--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.
PCB layout process tutorial includes:
• PCB design process overview
• Schematic capture
• PCB layout using CAD
• PCB design guidelines
• Signal integrity on PCBs
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 process overview
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.
Schematic capture
--the stage in the PCB design process where the circuit or "schematic" is captured and entered into a computer.
With computer based PCB design and many more computer simulations being used in the design of electronics circuits, the schematic capture or schematic entry stage of an electronics circuit into a computer is now an integral part of the design process. Typically complete software suites are now used, and the schematic capture stage only needs to be undertaken once, and the information used not only for being able to produce a drawing of the circuit, but also data entry into the PCB design as well as circuit simulation, production of the Bill of Materials (BOM) and many other applications.
Schematic capture in the design process
The schematic capture part of the design process is today undertaken interactively. Prior to the schematic capture of the design, the initial high level design must be undertaken. Then in years gone by, breadboards of the circuit would be made up and made to work before committing to the schematic stage. Now with highly sophisticated circuit simulation software, the circuit is designed interactively during the schematic capture stage and the circuit simulated using software rather than building a hardware version of the circuit.
By using a computer based system for schematic capture, it is possible to enter very complicated circuits into a computer relatively quickly. It is also possible to undertake the design of the board and perform circuit simulations while the basic design is underway. In addition to this, many circuit capture systems provide a means by which the circuit revisions can be managed and configuration controlled properly. Where a circuit is being repeatedly updated, and there may be the possibility of several people working on different areas, this is of great importance.
Elements entered into a schematic have a shape associated with them for the schematic. In this way a shape designed for a particular part will be pre-drawn and appear on the circuit every time that particular type of part appears on the circuit. When using an end-to-end design suite, the full shape may also include the PCB outline, pads and the like. In this way the part number for that part defines all the elements of the part for the design.
Practical aspects of schematic capture
One of the big problems with computer based schematic capture systems now is that the circuits are often very large and they can become unreadable and difficult to follow. There is a trend to print the circuits out on A4 or letter sized paper, and each sheet may only have a few components.
Schematic error checking
It is very important to ensure that any schematic that has been captured is fully checked. While the simulation and other applications now available as part of an end-to-end design suite will trap and highlight many errors, some can still get through. Errors that creep through can be quite subtle. One that has been seen is where a particular node may be given slightly different names on different sheets. As the names are different they will not be connected by the computer. For example a node may be labelled "0v" on one sheet, but could appear as "gnd" on another. Accordingly it is very important to ensure that errors such as these do not creep through. Discipline in naming is essential.
Computer based schematic capture has greatly simplified the process of drawing circuit diagrams. Circuit schematics can be drawn as the circuit is designed, and managed in such a way that there is little room for error. While errors can occur, the level of errors has fallen dramatically with the introduction of circuit schematic capture software.
PCB layout tutorial
--an overview, information, tutorial about PCB layout, PCB CAD and PCB software for designing printed circuit boards.
Today, PCB CAD design systems offer a tremendous advance since the days many years ago when PCB design was undertaken using tape on a translucent sheet. Now PCB layout software enables printed circuit boards to be designed in a far more efficient way. It is possible to PCB designs to be made using huge numbers of very small components. In addition to this very high levels of track density may also be present, and this would not have been possible using manual PCB design techniques.
PCB layout steps
There are a number of steps that should be followed in any PCB design:
1.Set up initial settings
This stage of the PCB design involves setting up the snap and visible grids. At this stage the default track and pad sizes should also be determined and set.
2.Set up the mechanical elements of the PCB design
It is necessary to import the details for the printed circuit board outline into the PCB layout software programme as soon as possible. It is also necessary to set up any reference marks and holes. These may be required for pick and place machines, or test fixtures during the production process.
3.Put all components onto the board
At this stage of the PCB layout, the components need to be placed onto the printed circuit board so that they are available to be moved and set in place later.
4.Create functional building blocks
At this stage of the PCB layout, the components should be moved into their functional blocks so that associated components are close to each other and the circuit can be routed easily later.
5.Identify and route layout critical tracks
Any tracks that are layout critical should be identified and then routed as they are required. By routing these tracks at this stage, then the remained of the design can be implemented around these tracks rather than trying to resolve problems later in the PCB layout.
6.Route power and earth rails
Often the earth and power rails may be included as planes, occupying a complete layer of the printed circuit board. This has significant advantages not only in terms of enabling the higher levels of current to be routed easily, but it also significantly reduces any problems with interference on the printed circuit board.
