PCB design process and precautions

小薇Emma

PCB design process and precautions [Copy link]

The basic design process of general PCB is as follows: preliminary preparation->PCB structure design->PCB layout->wiring->wiring optimization and silk screen->network and DRC inspection and structure inspection->plate making.

First: preliminary preparation. This includes preparing component libraries and schematics. "If you want to do your work well, you must first sharpen your tools." To make a good board, in addition to designing the principle, you must also draw it well. Before designing the PCB, you must first prepare the component library of the schematic SCH and the component library of the PCB. The component library can use the library that comes with peotel, but it is generally difficult to find a suitable one. It is best to make your own component library based on the standard size information of the selected device. In principle, make the component library of PCB first, and then make the component library of SCH. The component library of PCB has higher requirements, which directly affects the installation of the board; the component library of SCH has relatively loose requirements, as long as you pay attention to defining the pin attributes and the corresponding relationship with the PCB components. PS: Pay attention to the hidden pins in the standard library. After that, it is the design of the schematic diagram. After that, you are ready to start PCB design.

Second: PCB structure design. In this step, according to the determined circuit board size and various mechanical positioning, draw the PCB board surface in the PCB design environment, and place the required connectors, buttons/switches, screw holes, assembly holes, etc. according to the positioning requirements. And fully consider and determine the wiring area and non-wiring area (such as how much area around the screw hole belongs to the non-wiring area).

Third: PCB layout. Layout is simply to place devices on the board. At this time, if the preparations mentioned above are done, you can generate a network table on the schematic diagram (Design->Create Netlist), and then import the network table on the PCB diagram (Design->Load Nets). You can see that the devices are all stacked up, and there are flying wires between the pins. Then you can layout the devices. The general layout is carried out according to the following principles:

①. According to the reasonable division of electrical performance, it is generally divided into: digital circuit area (that is, afraid of interference and generates interference), analog circuit area (afraid of interference), power drive area (interference source); ②． Circuits that complete the same function should be placed as close as possible, and the components should be adjusted to ensure the simplest connection; at the same time, the relative positions of the functional blocks should be adjusted to make the connections between the functional blocks simplest; ③． For components with large mass, the installation position and installation strength should be considered; heating components should be placed separately from temperature-sensitive components, and thermal convection measures should be considered when necessary; ④． The I/O driver components should be placed as close to the edge of the printed circuit board and the lead-out connector as possible; ⑤． The clock generator (such as crystal oscillator or clock oscillator) should be as close as possible to the device that uses the clock;

⑥. A decoupling capacitor (usually a monolithic capacitor with good high-frequency performance) should be added between the power input pin and the ground of each integrated circuit; when the circuit board space is dense, a tantalum capacitor can also be added around several integrated circuits.

⑦. A discharge diode (1N4148 is sufficient) should be added to the relay coil; ⑧. The layout should be balanced, orderly and not top-heavy or one-sided. —— Special attention should be paid to the actual size (area and height) and relative position of components when placing them, so as to ensure the electrical performance of the circuit board and the feasibility and convenience of production and installation. At the same time, the placement of components should be appropriately modified to make them neat and beautiful, while ensuring that the above principles can be reflected. For example, the same components should be placed neatly and in the same direction, and should not be placed in a "staggered" manner.

This step is related to the overall image of the board and the difficulty of the next step of wiring, so you need to spend a lot of effort to consider it. When laying out, you can do preliminary wiring for places that you are not sure about and give them full consideration.

Fourth: wiring. Wiring is the most important process in the entire PCB design. This will directly affect the performance of the PCB board. In the design process of PCB, wiring is generally divided into three levels: the first is wiring, which is the most basic requirement for PCB design.

If the circuits are not connected and there are flying wires everywhere, it will be an unqualified board, which can be said to be not yet started. The second is the satisfaction of electrical performance. This is the standard for measuring whether a printed circuit board is qualified. This is after the wiring is connected, carefully adjust the wiring to achieve the best electrical performance. The next is beauty. If your wiring is connected, there is nothing that affects the electrical performance, but it looks messy at first glance, plus colorful and colorful, then no matter how good your electrical performance is, it is still a piece of garbage in the eyes of others. This brings great inconvenience to testing and maintenance. The wiring should be neat and uniform, and it should not be criss-crossed and disorderly. These must be achieved while ensuring the electrical performance and meeting other individual requirements, otherwise it will be a waste of effort. When wiring, it is mainly carried out according to the following principles:

①. In general, the power line and ground line should be routed first to ensure the electrical performance of the circuit board. Within the scope of conditions, try to widen the width of the power and ground lines. It is best that the ground line is wider than the power line. Their relationship is: ground line > power line > signal line. Usually the signal line width is: 0.2~0.3mm, the thinnest width can reach 0.05~0.07mm, and the power line is generally 1.2~2.5mm. For the PCB of digital circuits, a wide ground conductor can be used to form a loop, that is, to form a ground network for use (the ground of analog circuits cannot be used in this way)

②. Pre-route the lines with stricter requirements (such as high-frequency lines). The edge lines of the input and output ends should avoid being adjacent and parallel to avoid reflection interference. When necessary, ground isolation should be added. The wiring of two adjacent layers should be perpendicular to each other. Parallel wiring is prone to parasitic coupling.

