Practical Knowledge | PCB Design Experience
Latest update time：2022-08-26
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layout
Layout is an important part in design. The quality of layout will directly affect the effect of wiring, so it can be said that reasonable layout is the first step to successful PCB design.
Especially pre-layout is a process of thinking about the entire circuit board, signal flow, heat dissipation, structure and other architectures. If the pre-layout fails, all subsequent efforts will be in vain.
1. Consider the overall
success of a product. First, we must pay attention to the internal quality, and second, we must take into account the overall beauty. Only when both are perfect can the product be considered successful.
On a PCB board, the layout of components must be balanced, dense and orderly, and cannot be top-heavy or heavy.
success of a product. First, we must pay attention to the internal quality, and second, we must take into account the overall beauty. Only when both are perfect can the product be considered successful.
On a PCB board, the layout of components must be balanced, dense and orderly, and cannot be top-heavy or heavy.
Will the PCB be deformed?
Is a process edge reserved?
Do you want to reserve MARK points?
Is paneling required?
How many layers of boards are needed to ensure impedance control, signal shielding, signal integrity, economy, and feasibility?
2. Eliminate low-level errors.
Does the size of the printed board match the size of the processing drawing? Can it meet PCB manufacturing process requirements? Are there any location markers?
Are there any conflicts between components in two-dimensional and three-dimensional space?
Is the component layout dense and orderly, arranged neatly? Have they all been laid out?
Can components that need to be replaced easily be replaced? Is it easy to plug the plug-in board into the device?
Is there an appropriate distance between the thermal element and the heating element?
Is it easy to adjust the adjustable elements?
Are there any radiators installed where heat dissipation is required? Is the air flow clear?
Is the signal flow smooth with minimal interconnections?
Are plugs, sockets, etc. inconsistent with the mechanical design?
Have you considered the line interference problem?
Does the size of the printed board match the size of the processing drawing? Can it meet PCB manufacturing process requirements? Are there any location markers?
Are there any conflicts between components in two-dimensional and three-dimensional space?
Is the component layout dense and orderly, arranged neatly? Have they all been laid out?
Can components that need to be replaced easily be replaced? Is it easy to plug the plug-in board into the device?
Is there an appropriate distance between the thermal element and the heating element?
Is it easy to adjust the adjustable elements?
Are there any radiators installed where heat dissipation is required? Is the air flow clear?
Is the signal flow smooth with minimal interconnections?
Are plugs, sockets, etc. inconsistent with the mechanical design?
Have you considered the line interference problem?
3. Bypass or decoupling capacitor
When wiring, both analog and digital devices require these types of capacitors, and both require a bypass capacitor close to its power pin. This capacitor value is usually 0.1μF. Keep the pins as short as possible to reduce the inductive reactance of the traces, and keep them as close to the device as possible
The addition of bypass or decoupling capacitors on a circuit board and the placement of these capacitors on the board are basic common sense for digital and analog designs, but their functions are different. In analog wiring design, bypass capacitors are usually used to bypass high-frequency signals on the power supply. If no bypass capacitor is added, these high-frequency signals may enter sensitive analog chips through the power pins. Generally, these high-frequency signals have frequencies that exceed the ability of the analog device to reject the high-frequency signals. If bypass capacitors are not used in analog circuits, noise may be introduced on the signal path, and in more serious cases, vibration may even occur. For digital devices such as controllers and processors, decoupling capacitors are also needed, but for different reasons. One function of these capacitors is to act as a "tiny" charge reservoir. This is because in digital circuits, performing a switching of gate states (i.e., switching) usually requires a large current. When switching, a switching transient current is generated on the chip and It is advantageous to have this extra "spare" charge flowing through the circuit board. If there is not enough charge when performing the switching action, the supply voltage will change significantly. If the voltage changes too much, it will cause the digital signal level to enter an uncertain state and may cause the state machine in the digital device to operate incorrectly. The switching current flowing through the circuit board traces will cause the voltage to change. Since the circuit board traces have parasitic inductance, the following formula can be used to calculate the voltage change: V=Ldl/dt where V=change in voltage L=circuit board traces Line inductance dI = change in current flowing through the trace dt = time for current change Therefore, for many reasons, it is very good to apply a bypass (or decoupling) capacitor at the power supply or at the power pin of the active device. way of doing.