7.Route the remaining lines
Usually it is necessary to use the auto-route function on the PCB layout software. Although there are manual routing options on PCB layout software, it is normal to use the auto-route function as this may save many days trying to route the PCB layout manually. The auto-route functions have been very well developed in recent years and normally provide very good results. It is possible to set up various parameters to ensure the PCB layout software routes the circuit according to the requirements.
8.Manually route any final lines on the PCB layout
After the PCB layout software has completed the auto-routing, there may be a few tracks that would not route. These canoften be routed manually. Alternatively if the design has become too complicated for the space and the available number of layers, it may be necessary to make some fundamental changes to the board.
9.Undertake final tidy up
Once all the lines have been routed, it is complete any small items that may need completing at this stage.
10.Complete a design rule check
While all the design rules should have been followed during the design, it is necessary to do a final check. It is better to catch any problems at this stage rather than once a prototype PCB has been made.
11.Have the work checked by an independent party
However much care has been taken designing using the PCB layout software, there is always room for possible errors. These are not easy to spot having worked intimately with the job. It is therefore always good practice to have the work checked by an independent party who has not been involved on the PCB layout in question.
12.Release the design for prototype PCB manufacture
With the PCB layout complete and checked, it is necessary to send it to the PCB manufacturer for the manufacture of the bare prototype PCB. At this stage it is necessary to ensure that all the correct files are sent. In order to ensure that there is no confusion, the files should be formally released. Although many design checks will have been carried out, it is still necessary to undertake the manufacture of a prototype PCB because there is always the risk of an unforeseen problem. Committing to large quantities of PCBs could be costly is a problem is found and as a result it is always wise to undergo a prototype PCB stage.
PCB layout and design should be a relatively straightforward process in terms of the steps to take. The real challenges can test the PCB layout engineer in terms of achieving a layout that works well first time, and one that fits within the given mechanical constraints. If the process is correct, then this will enable the PCB layout engineer to focus on what is important and where his skills reallylay.
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 usedIt 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.
•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.mThere 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.
Signal Integrity
--an overview the methods used during PCB and circuit design to ensure signal integrity.
Signal integrity is becoming an increasingly important element of circuit and PCB design. As frequencies used within digital circuits rise, even comparatively short connections act as transmission lines, and they have an effect of the integrity of the signals being carried. Signals that might otherwise be considered as purely digital are modified by effects that may be thought of as applying to the analogue domain. These effects can cause circuits not to work, and accordingly signal integrity is now a major issue for any circuit design.
In view of the importance of signal integrity in any of today's high speed processor designs, it is necessary to incorporate design simulations and checks during the PCB design and layout process. Circuit boards effectively need to undergo signal integrity engineering. If it is not carried out at during the design, then there is little that can be done once a completed board has been built. In view of this the top PCB design software packages incorporate options for including signal integrity engineering and checking software, and this will enable checks to be carried out as the design proceeds. In this way the PCB layout can be optimised to ensure that the signal integrity is correctly engineered and problems occurring once the finished PCB is available for its test will be minimised.
Signal integrity issues
There are four main areas of circuit design and layout that must be taken into consideration to ensure that the signal integrity of a board or circuit design are maintained:
•Transmission line effects
•Impedance matching
•Simultaneous switching effects
•Crosstalk
To ensure that signal integrity is maintained, all the issues must be addressed to ensure that the signal is not distorted in any way and the data is corrupted. In this way the system is able to operate satisfactorily without errors and at the required speed.
Transmission line effects
At low frequencies a length of track may be considered purely by its DC characteristics. However as frequencies rise, effects including the capacitance and inductance associated with the track start to have a significant impact on the performance of the line. Accordingly it is necessary to consider the tracks as transmission lines, and treat them accordingly, looking at aspects such as the line impedance.
As a result it is necessary to ensure that the line maintains the same characteristic impedance along the length of the line, otherwise discontinuities will be introduced. This may result in signal reflections being created that may give rise to ringing and poor signal integrity.
In order to ensure that the transmission lines are treated correctly. First it is necessary for the lines to have a ground plane underneath them. It is also necessary to calculate the impedance of the line. This is determined from a combination of the line thickness, the distance between the line and the ground plane, and the dielectric constant of the board. If as often may happen, the line needs to traverse between layers and therefore the distance between the line and the ground plane changes. It will be necessary to ensure the line impedance remains the same, possibly by changing the line thickness.
