Analysis of wiring rules for PCB traces

General wiring rules:

1. Draw wiring area, within ≤1mm from the edge of the PCB board, and within 1mm around the mounting hole, wiring is prohibited.

2. The power cord should be as wide as possible, should not be lower than 18mil, the signal line width should not be lower than 4mil, the cpu access line should not be lower than 4mil (or 6mil), the line spacing should be no less than 8mil; the high density board can be used 4 /6mil line width / pitch, low density version, try to use 6 / 8mil line width / spacing. Signal line spacing must follow the 3W principle.

3. The normal via hole is not less than 12 mil; the high density board can be considered to have a via of 8/12 mil or more of the inner and outer diameter, and the low density board is more than 12/24 mil.

4. The traces on the printed board are as short as possible.

5. The line should avoid acute angle, right angle, adopt 45° angle of travel; the plate is FR4 high-speed board, consider the cross-weaving method of glass fiber, and adopt 10 degree wiring to achieve better impedance control when the signal rate reaches 4GHz. Or let the length of the board rotate the glass substrate by 10 degrees (increased cost, not recommended).

6. The direction of the signal line trace of each layer is different from the trace direction of the adjacent layer, and it is preferable that the adjacent layer signal lines are orthogonal.

7. Prevent signal lines from forming self-loops between different layers.

8. Normally, wiring with one end floating is not allowed. When designing jumpers, jumper resistors/capacitors should be added across the jumper, not just at one end.

9. The power cable and ground wire should be oriented in the same direction as the data flow to enhance the anti-noise capability.

10. For differential signal lines, the wires should be routed in pairs. Try to make them parallel and close to each other, and the length difference is not big. Try to make fewer holes. When you have to punch holes, you should punch the two lines together.

11. A group of buses with the same attributes should be routed side by side as much as possible.

12. The wires on the input and output of the PCB should be avoided as far as possible, and the ground wire should be placed between the two wires to avoid circuit feedback.

13. The digital ground and analog ground should be separated. For low-frequency circuits, the ground should be grounded in a single point and parallel; the high-frequency circuit should adopt multi-point series grounding. For digital circuits, the ground wire should be closed into a loop to improve noise immunity.

14. The whole circuit board should be routed and punched evenly to avoid obvious unevenness. When the outer layer signal of the printed board has a large blank area, an auxiliary line should be added to make the distribution of the metal line on the board surface substantially balanced.

15. The low-frequency circuit can be grounded in parallel with a single point. The actual wiring can be connected in series and then connected in parallel, and the high-frequency circuit is connected by multiple points. The ground wire should be short and thick. For large-frequency foil around the high-frequency components, the ground wire should be as thick as possible. If the ground wire is a very thin wire, the ground potential changes with current, which reduces the noise immunity.

16. The wiring width of the same network should be consistent. The variation of the line width will cause the characteristic impedance of the line to be uneven. When the transmission speed is high, reflection will occur. This should be avoided in the design. Under certain conditions, such as the connector lead-out line and the similar structure of the lead-out line of the BGA package, the line width variation may not be avoided, and the effective length of the inconsistent portion in the middle should be minimized.

17. Multilayer boards should be as symmetrical as possible and follow the 20H principle when designing the laminated structure. The trace density and copper plating of each layer should be as symmetrical as possible to reduce warpage and improve EMI during soldering of the board.

18. The signal line should not be divided across the power supply and ground. The signal reference plane should be as complete as possible.

19. Impedance control: For the signal line that needs impedance control, it must be strictly in accordance with the calculated data wiring, and the board manufacturer is required to do impedance control in the board specification. For signal lines that do not require impedance control, it is also necessary to calculate the impedance after wiring to develop a good habit. Generally speaking, the single-ended signal is wired with a 50 ohm impedance. In the double panel, the impedance is calculated according to the conventional model, and the line width is too large, which is difficult to achieve. The following model can be used to calculate the impedance:

20. Grid copper should be used with caution in low frequency circuits. Grid copper can effectively improve the problem of large area copper blistering, but grid copper can be seen as having a number of traces. When using grid copper, you need to consider the electrical length of the grid line and the work of the board. The relationship of frequency. The power supply should also be copper-plated as much as possible, and the power supply copper should be solid copper. After the PCB design is completed, the board needs to be proofed, and the blogger has a discount.

Special wiring rules:

1. Power and ground wiring

(1) Try to increase the power line and ground line width as much as possible to reduce the loop resistance. In particular, it is necessary to make the power supply direction in the power line and the ground line opposite to the data and signal transmission direction, that is, the power supply mode that is advanced from the final stage to the front stage, which helps to enhance the anti-noise capability.

(2) Preferably, the ground line is wider than the power line, and their relationship is: ground line > power line > signal line.

(3) Use a large-area copper layer for grounding, and connect the unused areas to the ground on the printed board for grounding. Or make a multi-layer board, power supply, ground line each occupy a layer.

