PCB Tech - General Principles of Multilayer PCB board Layout

General principles of multilayer printed circuit boards layout and wiring. The general principles that designers need to follow in the circuit board layout process are as follows:

(1) The principle of setting the spacing of component printed traces. The spacing constraint between different networks is determined by the principles of electrical insulation, manufacturing process, and spacing of component printed traces. size and other factors. For example, the pin spacing of a chip component is 8mil, then the [Clearance Constraint] of the chip cannot be set to 10mil, and the designer needs to set a 6mil design rule for the chip. At the same time, the setting of the spacing should also take into account the production capacity of the manufacturer. In addition, an important factor affecting components is electrical insulation. If the potential difference between two components or networks is large, electrical insulation issues need to be considered. The clearance safety voltage in general environment is 200V/mm, which is 5.08V/mil. Therefore, when there are both high-voltage circuits and low-voltage circuits on the same circuit board, it is necessary to pay special attention to sufficient safety distance. When there are high-voltage circuits and low-voltage circuits, it is necessary to pay special attention to sufficient safety distance.

(2) The choice of the line corner routing form. In order to make the printed circuit boards easy to manufacture and beautiful, it is necessary to set the corner mode of the line and the selection of the line corner routing form during the design. 45°, 90° and Arc can be selected. Generally, sharp corners are not used, and arc transitions or 45° transitions are used, and 90° or sharper corner transitions are avoided. The connection between the wire and the pad should also be as smooth as possible to avoid the appearance of small pointed feet, which can be solved by the method of filling teardrops. 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 the diameter of the pad. diameter. When the wire passes between the two pads without being connected to it, it should maintain and equal spacing with them. Similarly, when the wire and wire pass between the two pads without connecting with it, it should keep and equal to them. The spacing between, and the spacing between should also be uniform and equal and maintained. The spacing should also be uniform and equal and maintained.

(3) How to determine the width of printed traces. The width of the trace is determined by factors such as the current level and anti-interference flowing through the wire. The larger the current flowing through the wire, the wider the trace should be. The power lines should be wider than the signal lines. In order to ensure the stability of the ground potential (the larger the ground current is, the wider the trace should be. Generally, the power line should be wider than the signal line, and the power line should be less affected by the width of the signal line), and the ground wire should also be wider than the signal line. The wide ground wire should also be wider. Experiments have shown that when the copper film thickness of the printed wire is 0.05mm, the current-carrying ground wire of the printed wire should also be wider and can be calculated according to 20A/mm2, that is, a wire with a thickness of 0.05mm and a width of 1mm can flow through a 1A wire. current. Therefore, the general width can meet the requirements; for high-voltage and high-voltage signal lines, the width of 10-30mil can meet the requirements for high-voltage, high-current signal lines with a line width greater than or equal to 40mil. The spacing between lines is greater than 30mil. In order to ensure the anti-stripping strength and working reliability of the wire, within the allowable range of board area and density, the wire as wide as possible should be used to reduce the line impedance and improve the anti-interference performance. For the width of the power line and the ground line, in order to ensure the stability of the waveform, the width of the circuit board should be thickened as much as possible when the wiring space of the circuit board allows. Generally, at least 50mil is required.

(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) the anti-interference and electromagnetic shielding of the printed wire. The interference on the wires mainly includes the interference introduced between the wires, the crosstalk between the signal wires, etc., and the crosstalk between the signal wires, etc. Reasonable arrangement and layout of the wiring and grounding methods can effectively reduce the interference source, so that the designed The circuit board has better electromagnetic compatibility performance. For high frequency or some other important signal lines, such as clock signal lines, on the one hand, the traces should be as wide as possible. For high frequency or some 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 adopted (that is, wrapping the signal line with a closed ground wire, and wrapping it is equivalent to adding a pack of ground to isolate it from the surrounding signal lines, which is to use a closed ground wire to wrap the signal line " wrap it up, layer ground shield). layer ground shield). The analog ground and digital ground should be wired separately and cannot be mixed. The analog ground and digital ground should be wired separately and cannot be mixed. If the analog ground and digital ground need to be unified into one potential, usually one point grounding method should be adopted, that is, only one point should be selected to connect the analog ground and digital ground to prevent the formation of a ground loop and cause ground potential offset.

After the wiring is completed, a large area of grounding copper film, also known as copper coating, should be applied on the top and bottom layers where no wires are laid. The area of grounding copper film, also known as copper coating, is used to effectively reduce the impedance of the ground wire, thereby weakening the high-frequency signal in the ground wire, and at the same time, a large area of grounding can suppress electromagnetic interference. The impedance of the ground wire is small, thereby weakening the high-frequency signal in the ground wire, and the large area of grounding can suppress the electromagnetic interference. A large area of grounding can suppress the parasitic capacitance of electromagnetic interference, which is especially harmful to high-speed circuits; at the same time, one via in a circuit board with too many vias will bring about 10pF of parasitic capacitance, which is especially harmful for high-speed circuits. Saying it is especially harmful also reduces the mechanical strength of the board. Therefore, when routing, the number of vias should be minimized. In addition, when using through-hole vias in routing, the number of vias should be minimized. When routing (through holes), pads are usually used instead. This is because when the circuit board is made, some penetrating vias (through holes) may not be penetrated due to processing, and the pads can definitely be penetrated during processing, which is also equivalent to giving Production brings convenience.

The above is the general principle of PCB board layout and wiring, but in actual operation, the layout and wiring of components is still a very flexible work.