1. 10 rules for component layout

1. Follow the layout principle of "big first, then small, first difficult and then easy", that is, important unit circuits and core components should be placed first.

2. Refer to the schematic block diagram in the layout, and arrange the main components according to the flow direction of the main signal of the single board.

3. The arrangement of components should be convenient for debugging and maintenance, that is, large components cannot be placed around small components, and there should be enough space around the components and components to be debugged.

4. For the circuit parts of the same structure, use the "symmetrical" standard layout as much as possible;

5. Optimize the layout according to the standards of uniform distribution, balanced center of gravity and beautiful layout;

6. The same type of plug-in components should be placed in one direction in the X or Y direction. The same type of polarized discrete components should also strive to be consistent in the X or Y direction, which is convenient for production and inspection.

7. Generally, the heating elements should be evenly distributed to facilitate the heat dissipation of the single board and the whole machine. The temperature-sensitive devices other than the temperature detection element should be kept away from the components with large heat generation.

8. The layout should meet the following requirements as far as possible: the overall connection is as short as possible, and the key signal lines are the shortest; high-voltage, high-current signals are completely separated from low-current, low-voltage weak signals; analog signals are separated from digital signals; high-frequency signals Separate from low frequency signals; high frequency components should be adequately spaced.

9. The decoupling capacitor should be placed as close as possible to the power supply pins of the IC, with the shortest loop between it and the power supply and ground.

10. During component layout, due consideration should be given to placing devices using the same power supply as close as possible to facilitate future power supply separation.

2. Wiring

(1) Wiring priority

Priority of key signal lines: priority wiring of key signals such as analog small signals, high-speed signals, clock signals and synchronization signals

Density priority principle: Start wiring from the device with the most complex connection relationship on the single board. Start routing from the densest area of the board

be careful:

Try to provide a dedicated wiring layer for key signals such as clock signals, high-frequency signals, and sensitive signals, and ensure the smallest loop area. When necessary, methods such as manual priority wiring, shielding and increasing safety distance should be adopted. Guaranteed signal quality.

The EMC environment between the power plane and the ground plane is poor, and signals sensitive to interference should be avoided.

Networks with impedance control requirements should be wired according to the line length and line width requirements as much as possible.

(2) Four specific wiring methods

1). The wiring of the clock:

The clock line is one of the biggest influencers on EMC. There should be fewer vias on the clock line, try to avoid running parallel with other signal lines, and should be far away from general signal lines to avoid interference to the signal lines. At the same time, the power supply part on the board should be avoided to prevent the power supply and the clock from interfering with each other.

If there is a special clock generation chip on the board, no wires can be routed below it, and copper should be laid below it, and if necessary, it can be specially cut off. For many chips that have reference crystal oscillators, there should be no traces under these crystal oscillators, and copper should be laid for isolation.

2). Right-angle wiring:

Right-angle wiring is generally a situation that needs to be avoided as much as possible in PCB wiring, and it has almost become one of the standards for measuring the quality of wiring. So how much impact does right-angle wiring have on signal transmission? In principle, right-angle traces will change the line width of the transmission line, resulting in discontinuities in impedance. In fact, not only right-angle traces, but also sharp-angle traces may cause impedance changes.

The impact of the right-angle trace on the signal is mainly reflected in three aspects:

The corner can be equivalent to a capacitive load on the transmission line, slowing down the rise time;

Impedance discontinuity will cause signal reflection;

EMI from right-angled tips.

3). Differential traces:

Differential signal (Differential Signal) is more and more widely used in high-speed circuit design, and the most critical signals in the circuit are often designed with differential structure. Definition: In layman's terms, the driving end sends two signals of equal value and opposite phase, and the receiving end judges the logical state "0" or "1" by comparing the difference between the two voltages. The pair of traces that carry the differential signal is called a differential trace.

Compared with ordinary single-ended signal traces, differential signals have the most obvious advantages in the following three aspects:

The anti-interference ability is strong, because the coupling between the two differential traces is very good. When there is noise interference in the outside world, they are almost coupled to the two lines at the same time, and the receiving end only cares about the difference between the two signals, so the external The common mode noise can be completely canceled.