(4) The digital ground is separated from the analog ground. If there are both logic circuits and linear circuits on the circuit board, they should be separated as much as possible. The ground of the low-frequency circuit should be grounded in parallel at a single point. If the actual wiring is difficult, it can be partially connected. Parallel grounding; high-frequency circuit should adopt multi-point series grounding, the ground wire should be short and thick, and the grid-like large-area foil should be used as much as possible around the high-frequency components.

(5) The grounding wire of the digital circuit system should form a closed loop circuit, which can improve the anti-noise ability.

2. The signal wire is laid on the electrical layer

When there are not many lines left on the signal line layer, it is laid on the power layer. When wiring the power plane, consider the integrity of the power plane as the reference layer of the adjacent signal layer.

Do not form a loop on any signal line. If it is unavoidable, the loop should be as small as possible, and the signal line should have as few vias.

3. Clock wiring

When routing the clock line, you should make fewer holes. Try to avoid running the line with other signal lines, and keep away from the general signal line to avoid interference with the signal line. Also avoid the power section on the board to prevent the power supply and clock from interfering with each other. When multiple clocks of different frequencies are used on one circuit board, two clock lines of different frequencies cannot be paralleled. The clock line should also be kept as close as possible to the output interface.

4. Crystal wiring

All lines connected to the input and output of the crystal are as short as possible to reduce noise interference and the effect of distributed capacitance on the crystal. The crystal can be surrounded by copper and ground the crystal case to improve the interference of the crystal with other components.

5. The effect of layout on wiring

Sometimes choosing a good layout will make wiring much easier. For example, in DDR3, if the fly bye topology is used, the lengths of the clock line and the data line do not need to be specially controlled. It is only necessary to note that the stubs of the clock and address lines need to be of equal length. If a T-type network is used, the equal length rule will be extremely troublesome, resulting in excessive winding. Excessive winding often has a negative impact.

One. PCB introduction:

1.PCB (Printing Circuit Board) material:

The printed circuit board is made of copper-clad laminate. The common copper-clad laminate is copper-clad phenolic paper laminate, copper-coated epoxy paper laminate, copper-clad epoxy glass laminate, copper-clad epoxy phenolic Glass cloth laminates, copper-clad polytetrafluoroethylene glass cloth laminates, and epoxy glass cloth for multilayer boards. Epoxy resin and copper foil have good adhesion, and the board made of epoxy resin can not be foamed in the tin furnace at 260 °C, and it is not easy to get wet, so the PCB made by this material is more used. . Ultra-high frequency PCBs are preferably laminated with copper-clad polytetrafluoroethylene glass cloth. Some flame retardant resin materials have also been added to the flame retardant PCB.

2.PCB (Printing Circuit Board) board layer: (for example, four-layer board) silk screen (Top overlay): silk screen layer Solder Mask (Top/Bottom): solder mask Paste Mask (Top/Bottom): solder paste layer Top: The top layer is the component layer Bottom: the bottom layer is the solder layer

Drill Guide (Drill Drawing): Drilling layer

Keep out layer: Disable the wiring layer for setting the PCB edge. Mechanical Layer: Mechanical layer for board layout Multi Layer: Penetration layer Vcc Layer: Intermediate power layer Gnd Layer: Intermediate layer 2. PCB overall layout:

About PCB trace

two. Various holes in the PCB:

The PCB has non-plated copper holes (NPTH), copper plated holes (PTH), via holes (VIA), buried holes (Buried), blind holes (Blind), and the like.

About PCB trace

(1). Plated Through Hole (PTH): A hole in which the wall of the hole is plated with metal to connect the intermediate layer and the outer conductive pattern.

(2). Non-plated through hole (NPTH): A hole in which the wall of the hole is not plated with metal to mechanically mount and mechanically secure the component. (such as screw holes) (3). Via (VIA): Used for electrical connections between unused layers of the PCB (such as blind vias and buried vias), plated vias that do not have component pins or other reinforcement materials.

Buried: Used for electrical connection between the inner and outer layers of a multilayer PCB.

Blind: Used for electrical connection between the inner and inner layers of a multilayer PCB.

About PCB traces About PCB traces About PCB traces

About PCB trace

three. Dimensions of the PCB trace PCB:

1. There are two types of units in the PCB: the conversions for Imperial and Metric are as follows:

1 meter (m) = 3.28 feet

1 foot = 12 inches (inch)

1 inch = 1000 mils = 2.54 cm



1um = 39.37 micro inches (mill)

1 oz = 35 microns (um) This unit represents the thickness of the copper foil. 2. This unit can also represent the size of the TV and Monitor. For example, for a 37-inch, 42-inch TV, the size refers to the length of the TV diagonal, which can be expressed. for:

About PCB trace

Fifth, the safe distance of PCB:

The safe distance for PCB traces is copper foil and copper foil wire (Track to Track), via and copper foil (Via to Track) vias and vias (Via to Via), copper foil wires and pads (Track to Pad), Pad to Pad, Via to Pan, etc.

The minimum distance (clearance).