4. For input power, if the current is relatively large, it is recommended to reduce the length and area of the wiring and avoid running all over the field.
The switching noise on
the input
is coupled to the plane
of the power output
.
The switching noise of
the MOS
tube of the
output
power
affects the input power of the previous stage.
If there are a large number of high-current DCDCs on the circuit board, there will be interference from different frequencies, high current and high voltage jumps.
Therefore, we need to reduce the area of the input power supply to meet the current flow. Therefore, when laying out the power supply, we must consider avoiding the input power supply running all over the board.
5. Power cord and ground wire
Good placement of power cords and ground wires can reduce the possibility of electromagnetic interference (EMl). If the power and ground wires are not matched properly, system loops will be designed and noise will likely be generated. An example of a PCB design with improper power and ground connections is shown in the figure. On this circuit board, different routes are used to route power lines and ground wires. Due to this improper coordination, the electronic components and circuits of the circuit board are more likely to be affected by electromagnetic interference (EMI).
6. Digital and analog separation
In every PCB design, noisy portions of the circuit are separated from "quiet" (non-noisy) portions.
Generally speaking, digital circuits can tolerate noise interference and are not sensitive to noise (because digital circuits have a larger voltage noise tolerance);
conversely, analog circuits have a much smaller voltage noise tolerance.
Of the two, analog circuits are the most sensitive to switching noise.
In the wiring of mixed-signal systems, these two circuits should be separated.
The basics of circuit board layout apply to both analog and digital circuits. A basic rule of thumb is to use an uninterrupted ground plane. This basic rule reduces the dI/dt (current variation with time) effect in digital circuits, because the dI/dt effect causes ground potential and allows noise to enter. Analog circuits. The techniques for wiring digital and analog circuits are basically the same, with one exception. For analog circuits, another point to note is to keep the digital signal lines and loops in the ground plane as far away from the analog circuits as possible. This can be accomplished by connecting the analog ground plane separately to the system ground connection, or by placing the analog circuitry at the far end of the board, at the end of the trace. This is done to keep external interference to the signal path to a minimum. This is not necessary for digital circuits, which can tolerate a lot of noise on the ground plane without problems.
7. Heat dissipation considerations
During the layout process, cooling air ducts and dead corners for cooling need to be considered;
Do not place heat-sensitive components behind the heat source. Give priority to the layout of DDR and other devices that are difficult to dissipate heat. Avoid repeated adjustments due to thermal simulation failure.
wiring
In PCB design, wiring is an important step to complete product design. It can be said that all the previous preparations are done for it. In the entire PCB, the wiring design process has the highest restrictions, the most delicate skills, and the largest workload.
PCB wiring includes single-sided wiring, double-sided wiring and multi-layer wiring. There are also two ways of wiring: automatic wiring and interactive wiring. Before automatic wiring, you can use interactive wiring to pre-wire the lines with strict requirements. The edge lines of the input and output ends should avoid being adjacent and parallel to avoid reflection interference. Ground isolation should be added when necessary. The wiring of two adjacent layers should be perpendicular to each other. Parallel wiring is prone to parasitic coupling.
The routing rate of automatic wiring depends on good layout. The wiring rules can be preset, including the number of bends of the traces, the number of via holes, the number of steps, etc. Generally, exploratory warp wiring is first performed to quickly connect short wires, and then labyrinth wiring is performed. The global wiring path optimization of the wires to be wired is performed first, and the wires that have been wired can be disconnected as needed. And try rewiring to improve the overall effect.
The through hole is not suitable for the current high-density PCB design, which wastes many precious wiring channels. To solve this contradiction, blind and buried via technologies have emerged. They not only complete the role of the through hole, but also save many wiring channels to make the wiring process more convenient, smoother and more complete. The design process of the PCB board is a complex and simple process. In order to master it well, the majority of electronic engineering designers need to experience it themselves to get the true meaning.
1. Handling of power supply and ground wires
Even if the wiring in the entire PCB board is completed well, interference caused by insufficient consideration of power supply and ground wires will degrade the performance of the product and sometimes even affect the success rate of the product.