It can effectively suppress EMI. In the same way, because the polarities of the two signals are opposite, the electromagnetic fields radiated by them can cancel each other. The tighter the coupling, the less electromagnetic energy is released to the outside world.

The timing positioning is accurate, because the switching change of the differential signal is located at the intersection of the two signals, unlike ordinary single-ended signals that rely on two threshold voltages, high and low, so it is less affected by process and temperature, which can reduce timing errors. Also more suitable for circuits with low amplitude signals. The current popular LVDS (low voltage differential signaling) refers to this small-amplitude differential signaling technology.

For PCB engineers, the most important concern is how to ensure that these advantages of differential routing can be fully utilized in actual routing. Maybe anyone who has been in contact with Layout will understand the general requirements of differential routing, that is, "equal length, equal distance".

Equal length is to ensure that the two differential signals maintain opposite polarities at all times and reduce common mode components; equal distance is mainly to ensure that the differential impedance of the two is consistent and reduce reflection. The "as close as possible principle" is sometimes one of the requirements for differential routing.

4). Serpentine:

Serpentine line is a type of routing method often used in Layout. Its main purpose is to adjust the delay to meet the system timing design requirements. The designer must first have this understanding: the serpentine line will destroy the signal quality, change the transmission delay, and try to avoid using it when wiring. However, in actual design, in order to ensure that the signal has sufficient holding time, or to reduce the time offset between the signals in the same group, it is often necessary to deliberately perform wiring.

be careful:

The differential signal lines that appear in pairs are generally routed in parallel, and holes should be drilled as little as possible. When holes must be drilled, holes should be drilled for both lines to achieve impedance matching.

A group of buses with the same attributes should be routed side by side as far as possible, and should be as long as possible. The vias drawn from the SMD pads should be as far away from the pads as possible.

(3) Common rules for wiring

1). The direction control rules of the trace:

That is, the wiring directions of adjacent layers form an orthogonal structure. Avoid running different signal lines in the same direction on adjacent layers to reduce unnecessary interlayer 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, It should be considered to isolate each wiring layer with a ground plane, and isolate each signal line with a ground signal line.

2). Open-loop inspection rules for traces:

Generally, Dangling Line with one end floating is not allowed, mainly to avoid "antenna effect" and reduce unnecessary interference radiation and reception, otherwise it may bring unpredictable results.

3). Impedance matching check rules:

The wiring width of the same network should be kept the same. The change of the line width will cause the uneven characteristic impedance of the line. When the transmission speed is high, reflection will occur. This situation should be avoided as much as possible in the design. Under certain conditions, such as the lead-out line of the connector, when the lead-out line of the BGA package has a similar structure, the change of the line width may not be avoided, and the effective length of the inconsistent part in the middle should be minimized.

4). Route length control rules:

That is, the short-circuit rule, in the design, the wiring length should be as short as possible to reduce the interference problem caused by the long trace, especially for some important signal lines, such as the clock line, the oscillator must be placed very close to the device The place. In the case of driving multiple devices, the network topology to be used should be decided on a case-by-case basis.

5). Chamfering rules:

In the PCB design, acute and right angles should be avoided, resulting in unnecessary radiation and poor process performance.

6). Device decoupling rules:

Add necessary decoupling capacitors on the printing plate to filter out the interference signal on the power supply and stabilize the power supply signal. In a multi-layer board, the position of the decoupling capacitor is generally not too high, but for a double-layer board, the layout of the decoupling capacitor 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.

In the double-layer board design, the current should generally be filtered by the filter capacitor before being used by the device.

In high-speed circuit design, the correct use of decoupling capacitors is related to the stability of the entire board.

7). Device layout partition/layering rules:

The main purpose is 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.

For hybrid circuits, there are also ways of arranging analog and digital circuits on both sides of the printed board, using different layers of wiring, and isolating them with ground layers in the middle.

