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Multi-layer PCB Design Layout and Routing Principles

Posted:03:38 PM July 14, 2021 writer: nextpcb


General principles of component layout

The general principles that designers need to follow in the circuit board layout process are as follows.
(1) The components are best placed on one side. If you need to place components on both sides and place pin-type components on the bottom layer, it may be difficult to place the circuit board and not conducive to soldering. Therefore, it is best to place only SMD components on the bottom layer, similar to common computer graphics PCB boards. Layout method of components. For single-sided placement, only one side of the circuit board needs to be printed with a silk screen layer, which is convenient for reducing costs.
(2) Reasonably arrange the position and direction of the interface components. Generally speaking, the connector components that connect the circuit board to the outside world (power supply, signal line) are usually arranged on the edge of the circuit board, such as serial ports and parallel ports. If it is placed in the center of the circuit board, it is obviously not conducive to wiring, and it may also be unable to connect due to obstacles from other components. In addition, pay attention to the direction of the interface when placing the interface, so that the connection line can be smoothly led out, away from the circuit board. After the interface is placed, the type of the interface should be clearly marked with the String of the interface component; for the power supply interface, the voltage level should be marked to prevent the circuit board from being burned due to wiring errors.
(3) It is better to have a wide electrical isolation band between high-voltage components and low-voltage components. That is to say, do not place components with very different voltage levels together. This is not only beneficial to electrical insulation, but also has great advantages in signal isolation and anti-interference.
(4) Components with close electrical connections are best placed together. This is the modular layout idea.
(5) For components that are prone to noise, such as high-frequency components such as clock generators and crystal oscillators, they should be placed as close to the clock input of the CPU as possible when placing them. High-current circuits and switching circuits are also prone to noise. These components or modules should also be kept away from high-speed signal circuits such as logic control circuits and storage circuits during layout. If possible, try to use the control board combined with the power board and use the interface To improve the overall anti-interference ability and working reliability of the circuit board. 
(6) Try to place decoupling capacitors and filter capacitors around the power supply and the chip. The arrangement of decoupling capacitors and filter capacitors is an important measure to improve the power quality of the circuit board and improve the anti-interference ability. In practical applications, the traces, pin connections, and wiring of the printed circuit board may bring about large parasitic inductances, resulting in high-frequency ripples and burrs in the power supply waveform and signal waveform, and the power and ground Placing a 0.1µF decoupling capacitor between them can effectively filter out these high frequency ripples and glitches. If the chip capacitor is used on the circuit board, the chip capacitor should be close to the power supply pin of the component. For power conversion chips, or power input terminals, it is best to arrange a 10µF or larger capacitor to further improve the power quality.
(7) The number of the components should be arranged close to the frame of the components, the size should be uniform, the direction should be neat, and it should not overlap with the components, vias and pads. The first pin of the component or connector indicates the direction; the positive and negative signs should be clearly marked on the PCB and are not allowed to be covered; power conversion components (such as DC/DC converters, linear conversion power supplies and switching power supplies) There should be enough space for heat dissipation and installation, and enough space for welding on the periphery.