Impedance matching
In view of the fact that lines on PCBs act more like transmission lines as the frequencies increase, so too it is necessary to consider the way in which the impedances need to be matched to ensure good signal integrity. When there is a mismatch between the line and the load, not all the energy of the waveform is absorbed by the load. That which is not absorbed is reflected back along the line where it may again not be absorbed if there is a mismatch between the transmitter and the line. This can cause overshoot and ringing which leads to poor signal integrity and giving rise to signal errors.To overcome this problem it is necessary to match the transmission line to the line drivers or transmitters and the line receivers. Many drivers and receivers exits that have suitable input and output impedances. Where this is not possible, say between the transmission line and the receiver, it is possible to put a resistor down to ground. In this way the parallel combination of the line receiver and the resistor can equal the line impedance.
In view of the high speeds involved and the length of some lines, the drive capability of the drivers needs to be higher than some "logic only" chips and special line drivers should be used. They will be able to supply the current required to properly drive the lines.
In some applications, it may be possible to add clamping diodes to reduce the level of overshoot and undershoot, and in this way maintain the levels of signal integrity. However wherever possible it is far better to ensure that proper matching is achieved.
Simultaneous switching effects
One effect that can disrupt the signal integrity on a circuit board occurs when several output lines are switched simultaneously. As stored charge on the outputs needs to be discharged, this gives rise to high levels of transient currents. While the levels of transients are normally adequate for single outputs changing, if several lines are switched simultaneously, especially on the same chip, the transient currents are larger, and this can give rise to problems. Problems with signal integrity arise because a voltage arises between the device ground and the board ground. If the chip ground rises sufficiently it can cause the signal switching levels to be exceeded, thereby causing spurious switching to occur.
To overcome this problem there are a number of measures that can be incorporated. One is to ensure that simultaneous switching does not occur, but this is not always possible, especially when circuits are operated in a synchronous manner. Good grounding is essential: a ground plane must be used to ensure a low resistance ground return. Additionally, sufficient decoupling directly across the chip can assist with some of the related effects.
Crosstalk
This aspect of signal integrity arises from the fact that signals appearing on one line appear on nearby lines. This can result in spurious spikes and other signal appearing on nearby lines. This can cause erroneous data or clocking pulses to appear, and these can be very difficult to track down in some circumstances. Poor signal integrity from crosstalk arises from two causes, namely mutual inductance, and mutual capacitance.
The mutual inductance is the effect that is used in transformers. It arises from the fact that a current in one track sets up a magnetic field. Changes in this field then induce a current in a nearby track.
Mutual capacitive occurs as a result of the coupling of the electric fields between two tracks. A voltage appearing on one track creates an electric field which can couple to a second line. Changing voltages, especially fast edges can result in similar edges appearing on nearby lines.
There are several techniques that can be used to overcome these effects. As poor signal integrity from crosstalk arises from mutual inductance and capacitance, the solutions involve taking steps to reduce them. This can be achieved in a number of ways by arranging the layout accordingly. The routing should avoid lines that run parallel to one another. If lines have to cross, this should be achieved at right angles, and using layers as far apart as possible. Line spacing should be as wide as possible, and to reduce mutual capacitance lines should be as thin as possible. Finally, where transmission lines are used, they should be as close to the ground plane as possible. This will reduce coupling to other nearby lines.
Further ideas
There are a number of other ideas that can be implemented to assist maintaining good levels of signal integrity. One area to which particular attention should be paid is the clocking circuitry. As it generates a regular clocking pulse, this can create a background noise if the signal integrity measures are not incorporated. Accordingly it is necessary to ensure that measures to reduce crosstalk on the clock lines are implemented. In particular, signal lines should be kept away from the clock lines, and they should not be routed underneath each other. If this is necessary then the ground or earth plane should be between them. To ensure the signal integrity, it is also necessary to ensure the lines are well matched so that ringing is prevented. This can add additional spikes that may be transmitted around the circuitry.
Another method of improving signal integrity is to ensure that all chips are adequately decoupled. Poor decoupling will add to the noise present on the circuits and this may impact the signal integrity. Each chip should be decoupled in line with the manufacturers guidelines. The decoupling capacitors should also be placed as close to the chips as possible.
Signal integrity engineering is now an integral and essential part of the printed circuit board design process. With the high speeds employed in many of today's circuits, it is no longer possible to design the basic circuit in isolation from the PCB. Instead the PCB design must be part of the overall electrical design. When this approach is adopted, then the possibility of problems arising from poor signal integrity will be minimised.
This areas investigates the electronics design considerations, concepts, techniques, tools and processes that can be used.
Topics from electronics design concepts and methodologies, through to more practical elements of PCB layout, ESD, EMC, verification and validation testing, etc.
Process management within the engineering function is a key tool to ensure that the development process progresses in an order controlled fashion allowing projects to have a much better chance of delivering the right product, on time and within budget.
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