PCB layout and routing rules analysis

Do you know what a PCB is? So do you know what PCB layout and routing rules are? 1. 10 rules for component layout: Follow the layout principle of “large first, then small, first difficult, then easy”, that is, important unit circuits and core components should be laid out first. The layout should refer to the schematic block diagram and arrange the main components according to the main signal flow pattern of the single board. The arrangement of components should be convenient for debugging and maintenance, that is, large components should not be placed around small components, and there should be enough space around components and devices that need to be debugged.

For circuit parts with the same structure, the "symmetrical" standard layout should be adopted as much as possible, and the layout should be optimized according to the standards of even distribution, center of gravity balance, and beautiful layout. Plug-in components of the same type should be placed in the same 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 to facilitate production and inspection.

Heating components should generally be evenly distributed to facilitate heat dissipation of the single board and the entire machine. Temperature-sensitive components other than temperature detection components should be kept away from components that generate large amounts of heat. The layout should try to meet the following requirements: the total wiring is as short as possible, and the key signal lines are the shortest; high voltage and high current signals are completely separated from small current and low voltage weak signals; analog signals are separated from digital signals; high frequency signals are separated from low frequency signals. Signals should be separated; high-frequency components should be adequately spaced.

The layout of the decoupling capacitor should be as close as possible to the power pin of the IC, and the loop formed between it and the power supply and ground should be the shortest. When laying out components, due consideration should be given to placing devices using the same power supply together as much as possible to facilitate future power supply separation.

2. Wiring

1. Wiring priority

Key signal line priority: Analog small signals, high-speed signals, clock signals, synchronization signals and other key signals are routed first. Density priority principle: Start wiring from the devices with the most complex connections on the board. Start routing from the most densely connected areas on the board.

be careful:

Try to provide dedicated wiring layers for key signals such as clock signals, high-frequency signals, and sensitive signals, and ensure the smallest loop area. If necessary, methods such as manual priority wiring, shielding, and increasing safety distances should be adopted. Ensure signal quality. The EMC environment between the power layer and the ground layer is poor, so signals that are sensitive to interference should be avoided. Networks with impedance control requirements should be laid out according to line length and width requirements as much as possible.

2. Four specific wiring methods

1) Clock wiring:

The clock line is one of the factors that has the greatest impact on EMC. There should be as few holes as possible on the clock line, try to avoid running them in parallel with other signal lines, and stay away from general signal lines to avoid interference with signal lines. At the same time, the power supply part of the board should be avoided to prevent the power supply and clock from interfering with each other. If there is a special clock generation chip on the board, no traces can be routed underneath it. Copper should be laid underneath it, and ground can be specially cut for it if necessary. For crystal oscillators that are referenced by many chips, traces should not be routed under these crystal oscillators, and copper should be laid for isolation.

2) Right angle routing:

Right-angle routing is generally a situation that needs to be avoided in PCB wiring, and it has almost become one of the standards for measuring the quality of wiring. So how much impact will right-angle routing have on signal transmission? In principle, right-angle routing will The line width of the transmission line changes, causing impedance discontinuity. In fact, not only right-angle wiring, but also round-corner and acute-angle wiring may cause impedance changes.

The impact of right-angle wiring on signals is mainly reflected in three aspects: the corners can be equivalent to capacitive loads on the transmission line, slowing down the rise time; impedance discontinuity will cause signal reflection; and EMI generated by right-angle tips.

3) Differential wiring:

Differential Signal is increasingly used in high-speed circuit design, and the most critical signals in the circuit often adopt differential structure design. Definition: In layman's terms, the driving end sends two equal and opposite signals, and the receiving end determines the logic state "0" or "1" by comparing the difference between the two voltages. The pair of traces that carry differential signals are called differential traces.

Compared with ordinary single-ended signal traces, the most obvious advantages of differential signals are reflected in the following three aspects: strong anti-interference ability, because the coupling between the two differential traces is very good, when there is noise interference from the outside, almost are coupled to two lines at the same time, and the receiving end only cares about the difference between the two signals, so the external common mode noise can be completely offset. It can effectively suppress EMI. In the same way, since the polarity of the two signals is opposite, the electromagnetic fields radiated by them can cancel each other out. The closer the coupling, the less electromagnetic energy released to the outside world.