③. The oscillator shell is grounded. The clock line should be as short as possible and should not be led everywhere. The area of the ground should be increased under the clock oscillation circuit and the special high-speed logic circuit part, and other signal lines should not be routed to make the surrounding electric field approach zero;

④. ⑤． Any signal line should not form a loop. If it is unavoidable, the loop should be as small as possible; the number of vias of the signal line should be as small as possible; ⑥． The key line should be as short and thick as possible, and add protective ground on both sides. ⑦． When transmitting sensitive signals and noise field band signals through flat cables, they should be led out in the form of "ground wire-signal-ground wire". ⑧． Test points should be reserved for key signals to facilitate production and maintenance inspection. ⑨． After the schematic wiring is completed, the wiring should be optimized; at the same time, after the preliminary network check and DRC check are correct, the ground wire is filled in the unwired area, and a large area of copper layer is used as the ground wire. On the printed circuit board, all the unused areas are connected to the ground as the ground wire. Or make a multi-layer board, with the power supply and ground wire occupying one layer each.

——PCB wiring process requirements

①. Line

In general, the signal line width is 0.3mm(12mil), and the power line width is 0.77mm(30mil) or 1.27mm(50mil); the distance between lines and between lines and pads is greater than or equal to 0.33mm(13mil). In practical applications, the distance should be increased when conditions permit;

When the wiring density is high, you can consider (but not recommend) using two lines between the IC feet. The width of the line is 0.254mm(10mil), and the line spacing is not less than 0.254mm(10mil). In special cases, when the device pins are dense and the width is narrow, the line width and line spacing can be appropriately reduced.

②. The basic requirements for pads (PADs) and vias (VIAs) are that the pad diameter must be 0.6mm larger than the via diameter; for example, general-purpose pin-type resistors, capacitors, and integrated circuits use pad/hole sizes of 1.6mm/0.8mm (63mil/32mil), and sockets, pins, and diodes 1N4007 use 1.8mm/1.0mm (71mil/39mil). In practical applications, the size of the actual components should be determined. If conditions permit, the pad size can be appropriately increased; the component mounting aperture designed on the PCB board should be 0.2 to 0.About 4mm.

③. Via (VIA)

is generally 1.27mm/0.7mm(50mil/28mil);

when the wiring density is high, the via size can be appropriately reduced, but it should not be too small, and 1.0mm/0.6mm(40mil/24mil) can be considered.

④. Pad, line, via spacing requirements

PAD and VIA: ≥ 0.3mm (12mil)

PAD and PAD: ≥ 0.3mm (12mil)

PAD and TRACK: ≥ 0.3mm（12mil）

TRACK and TRACK ： ≥ 0.3mm（12mil）

When the density is higher:

PAD and VIA ： ≥ 0.254mm（10mil）

PAD and PAD ： ≥ 0.254mm（10mil）

PAD and TRACK ： ≥ 0.254mm（10mil）

TRACK and TRACK ： ≥ 0.254mm (10mil)

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Fifth: wiring optimization and silk screen printing. "There is no best, only better"! No matter how hard you try to design, after you finish drawing, you will still find that many places can be modified. The general design experience is: the time for optimizing wiring is twice the time for initial wiring. When you feel that there is no need to modify, you can lay copper (Place->polygon Plane). Copper is usually laid for ground wires (note the separation of analog ground and digital ground), and it may also be necessary to lay power for multi-layer boards. When it comes to silk screen printing, be careful not to be blocked by components or removed by vias and pads. At the same time, when designing, look directly at the component side, and the bottom layer of words should be mirrored to avoid confusing the layers.

Sixth: Network and DRC inspection and structure inspection. First, on the premise of confirming that the circuit schematic design is correct, the generated PCB network file and the schematic network file are subjected to a network check (NETCHECK) of the physical connection relationship, and the design is promptly modified according to the output file results to ensure the correctness of the wiring connection relationship; after the network check is passed correctly, the PCB design is subjected to a DRC check, and the design is promptly modified according to the output file results to ensure the electrical performance of the PCB wiring. Finally, the mechanical installation structure of the PCB needs to be further checked and confirmed.

Seventh: plate making. Before this, it is best to have an audit process.

2px]PCB design is a job that requires careful thinking. The more careful and experienced you are, the better the board you will design. So you must be extremely careful when designing, and take all factors into consideration (for example, many people don't consider the convenience of maintenance and inspection). If you keep improving, you will definitely be able to design a good board.

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Yes, some details do need to be paid attention to, otherwise it will be not fun if mistakes are made later. Please support it.