Therefore, the wiring of electricity and ground wires must be taken seriously to minimize the noise interference generated by electricity and ground wires to ensure the quality of the product.
Every engineer who is engaged in the design of electronic products understands the causes of noise between the ground wire and the power wire. Now we only describe the reduction type noise suppression:
(1) It is well known that decoupling capacitors are added between the power supply and the ground wire.
(2) Try to widen the width of the power supply and ground wire. It is best that the ground wire is wider than the power supply wire. The relationship between them is: ground wire > power supply wire > signal wire. Usually the signal wire width is: 0.2~0.3mm, the thinnest width can reach 0.05~0.07mm, and the power wire is 1.2~2.5mm.
For the PCB of digital circuits, a wide ground wire can be used to form a loop, that is, to form a ground network for use (the ground of analog circuits cannot be used in this way)
(3) Use a large area of copper layer as a ground wire, and connect all unused areas on the printed circuit board to the ground as a ground wire. Or make a multi-layer board, with the power supply and ground wire occupying one layer each.
(2) Try to widen the width of the power supply and ground wire. It is best that the ground wire is wider than the power supply wire. The relationship between them is: ground wire > power supply wire > signal wire. Usually the signal wire width is: 0.2~0.3mm, the thinnest width can reach 0.05~0.07mm, and the power wire is 1.2~2.5mm.
For the PCB of digital circuits, a wide ground wire can be used to form a loop, that is, to form a ground network for use (the ground of analog circuits cannot be used in this way)
(3) Use a large area of copper layer as a ground wire, and connect all unused areas on the printed circuit board to the ground as a ground wire. Or make a multi-layer board, with the power supply and ground wire occupying one layer each.
2 Common ground processing for digital circuits and analog circuits
Nowadays, many PCBs are no longer single-function circuits (digital or analog circuits), but are composed of a mixture of digital and analog circuits. Therefore, it is necessary to consider the mutual interference between them when wiring, especially the noise interference on the ground line.
The frequency of digital circuits is high, and the sensitivity of analog circuits is strong. For signal lines, high-frequency signal lines should be as far away from sensitive analog circuit devices as possible. For ground lines, the entire PCB has only one node to the outside world, so The problem of digital and analog common ground must be dealt with inside the PCB. However, the digital ground and analog ground are actually separated inside the board. They are not connected to each other, but are only at the interface where the PCB connects to the outside world (such as plugs, etc.). The digital ground is shorted a little bit to the analog ground, note that there is only one connection point. There are also different ground on the PCB, which is determined by the system design.
3. Signal lines are laid on the electrical (ground) layer.
When wiring multi-layer printed boards, there are not many unfinished lines left on the signal line layer. Adding more layers will cause waste and increase the workload of production, and the cost will also increase accordingly. To resolve this contradiction, you can consider wiring on the electrical (ground) layer. The power layer should be considered first, followed by the ground layer. Because it is best to preserve the integrity of the formation.
4 Treatment of connecting legs in large area conductors
In large-area grounding (electricity), the legs of commonly used components are connected to it. The handling of the connecting legs needs to be comprehensively considered. In terms of electrical performance, it is better for the pads of the component legs to be fully connected to the copper surface, but for There are some hidden dangers in the welding assembly of components, such as: ① Welding requires a high-power heater. ②It is easy to cause virtual solder joints. Therefore, taking into account the electrical performance and process requirements, a cross-shaped solder pad is made, which is called heat shield, commonly known as thermal pad (Thermal). In this way, the possibility of virtual solder joints due to excessive cross-section heat dissipation during welding can be eliminated. Sex is greatly reduced. The treatment of the power (ground) layer legs of multi-layer boards is the same.
5 The role of network systems in wiring
In many CAD systems, wiring is determined by the network system. If the grid is too dense, the number of paths will increase, but the step size will be too small, and the amount of data in the drawing field will be too large, which will inevitably place higher requirements on the storage space of the equipment, and will also have a great impact on the computing speed of computer-related electronic products. Some paths are invalid, such as those occupied by the pads of the component legs or by the mounting holes and fixed holes. If the grid is too sparse, too few paths will have a great impact on the wiring rate. Therefore, a grid system with reasonable density is required to support the wiring process.