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 high and low threshold voltages to judge, so it is less affected by process and temperature, and can reduce timing errors. At the same time It is also more suitable for circuits with low amplitude signals. The currently 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 the advantages of differential routing can be fully utilized in actual routing. Perhaps anyone who has been exposed to Layout will understand the general requirements for differential routing, which is "equal length and equal distance". The equal length is to ensure that the two differential signals maintain opposite polarity at all times and reduce the common mode component; the equal distance is mainly to ensure that the differential impedance of the two is consistent and reduce reflection. The "principle of getting as close as possible" is sometimes also one of the requirements for differential routing.

4) Serpentine line:

Snake lines are a type of wiring method often used in Layout. Its main purpose is to adjust the delay and meet the system timing design requirements. Designers must first understand that serpentine lines will destroy signal quality and change transmission delays, so they should be avoided 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 same group of signals, the wiring often has to be deliberately wound.

be careful:

Differential signal lines that appear in pairs are generally routed in parallel with as few holes as possible. When holes must be drilled, both lines should be drilled together to achieve impedance matching. A group of buses with the same attributes should be routed side by side as much as possible and have the same length as possible. The via holes leading from the patch pad should be as far away from the pad as possible.

3. Common rules for wiring

1) Direction control rules for wiring:

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 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, Consider using ground planes to isolate wiring layers and ground signal lines to isolate signal lines.

2) Open-loop inspection rules for wiring:

Generally, wiring with one end floating (Dangling Line) is not allowed, mainly to avoid the "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 consistent. Changes in line width will cause uneven line characteristic impedance. When the transmission speed is high, reflection will occur. This situation should be avoided in the design. Under certain conditions, such as connector pinouts and BGA package pinouts with similar structures, it may be impossible to avoid changes in line width, and the effective length of the inconsistent parts in the middle should be minimized.

4) Trace length control rules:

That is, the short wire rule. When designing, the wiring length should be kept as short as possible to reduce interference problems caused by too long wiring. Especially for some important signal lines, such as clock lines, the oscillator must be placed very close to the device. The place. For driving multiple devices, the network topology should be decided based on the specific situation.

5) Chamfering rules:

Sharp angles and right angles should be avoided in PCB design, which will produce unnecessary radiation and poor process performance.

6) Device decoupling rules:

Add necessary decoupling capacitors to the printed plate 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. In a 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, whether decoupling capacitors can be used correctly is related to the stability of the entire board.

7) Device layout partitioning/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 is also a method of arranging analog and digital circuits on both sides of the printed board, using different layers for wiring, and using ground layers in the middle to isolate them.

8) 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 external interference received.

9) 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 area of ​​the signal line in the ground layer. .

10)3W rule:

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.

11) 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 signals with particularly high frequencies, the copper-axis cable shielding structure design should be considered, that is, the laid lines are isolated with ground wires, up, down, left, and right, and how to effectively combine the shielding ground with the actual ground plane should also be considered.

12) Cabling 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.

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. For point-to-multipoint (one output corresponds to multiple outputs) connections, when the network topology 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. 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.

13) Cabling 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) Control rules for branch length of traces:

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

15) Resonance rules for wiring:

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

16) 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.

17) Rules for overlapping power and ground layers:

Different power layers should avoid spatial overlap. The main purpose is to reduce interference between different power supplies, especially between power supplies with widely different voltages. The overlapping problem of power supply planes must be avoided. If it is difficult to avoid, an intermediate ground layer can be considered.

18)20H rule:

Since the electric field between the power layer and the ground layer changes, electromagnetic interference will be radiated outward at the edge of the board. called edge effect. The solution is to shrink the power layer so that the electric field is only conducted within the ground layer. Taking one H (the thickness of the medium between the power supply and the ground) as a unit, if the indentation is 20H, 70% of the electric field can be restricted within the edge of the ground layer; if the indentation is 100H, 98% of the electric field can be restricted within the edge.

4. Others

For single and double-layer boards, the power cord should be as thick and short as possible. The width requirements of the power line and the ground line can be calculated based on the line width of 1mm corresponding to the maximum current of 1A. The loop formed by the power supply and the ground should be as small as possible. In order to prevent the coupled 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. In order to prevent them from interfering with each other, the power supply of each load is independently decoupled and filtered first before entering the load. These are the rules for PCB layout and routing, and everyone needs to be more standardized when designing.