8). Ground loop rules:

The minimum loop rule is that the area of the ring formed by the signal line and its loop should be as small as possible. The smaller the ring area, the less external radiation and the less external interference it receives.

9). Integrity rules for power and ground layers:

For areas with dense via holes, care should be taken to avoid the holes being connected to each other in the hollow area of the power supply and the ground layer, forming a division of the plane layer, thereby destroying the integrity of the plane layer, and thus causing the loop area of the signal line in the ground layer to increase. .

10). 3W rules:

In order to reduce the crosstalk between lines, the line spacing should be large enough. When the line center spacing is not less than 3 times the line width, 70% of the electric field can be kept from interfering with each other, which is called the 3W rule. To achieve 98% of the electric fields that do not interfere with each other, a 10W spacing can be used.

11). Shield protection

Corresponding to the ground loop rule, it is actually to minimize the loop area of the signal, which is more common in some more important signals, such as clock signals, synchronization signals; for some particularly important signals with particularly high frequencies, copper coaxial cables should be considered. The design of the shielding structure is to separate the line, the upper, the lower, the left and the right with the ground wire, and it is also necessary to consider how to effectively combine the shielding ground with the actual ground plane.

12). Route 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 and the impedance of the transmission line are correctly matched, Various forms of matching methods can be used, and the selected matching method is related to the connection mode of the network and the topology of the wiring.

For point-to-point (one output to one input) connection, you can choose either series matching at the beginning or parallel matching at the end. The former has a simple structure and low cost, but has a large delay. The latter has a good matching effect, but has a complex structure and a high cost.

For point-to-multipoint (one output corresponds to multiple outputs) connections, when the topology of the network is a 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, and other structures can be regarded as the deformation of the basic structure, and some flexible measures can be taken to match. In actual operation, factors such as cost, power consumption, and performance should be taken into account. Generally, complete matching is not pursued, as long as the interference such as reflection caused by mismatch is limited to an acceptable range.

13). Route closed-loop inspection rules:

Prevent signal lines from forming self-loops between different layers. Such problems are prone to occur in multilayer board designs, and self-loops will cause radiated interference.

14). The branch length control rule of the trace:

Try to control the length of the branch, the general requirement is Tdelay<=Trise/20.

15). Resonance rules for traces:

Mainly for high-frequency signal design, that is, the wiring length should not be an integer multiple of its wavelength, so as to avoid resonance.

16). Control rules for isolated copper areas:

The appearance of the isolated copper area will bring some unpredictable problems. Therefore, connecting the isolated copper area with other signals will help to improve the signal quality. Usually, the isolated copper area is grounded or deleted. In the actual production, the PCB manufacturer adds some copper foils to the vacant parts of some boards, which is mainly to facilitate the processing of the printed board, and also has a certain effect on preventing the warping of the printed board.

17). Rules for overlapping power and ground layers:

Different power layers should avoid overlapping in space. The main purpose is to reduce the interference between different power supplies, especially between some power supplies with large voltage differences, the overlapping problem of power supply planes must be avoided, and intermediate ground layers can be considered when it is unavoidable.

18). 20H Rules:

Due to the varying electric field between the power plane and the ground plane, electromagnetic interference is radiated outward from the edge of the plate. called the edge effect.

The solution is to shrink the power plane so that the electric field is conducted only within the confines of the ground plane. Taking one H (the thickness of the medium between the power supply and the ground) as the unit, if it is retracted by 20H, 70% of the electric field can be confined within the edge of the ground layer; if it is retracted by 100H, 98% of the electric field can be confined within.

(4) Others

For single and double-layer boards, the power cables should be as thick and short as possible. The width requirements of the power line and the ground line can be calculated according to the maximum current of 1A corresponding to the line width of 1mm, and the loop formed by the power supply and the ground is as small as possible.

In order to prevent the coupling noise on the power line from directly entering the load device when the power line is long, the power supply should be decoupled before entering each device. And in order to prevent them from interfering with each other, the power supply of each load is decoupled independently, and it is filtered first before entering the load.