The distance between the legs of a standard component is 0.1 inch (2.54 mm), so the basis of the grid system is generally set at 0.1 inch (2.54 mm) or an integer multiple of less than 0.1 inch, such as 0.05 inch, 0.025 inch, 0.02 inch, etc.
6 Design Rule Check (DRC)
After the wiring design is completed, it is necessary to carefully check whether the wiring design complies with the rules set by the designer. It is also necessary to confirm whether the rules set meet the needs of the printed board production process. General inspections include the following aspects:
(1) Whether the distance between wires and wires, wires and component pads, wires and through holes, component pads and through holes, and through holes and through holes is reasonable and meets production requirements.
(2) Are the widths of the power and ground wires appropriate, and are the power and ground wires tightly coupled (low wave impedance)? Is there any place in the PCB where the ground wire can be widened?
(3) Whether the best measures have been taken for key signal lines, such as keeping them to the shortest length, adding protective lines, and clearly separating input lines and output lines.
(4) Whether the analog circuit and digital circuit parts have independent ground wires.
(5) Whether graphics (such as icons, labels) added to the PCB later will cause signal short circuits.
(6) Modify some unideal line shapes.
(7) Is there a process line added to the PCB? Whether the solder mask meets the requirements of the production process, whether the size of the solder mask is appropriate, and whether the character mark is pressed on the device pad to avoid affecting the quality of the electrical assembly.
(8) Whether the edge of the outer frame of the power supply ground layer in the multilayer board is reduced. If the copper foil of the power supply ground layer is exposed outside the board, it may easily cause a short circuit.
(2) Are the widths of the power and ground wires appropriate, and are the power and ground wires tightly coupled (low wave impedance)? Is there any place in the PCB where the ground wire can be widened?
(3) Whether the best measures have been taken for key signal lines, such as keeping them to the shortest length, adding protective lines, and clearly separating input lines and output lines.
(4) Whether the analog circuit and digital circuit parts have independent ground wires.
(5) Whether graphics (such as icons, labels) added to the PCB later will cause signal short circuits.
(6) Modify some unideal line shapes.
(7) Is there a process line added to the PCB? Whether the solder mask meets the requirements of the production process, whether the size of the solder mask is appropriate, and whether the character mark is pressed on the device pad to avoid affecting the quality of the electrical assembly.
(8) Whether the edge of the outer frame of the power supply ground layer in the multilayer board is reduced. If the copper foil of the power supply ground layer is exposed outside the board, it may easily cause a short circuit.
7 Check whether there are sharp corners, impedance discontinuities, etc.
(1) For high-frequency current, when the bend of the conductor is at a right angle or even an acute angle, the magnetic flux density and electric field strength are relatively high near the bend, which will radiate stronger electromagnetic waves. In addition, the inductance here will be relatively large, and the inductive reactance will also be larger than that of an obtuse or rounded angle.
(2) For bus wiring of digital circuits, the wiring turns are blunt or rounded, and the area occupied by the wiring is relatively small. Under the same line spacing conditions, the total line spacing width is 0.3 times less than that of right-angle turns.
8.
Check
the 3W and 3H principles
(1) The wiring of clock, reset, signals above 100M and some key bus signals and other signal lines must meet the 3W principle. There should be no long parallel lines on the same layer and adjacent layers, and there should be as few vias on the link as possible.
(2) The problem of the number of vias for high-speed signals. Some device instructions generally have strict requirements on the number of vias for high-speed signals. The principle of interconnection is that except for the necessary pin fanout vias, it is strictly prohibited to drill holes in the inner layer. For the extra vias, they laid out 8G PCIE 3.0 traces and drilled 4 vias, and there was no problem.
(3)
The center distance between clocks and high-speed signals on the same layer must strictly meet 3H (H is the distance from the wiring layer to the reflow plane); signals on adjacent layers are strictly prohibited from overlapping. It is recommended that the principle of 3H be also met. Regarding the above crosstalk problem, there are tools Can be checked.
routing constraints
Routing Constraints: Layer Distribution Routing Constraints: Layer Distribution
Each layer of the RF PCB has a large area of auxiliary ground, and there is no power plane. The two adjacent layers above and below the RF wiring layer should be ground planes. Even if it is a digital-analog hybrid board, the digital part can have a power plane, but the RF area still needs to meet the requirement of a large area of auxiliary ground for each layer.
