations of connectors, displays, mounting brackets, or any other external features.
The function of a PCB includes the thickness of the copper laminated to the surfaces. The amount of current carried by the board
dictates the thickness of this copper foil, which need to be defined before the design.
The parts to be mounted on the PCB should be detailed on the parts list. Each part should be identified by a unique reference
designator and a part description (i.e. a resistor might be shown as reference designator "R1" with a description of "1/2 Watt
Carbon Film resistor"). Any additional information useful to the assembly process can be included on this list, such as mounting
hardware, part spacers, connector shrouds, or any other material not shown in the schematic diagram.
Part manufacturers provide data sheets to be used by the circuit designer to select parts for the circuit. If we are to be able to
design the PCB, these sheets should also have the physical dimensions of the part included. If each part type to be used on the
board does not have a data sheet, you should procure a sample part you can measure to define this data yourself. This measurement
method is far less accurate than using the part manufacturer's information, especially if there is a large tolerance on the part,
but it is better than just guessing.
A schematic diagram must be made available that shows the connection of the parts on the board. Each part on the schematic should
have a reference designator that matches the one shown on the BOM. Many schematic layout programs will allow automatic generation
of the BOM.
A "netlist" file is usually required to design a board on an ECAD system. The file has the device names of the parts used on the
board and a list of "nets" (interconnections between the pins of those parts). The file should be generated by the system used to
generate the schematic, but may have to be typed in by hand if the schematic was manually drawn, or the schematic program can not
generate a netlist the ECAD system will accept. This file is critical to PCB function. It should be double checked whenever time
allows, since the smallest mistake can scrap the finished board.
The design starts by setting-up the physical board in the design database. The first step is to to draw the outline of the board.
Some ECAD systems require to create the outline as a separate symbol, others require it to be drawn in the same program as the
layout.
Now the insertion of the symbols for the mounting holes, smd pads and any fixed location parts at their required coordinates. Three
tooling holes need to be placed so that they are in three corners of the board, two on the same vertical axis and the third on the
same horizontal axis as one of the first. The hole at the intersection of the horizontal and vertical axis should be designated as
the datum (0,0) coordinate of the PCB. This allows extraction of placement, hole, and interconnection locations. All board
dimensions should be in reference to this hole.
Next, the netlist is read and associated to any pre-existing fixed parts on the board. Any errors which have been made in processes
before this usually become very apparent here.
Now the placing of the parts on the board can begin. There are a number of factors to be considered when placing parts - electrical
function, physical size, temperature factors, and routability are a few.
Many ECAD programs have auto-placement options that let the computer place the parts without human intervention. This is not always
suitable. The biggest problem is that the program can not look at many of the constraints that will be obvious to a human mind. If
a board has few parts on it, auto-placement may be a viable option.
Some general placement guidelines:
- Through hole parts go on one side of the board, referred to as the "primary" side, since SMD components may be placed on either side of the board.
- A standard grid is preferred while placing the board.
- Integrated circuits need, if possible, to be placed in an even matrix. This allows support parts to be placed around them and connect power and ground more easily. Place the ICs so that they are functionally grouped, and that they are near any fixed-location parts associated with them.
- Support parts such as resistors and capacitors need to be placed where they are needed (i.e. input ESD filters should go near their associated connectors, not the IC). The parts need to be arranged so they are evenly spaced, so they do not overlap, and so their pads are a sufficient distance apart to allow the interconnecting traces to pass between.
- Parts must be placed in only one or two orientations, vertical and horizontal. It is best to place polarized parts such as caps and diodes in the same orientation (i.e. all diode cathodes to the right or up) to keep assembly and repair errors to a minimum. Non-orthogonal angles shoul be avoided unless absolutely necessary to the function of the board as it is much harder to assemble these components.
- Axial parts such as resistors and diodes should be placed so there is an equal amount of lead wire on either side of the body, and the two pads are at a similar distance as other axial parts. A good general distance is .500" for the smaller parts
Once the parts have been placed on the board, they need to be connected together. This process is known as routing and can be done
manually or automatically.
A drawback using auto-routing software is one of modifications to a circuit. Since an autorouted board usually uses more space than
a manually routed board, changes can be very difficult, as can adding those last few traces that the autorouter didn't get in.
When routing a signal it is always a good idea to make the trace as short and direct as possible. Although it may be possible to
route a board with all signal traces starting and ending on a component pad, it would probably results in a trace that snakes
around so much that the function of the circuit is degraded. The solution to this is to use a pad without any component lead thru
it, called a via or feed-thru.
This is the final phase of the design. It prepares the data that will actually be used by the manufacturer to generate the finished
board. Three items have to be delivered to the board manufacturer.
The first step is printing the fabrication and assembly drawings. The fabrication drawing should show the dimensions of the board
in reference to the datum tool hole. It should also show a graphic representation for each hole on the board, using a different
symbol for each hole size and including a table showing the quantity of each hole size. An assembly drawing is helpful when
building and repairing the board. This should show the outlines of the parts on the board, including their reference designators.
It also should contain any special assembly instructions, such as mounting hardware and connector shells. Many companies require
these drawings, others just use copies of the silkscreen legend.
The second step is generating the NC drill file of hole positions. This file is in ascii (usually) so that the drilling
machine can read it to produce the board. The manufacturer will use this data to set up for drilling the boards.
The third step is generating the artwork for the board. Each "layer" of the finished board must have a master artwork with opaque
features on a clear background. The manufacturer will use copies of these master artworks to build the board. This step may be
done by pen plotting on matte mylar at a magnified scale, then sending the plots to a reprographic company who will
photographically reduce the plot on clear film The most accurate method is to generate photoplot files (also known as "gerber"
files, after the manufacturer of the first photoplotters) for each layer required by the manufacturer. Some manufacturers require
an aperture file to generate the photoplot files. This file tells the plotter which machine code is used for a particular size
dot on the film.
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