Six. PCB high frequency circuit wiring:

(1) Reasonable selection of the number of PCB layers. The intermediate power layer (vcc layer) and the ground layer (Gnd layer) can be used for shielding, effectively reducing parasitic inductance and parasitic capacitance, and also greatly shortening the length of the wiring and reducing cross-talk between signals.

(2), the way of routing. Must follow the 45° corner and do not use the 90° corner. Figure:

About PCB trace (2), interlayer wiring direction. Should be perpendicular to each other, the top layer is horizontal, and the bottom layer is vertical, which can reduce interference between signals.

(3), the package. The packet processing of important signals can significantly improve the anti-interference ability of the signal, and can also multi-interfere signals to cover the ground so that it cannot interfere with other signals.

(4) Add the tantalum capacitor. Add a tantalum capacitor to the power supply end of the IC.

(5), high frequency turbulence. When there is a common ground such as digital ground and analog ground, a high-frequency turbulence device is added between them, and a high-frequency ferrite bead through which a wire is passed through a center hole can be generally used.

(6), paving copper. Increasing the area of ​​the ground also reduces signal interference.

(7), the length of the line. The shorter the trace length, the better, especially when the two wires are parallel.

Seven, the wiring of special components:

(1) High-frequency components: The shorter the connection between the high-frequency components, the better. Try to reduce the distribution parameters of the wires and the electrical interference between them. The components that are easy to interfere cannot be too close.

(2) Components with high potential difference: The distance between components with high potential difference and wiring should be increased to avoid accidental short circuit damage to components. In order to avoid the occurrence of creepage, it is generally required that the copper foil line distance between potential differences of 2000V should be greater than 2mm.

(3) Heavy components: Heavy components should be fixed by brackets.

(4) Heat and thermal components: Note that the heating components should be kept away from the thermal components.

Eight, the distance of the component from the edge of the PCB:

All components should be placed within 3 mm of the edge of the panel, or at least the distance from the edge of the panel equal to the thickness of the panel, which is due to the use of the pipeline insert and wave soldering in mass production. Prevent the PCB edge from being damaged when the external processing is performed, and the Track line of the PCB is broken, resulting in scrapping. If there are too many circuit components and have to exceed the range of 3mm, a 3mm process edge can be added to the edge of the PCB, and a V-shaped groove can be opened on the edge of the process, which is opened by hand during production.

Important parameters about PCB traces and PCB design:

(1) Copper foil line (Track) line width: single panel 0.3mm, double panel 0.2mm (2) minimum gap between copper foil lines: single panel 0.3mm, double panel 0.2mm (3) copper foil line distance PCB board The edge is at least 1mm, the component is at least 5mm from the edge of the PCB, and the pad is at least 4mm from the edge of the PCB.

(4) The pad diameter of a general via mounting component is twice the diameter of the pad inner diameter.

(5) Electrolytic capacitors should not be close to heating elements, such as high-power resistors, transformers, high-power transistors, three-terminal regulated power supplies, and heat sinks. The distance between the electrolytic capacitor and these components is not less than 10mm.

(6) There should be no copper foil wire (except grounding) and components within 5mm of the screw hole radius.

Regarding the PCB trace (7) in the large-area PCB design (more than 500cm2 or more), in order to prevent the PCB from bending when the tin furnace is over, a gap of 5mm to 10mm wide should be left in the middle of the PCB without placing components for placement prevention. PCB curved beading.

(8) Each PCB uses a hollow arrow to indicate the direction of the tin furnace.

(9) When wiring, the IC placement direction of the DIP package should be perpendicular to the direction of the tin furnace. Try not to be parallel to avoid short circuit.

(10) When the wiring direction is turned from vertical to horizontal, it should enter from the 45° direction.

(11) The fuses, fuses, AC 220V filter capacitors, transformers and other components on the PCB should be marked with a warning mark on the top screen.

(12) The interval between the live and neutral lines of the AC 220V power supply section shall be not less than 3 mm. The distance between any one of the 220V circuit and the low voltage component and the pad and track should be no less than 6mm. The high voltage mark should be printed on the silk screen. The weak wire and the strong electricity should be separated by a thick wire mesh. .

X. Zero Power Technology PCB Design Rules

The above nine articles are the basic information on the Internet, and introduce the basic concepts and rules of PCB design. The following are the rules for PCB designers to follow for the current situation of the company:

1. Tools: Prioritize the use of Altium Designer Release 102, component library: use the company's unified component library LERLIB, the newly designed library needs to confirm that LERLIB does not contain this component, and needs to be updated into the library.

3. SCH rule 1 [Visual grid and electrical grid set to 5, 10, 5]

About PCB trace

4, PCB rules

ü minimum line spacing 10mil;

ü use teardrop Teardrops;

ü The copper-clad rule spacing is 25mil;

ü Conventional design uses 20mil line width according to 1A current. When the line width is insufficient, the soldering layer on the line can be added with tin; ü through hole requires solder mask; ü board thickness 1.6mm