RF
single board stacked structure
Routing Constraints: Basic Requirements
(1) The wiring is required to be as short as possible, no closed loops, no sharp or right angles, the width of the lines should be consistent, and there should be no floating lines.
(2) The wiring method of the soldering pad must be reasonable.
Basic wiring requirements diagram
(3) Differential signal lines are generally high-speed signals, and they must meet the symmetry of impedance. Differential lines cannot be crossed, and the difference in line length cannot exceed 100 mil. There must be a certain distance between differential lines and between a single differential line and the ground. Meet impedance requirements. There can be no more than 4 differential routing vias. The spacing between differential line pairs meets the 3W rule.
(4) Clock lines, control lines, and electromagnetic sensitive lines are prohibited under general crystal oscillators, PLL filter devices, analog signal processing chips, inductors, and transformers.
(5) Analog signals and digital signals, power lines and control signal lines, weak signals and any other signals cannot be routed side by side. They should be routed in layers (preferably with ground isolation) or routed far apart. If the lines of adjacent layers need to be routed across each other, they cannot be routed in parallel. In order to reduce crosstalk between lines, the distance between lines should be ensured to be large enough. When the distance between line centers is not less than 3 times the line width, 70% of the electric field can be maintained without interfering with each other, which is called the 3W rule. If you want to achieve 98% electric field without mutual interference, you can use a spacing of 10W.
Note: When routing the clock, be sure to pay attention to effective isolation from the data line and control signal line. The farther the distance, the better, and try not to route them on the same layer.
(6) Do not place strong radiation signal lines (high frequency, high speed, especially clock lines) close to interfaces, handles, etc. to prevent external radiation.
(7) Sensitive signals (mainly refer to: weak signals, reset signals, comparator input signals, AD reference power, phase-locked loop filter signals, and the filter part of the PLL circuit inside the chip.) The wiring should be as short as possible and not close to For strong radiation signals, do not place it on the edge of the board, more than 15mm away from the outer metal frame. When routing long distances, you can wrap the ground (it should be noted that wrapping the ground may cause impedance changes) and inner layer wiring. In addition, for the wiring of chips with weak ESD, inner layer wiring is recommended to reduce the probability of chip damage.
Wiring Constraints: Power Supply
(1) Pay attention to power decoupling and filtering to prevent interference from different units through power lines. Power lines should be isolated from each other during power wiring. The power line is isolated from other strong interference lines (such as CLK) with a ground wire.
(2) The power wiring of the small signal amplifier requires ground copper foil and ground via isolation to prevent other EMI interference from intruding and thus deteriorating the signal quality at this level.
(3) Different power supply layers should avoid overlapping in space. The main purpose is to reduce interference between different power supplies, especially between power supplies with large voltage differences. The overlapping problem of power supply planes must be avoided. If it is unavoidable, an intermediate ground layer can be considered.
Wiring Constraints: Power Supply Overcurrent Capacity
(1) The number of vias for the power supply conductors and printed wires between layers meets the current requirements (1A/Ф0.3mm hole)
(2) The size of the copper foil in the POWER part of the PCB conforms to the maximum current flowing through it, and the margin is taken into account (the general reference is 1A/mm line width)
Wiring Constraints: Grounding Methods
(1) The grounding wire should be short and straight to reduce distributed inductance and the interference caused by common ground impedance.
Adjust the direction of the filter capacitors in each group to reduce the ground loop. As shown in Figure 15, the three filter capacitors are grounded in the direction of the related RF devices, especially the high-frequency filter capacitors.
capacitor ground map
(2) When the grounding devices and power supply filter capacitors on the RF main signal path need to be grounded, they must be grounded nearby to reduce the grounding inductance of the devices.
(3) The bottom of some components is a grounded metal shell. Some grounding holes must be added in the projection area of the component. Signal lines and vias must not be arranged on the surface layer in the projection area;
(4) When the ground wire needs to run a certain distance, the line width should be increased and the length should be shortened. It is prohibited to approach or exceed 1/4 of the guide wavelength to prevent signal radiation caused by the antenna effect.