General principles of component wiring

The general principles that designers need to follow in the circuit board wiring process are as follows.
(1) The principle of setting the spacing of printed traces of components. The spacing constraints between different networks are determined by factors such as electrical insulation, manufacturing process, and component size. For example, the pin pitch of a chip component is 8mil, then the [Clearance Constraint] cannot be set to 10mil, the designer needs to set a 6mil design rule for the chip separately. At the same time, the spacing setting should also consider the production capacity of the manufacturer.
In addition, an important factor that affects components is electrical insulation. If the potential difference between two components or networks is large, electrical insulation needs to be considered. The gap safety voltage in a general environment is 200V/mm, which is 5.08V/mil. So when there are both high-voltage and low-voltage circuits on the same circuit board, you need to pay special attention to sufficient safety distance.
(2) The choice of the wiring form at the corner of the line. In order to make the circuit board easy to manufacture and beautiful, it is necessary to set the corner mode of the circuit when designing, and you can choose 45°, 90° and arc. Generally, sharp corners are not used. It is best to use arc transition or 45° transition, and avoid 90° or sharper corner transitions.
The connection between the wire and the pad should also be as smooth as possible to avoid small sharp feet, which can be solved by teardropping. When the center distance between the pads is less than the outer diameter D of a pad, the width of the wire can be the same as the diameter of the pad; if the center distance between the pads is greater than D, the width of the wire should not be greater than that of the pad diameter.
When the wires pass between the two pads but do not communicate with them, they should be kept at the largest and equal spacing. Similarly, the spacing between the wires and the wires should be uniform and equal and kept at the maximum.
(3) How to determine the width of printed traces. The width of the trace is determined by factors such as the level of current flowing through the wire and anti-interference. The larger the current flowing through, the wider the trace should be. Generally, the power line should be wider than the signal line. In order to ensure the stability of the ground potential (less affected by changes in the magnitude of the ground current), the ground wire should also be wider. Experiments show that when the copper film thickness of the printed wire is 0.05mm, the current carrying capacity of the printed wire can be calculated at 20A/mm2, that is, a wire with a thickness of 0.05mm and a width of 1mm can flow 1A current. Therefore, for general signal lines, a width of 10-30 mils can meet the requirements; for high-voltage, high-current signal lines, the line width is greater than or equal to 40 mils, and the line spacing is greater than 30 mils. In order to ensure the anti-stripping strength and working reliability of the wire, the widest possible wire should be used to reduce the line impedance and improve the anti-interference performance within the allowable range of the board area and density.
For the width of the power line and ground line, in order to ensure the stability of the waveform, if the wiring space of the circuit board allows, try to thicken it as much as possible. Generally, it needs at least 50mil.
(4) Anti-interference and electromagnetic shielding of printed wires. The interference on the wires mainly includes the interference introduced between the wires, the interference introduced by the power line and the string between the signal lines, etc. Reasonable arrangement and layout of the wiring and grounding method can effectively reduce the interference source, so that the designed circuit board has more Good electromagnetic compatibility performance. For high-frequency or other important signal lines, such as clock signal lines, on the one hand, the traces should be as wide as possible, on the other hand, it can be isolated from the surrounding signal lines in the form of a ground (that is, use a closed ground The wire "wraps" the signal wire, which is equivalent to adding a ground shielding layer).
The analog ground and digital ground must be wired separately and cannot be mixed. If you need to finally unify the analog ground and digital ground into one potential, you should usually use one point grounding method, that is, only select one point to connect the analog ground and digital ground to prevent the formation of a ground loop and cause ground potential shift.
After wiring is completed, a large area of ​​grounding copper film, also called copper, should be laid on the top and bottom layers where no wires are laid to effectively reduce the impedance of the ground wire, thereby weakening the high-frequency signal in the ground wire, and at the same time. Area grounding can inhibit electromagnetic interference.
A via in the circuit board will bring about 10pF of parasitic capacitance, which is particularly harmful to high-speed circuits; at the same time, too many vias will also reduce the mechanical strength of the circuit board. Therefore, when wiring, the number of vias should be reduced as much as possible. In addition, when penetrating vias (through holes) are used, pads are usually used instead. This is because when the circuit board is manufactured, some penetrating vias (through holes) may not be penetrated due to processing, and the pads can definitely be penetrated during processing, which is equivalent to The production brings convenience.
The above are the general principles of PCB board layout and wiring, but in actual operation, the layout and wiring of components is still a very flexible job. The layout and wiring methods of components are not unique, and the results of layout and wiring are very different. To a large extent, it still depends on the experience and ideas of the designer. It can be said that there is no standard to judge the right and wrong of layout and wiring schemes, only the relative advantages and disadvantages can be compared. Therefore, the above layout and wiring principles are only for design reference, and practice is the only criterion for judging the pros and cons.




Tag: PCB design pcb design Multi-layer PCB PCB Design PCB Design PCB Design Layout Routing Principles
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