(5) Except for special purposes, there must be no isolated copper sheets, and ground wire vias must be added to the copper sheets.
(6) For some sensitive circuits and circuits with strong radiation sources, place them in shielding cavities. The shielding cavity is pressed against the PCB surface during assembly. When designing the PCB, a "via shielding wall" must be added, which is to add a grounded via on the PCB at the location close to the shielding cavity wall. As shown in Figure 12 below, there must be more than two rows of via holes. The two rows of via holes are staggered from each other. The spacing between via holes in the same row is about 100 mils.
Routing Constraints: General Rules
(1) The top layer of PCB carries RF signals, and the plane layer below the RF signals must be a complete ground plane to form a microstrip line structure. As shown in Figure 13. To ensure the structural integrity of the microstrip line, it must be done: The microstrip lines in the same layer must be covered with ground copper. It is recommended that the edge of the ground copper should be 3H wide from the edge of the microstrip line. H represents the thickness of the dielectric layer. Within the 3H range, there must be no other signal vias. Do not allow RF signal traces to cross the ground plane gap on the second layer. Ground copper should be added between uncoupled microstrip lines, and ground vias should be added to the ground copper.
The distance between the microstrip line and the shielding wall should be kept above 3H. Microstrip lines must not cross the dividing line of the second layer ground plane.
Microstrip line structure diagram
(2) The distance between ground copper and signal traces is required to be ≥3H.
(3) Add ground wire holes to the edge of the ground copper sheet. The hole spacing is about 100 mils and arranged evenly and neatly;
(4) The edge of the ground wire copper sheet must be smooth and flat, and sharp burrs are prohibited;
(5) Except for special purposes, it is prohibited to extend excess wire ends on the RF signal wiring.
(6) If there are other RF signal lines around the RF signal wiring, a ground copper sheet must be added between the two, and a grounding via should be added at an interval of about 100 mils on the ground copper sheet for isolation.
(7) If there are other unrelated non-RF signals (such as passing power lines) around the RF signal wiring, a ground copper foil should be added between the two, and a ground via should be added every 100 mils or so.
(8) The RF signal via is close to other wiring in the inner layer. For example, the power line passing through is close to the RF signal via as shown in the left figure. The EMI interference on the power line will penetrate into the RF wiring. Therefore, the correct wiring method shown in the right figure of Figure 14 should be adopted. The ground and ground vias should be added between the power line and the RF signal via to play an isolation role. Sometimes the RF signal line in the inner layer is close to the vias of other signals with strong interference (such as the passing power line). The same method is also used to add ground and ground vias.
Power line and RF via wiring diagram
(9) When the device mounting hole is a non-metallized hole, the RF signal wiring should be kept away from the device mounting hole. It is necessary to insert a ground copper foil between the RF signal wiring and the mounting hole, and add a ground via.
1. Wiring priority
Prioritize key signal lines:
Key
signals such as power supply,
analog
small signals, high-speed signals, clock signals, and synchronization signals
should be routed first.
Density priority principle: Start routing from the device with the most complex connections on the board.
Start routing from the area with the most dense connections on the board.
2. Automatic wiring
When the wiring quality meets the design requirements, an automatic router can be used to improve work efficiency.
The following preparations should be completed before automatic wiring:
Automatic wiring control file (do file)
In order to better control the wiring quality, wiring rules must be defined in detail before running. These rules can be defined in the graphical interface of the software, but the software provides a better control method, that is, for Design situation, write the automatic wiring control file (do file), and the software runs under the control of this file.
3.
Try to provide dedicated wiring layers for key signals such as clock signals, high-frequency signals, and sensitive signals, and ensure
To ensure the minimum loop area.
If necessary, manual priority wiring, shielding and increased safety spacing should be adopted
to ensure signal quality.
4. The EMC environment between the power layer and the ground layer is poor, so avoid arranging signals that are sensitive to interference.
5. Networks requiring impedance control should be arranged on the impedance control layer.
6. Rules that should be followed when designing PCB
1) Ground loop rules:
The minimum loop rule means that the loop area formed by the signal line and its loop should be as small as possible. The smaller the loop area,
the less external radiation
and
the smaller the received external interference. In response to this rule, when dividing the ground plane,
the distribution of the ground plane and important signal traces must be considered to prevent problems caused by ground plane slotting; in double-layer board design, leave enough space for the power supply. In this case, the remaining part should be filled with reference ground, and some necessary holes should be added to effectively connect the double-sided ground signals. Ground isolation should be used as much as possible for some key signals. For some higher frequency designs, it is necessary to In particular, considering the problem of the ground plane signal loop, it is recommended to use a multi-layer board.
2)
Crosstalk
control:
Crosstalk refers to the mutual
interference . It is mainly due to the distributed capacitance and distributed inductance between parallel lines. The main measures to overcome crosstalk are:
Increase the spacing between parallel wirings and follow the 3W rule.
Insert grounded isolation wires between parallel wires.
Reduce the distance between wiring layers and ground planes.
3) Shielding protection
The corresponding ground loop rule is actually to minimize the loop area of the signal, which is more common in some
more important signals, such as clock signals and synchronization signals; for some particularly important and high-frequency signals,
copper axis cables should be considered The shielding structure design is to isolate the laid lines with ground wires, and also consider how to effectively combine the shielding ground with the actual ground plane.
4) Routing direction control rules:
That is, the wiring directions of adjacent layers form an orthogonal structure. Avoid running different signal lines in the same
direction
to reduce unnecessary inter-layer
interference
; when it is difficult to avoid
this situation due to board structure limitations (such as some backplanes), especially when the signal rate is high When doing this, you should consider using a ground plane to isolate each wiring layer, and using a ground signal line to isolate each signal line.
5) Open-loop inspection rules for wiring:
Generally, wiring with one end floating in the air (Dangling Line) is not allowed.
This is mainly to avoid the "antenna effect" and reduce unnecessary interference radiation and reception, otherwise it may bring unpredictable results.
6) Impedance matching check rules:
The wiring width of the same network should be consistent. Changes in line width will cause
uneven . When the transmission speed is high, reflection will occur. This situation should be avoided in the design. Under certain conditions, such as connector lead-out lines and BGA package lead-out lines with similar structures, it may be impossible to avoid changes in line
width
, and the effective length of the inconsistent portion in the middle should be minimized.
7) Routing termination network rules:
In high-speed digital circuits, when the delay time of PCB wiring is greater than
1/4 of the signal rise time (or fall time), the wiring can be regarded as a transmission line. In order to ensure that the input and output impedance of the signal correctly matches the impedance of the transmission line, Various forms of matching methods can be used, and the selected matching method is related to the connection method of the network and the topology of the wiring.
A. For point-to-point (one output corresponds to one input) connection, you can choose starting series matching or terminal
parallel matching. The former has a simple structure and low cost, but has a large delay. The latter has good matching effect, but has complex structure and
high cost.
B. For point-to-multipoint (one output corresponds to multiple outputs) connection, when the network topology is daisy chain
, terminal parallel matching should be selected. When the network is a star structure, you can refer to the point-to-point structure.
Star and daisy chain are two basic topological structures. Other structures can be regarded as deformations of the basic structure, and some flexible measures can be
taken for matching. In actual operations, factors such as cost, power consumption, and performance must be taken into consideration
. Complete matching is generally not pursued, as long as interference such as reflections caused by mismatch is limited to an acceptable range.
8) Cabling closed-loop inspection rules:
Prevent signal lines from forming self-loops between different layers. This problem is prone to occur in multi-layer board designs, and self-loops
will cause radiation interference.
9) Rules for controlling the branch length of the routing:
Try to control the length of the branch. The general requirement is Tdelay<=Trise/20.
10) Resonance rules for wiring:
Mainly for high-frequency signal design, that is, the wiring length must not be an integer multiple of its wavelength to avoid
resonance.
11) Trace length control rules:
That is, the short-line rule. When designing, the wiring length should be as short as possible to reduce
the interference caused by long wiring. In particular, for some important signal lines, such as clock lines, their oscillators must be placed
very close to the device. For driving multiple devices, the network topology should be determined according to the specific situation.
12) Chamfering rules:
Sharp angles and right angles should be avoided in PCB design to avoid unnecessary radiation and
poor process performance.
13) Device decoupling rules:
A. Add necessary decoupling capacitors to the printed circuit board to filter out interference signals on the power supply and stabilize the power supply signal.
In multi-layer boards, the location of decoupling capacitors is generally not very demanding, but for double-layer boards, the layout of decoupling capacitors and the wiring method of the power supply will directly affect the stability of the entire system, and sometimes even affect the design. success or failure.
B. In the double-layer board design, the current should generally be filtered by the filter capacitor before being used by the device. At the same time,
the impact of the power supply noise generated by the device on the downstream devices should also be fully considered. Generally speaking,
a bus structure design is used. Better yet, when designing, you should also consider the impact of voltage drops on devices due to long transmission distances, and add some power supply filtering loops if necessary to avoid potential differences.
C. In high-speed circuit design, whether decoupling capacitors can be used correctly is related to the stability of the entire board.
14) Device layout partition/layer rules:
A. Mainly to prevent mutual interference between modules with different operating frequencies and to
shorten
the wiring length
of
the high-frequency part as much as possible
. Usually the high-frequency part is arranged in the interface part to reduce the wiring length. Of course, such a
layout still needs to take into account the possible interference that low-frequency signals may receive. At the same time, the problem of dividing the ground plane of the high/low frequency part must also be considered. Usually, the ground planes of the two are divided and then connected at a single point at the interface.
B. For hybrid circuits, there is also a method of arranging analog and digital circuits on both sides of the printed board, using
different
layers
of wiring, and using ground layers in the middle to isolate them.
15) Isolated copper zone control rules:
The emergence of isolated copper areas will bring about some unpredictable problems. Therefore,
connecting the isolated copper areas with other signals will help improve the signal quality. Usually, the isolated
copper
areas are grounded or deleted. In actual production,
PCB manufacturers add some copper foil to the vacant parts of some boards. This is mainly to facilitate printed board processing, and it also plays a certain role in preventing printed board warpage.
The emergence of isolated copper areas will bring some unpredictable problems, so connecting the isolated copper areas to other signals will help improve signal quality.
Typically isolated copper areas are grounded or removed. In actual production, PCB manufacturers add some copper foil to the vacant parts of some boards. This is mainly to facilitate printed board processing, and it also plays a certain role in preventing printed board warpage.
16) Integrity rules for power and ground layers:
For areas with dense via holes, care should be taken to avoid holes connecting to each other in the hollowed-out areas of the power supply and ground layers,
forming a division of the plane layer, thereby destroying the integrity of the plane layer, and thereby causing an increase in the
loop big.
17) Overlapping power and ground layer rules:
Different power supply layers should avoid overlapping in space. This
is mainly to reduce interference between different power supplies, especially between power supplies with large voltage differences. The overlap of power supply planes must be avoided. If it is difficult to avoid, consider using a ground layer in the middle.
18) 3W rule:
In order to reduce crosstalk between lines, the line spacing should be large enough. When the line center spacing is not less than 3 times the line width,
If 70% of the electric fields do not interfere with each other, this is called the 3W rule. If 98% of the electric fields do not interfere with each other,
a spacing of 10W can be used.
19) 20H Rules:
Since the electric field between the power layer and the ground layer is changing, electromagnetic interference will be radiated outward at the edge of the board.
This is called the edge effect. The solution is to shrink the power layer so that the electric field is only conducted within the range of the ground layer.
Taking one H (the dielectric thickness between the power and the ground) as a unit, if the shrinkage is 20H, 70% of the electric field can be confined to the edge of the ground layer; if the shrinkage is 100H, 98% of the electric field can be confined.
20) Five, five rules:
The rule for selecting the number of layers of a printed circuit board is that if the clock frequency reaches 5MHz or the pulse rise time is less than 5ns, the PCB
board must use a multi-layer board. This is a general rule. Sometimes, due to cost and other factors, a double-layer
board structure is used. In this case, it is best to use one side of the printed circuit board as a complete ground plane layer.
Source: Hardware One Hundred Thousand Whys
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