Detailed explanation of the basic PCB design process

The general basic design process is as follows: preliminary preparation -> structural design -> layout -> optimization and silk screen -> network and DRC check and structural check -> plate making.

First: preliminary preparation.

This includes preparing component libraries and schematics. “If you want to do your job 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 provided by 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 the PCB first, and then the component library of the SCH. The component library of the PCB has higher requirements, which directly affects the installation of the board; the component library of the SCH is relatively loose, 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, and after it is done, 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, the PCB board surface is drawn in the PCB design environment, and the required connectors, buttons/switches, screw holes, assembly holes, etc. are placed according to the positioning requirements. And fully consider and determine the area and non-wiring area (such as how much area around the screw hole belongs to the non-wiring area).

Third: PCB layout.

To put it simply, layout is 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->CreateNetlist), and then import the network table on the PCB diagram (Design->LoadNets). You can see that the devices are all stacked up, and there are flying wires between the pins to prompt the connection. Then you can layout the devices. The general layout is carried out according to the following principles:

①. Reasonable division according to electrical performance, generally divided into: digital circuit area (i.e. afraid of interference and generating 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 between the functional blocks should be adjusted to make the connection between the functional blocks as simple as possible;

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 if necessary;

④. I/O driver devices should be as close to the edge of the printed board as possible and close to the lead-out connectors;

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 (generally 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 dense, and should not be top-heavy or heavy on one side.

Special attention should be paid:

When placing components, the actual size of the components (the area and height they occupy) and the relative position between the components should be considered 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 the 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 “staggered”. This step is related to the overall image of the board and the difficulty of the next wiring, so it is necessary to spend a lot of effort to consider it. When laying out, preliminary wiring can be done for places that are not sure, and full consideration should be given.

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 the PCB, wiring is generally divided into three realms: 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 cannot be criss-crossed and disorderly. All of these must be achieved while ensuring the performance of the electrical appliances and meeting other individual requirements, otherwise it will be a waste of effort. The wiring is mainly carried out according to the following principles:

①. In general, the power line and the ground line should be wired first to ensure the electrical performance of the circuit board.

Within the scope of conditions, try to widen the width of the power supply and ground wires. It is best that the ground wire is wider than the power wire. The relationship between them is: ground wire>power wire>signal wire. Usually the signal wire width is: 0.2~0.3mm, the thinnest width can reach 0.05~0.07mm, and the power wire is generally 1.2~2.5mm. For the PCB of digital circuits, a wide ground wire 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-wire the wires with stricter requirements (such as high-frequency wires).

The edge wires of the input and output ends should avoid adjacent parallel to avoid reflection interference. If necessary, ground wire isolation should be added. The wiring of two adjacent layers should be perpendicular to each other. Parallel is prone to parasitic coupling.

③. The oscillator shell is grounded, and the clock line should be as short as possible and cannot be led everywhere.

Under the clock oscillator circuit and the special high-speed logic circuit, the ground area should be increased, and other signal lines should not be used to make the surrounding electric field approach zero;

④. Use 45-degree folded line wiring as much as possible, and do not use 90-degree folded line to reduce the radiation of high-frequency signals; (high-requirement lines should also use double arc lines)

⑤. No signal line should form a loop. If it is unavoidable, the loop should be as small as possible; the vias of the signal line should be as few as possible;

⑥. The key line should be as short and thick as possible, and protective ground should be added 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”. Pagination

⑧. Key signals should reserve test points 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 inspection and DRC inspection are correct, the unwired area is filled with ground wires, and a large area of ​​copper layer is used as the ground wire. On the printed circuit board, all unused areas are connected to the ground as ground wires. Or make a multi-layer board, with the power supply and ground wire occupying one layer each.

——PCB wiring process requirements:

①. Line

Under normal circumstances, the signal line width is 0.3mm (12mil), 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, if conditions permit, the distance should be increased; when the wiring density is high, it can be considered (but not recommended) to use two lines between IC pins, the line width 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. Pads (PAD)

The basic requirements for pads (PAD) and transition holes (VIA) are: the diameter of the pad should be larger than the diameter of the hole by 0.6mm; for example, general 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, it should be determined according to the size of the actual components. If conditions permit, the pad size can be appropriately increased. The component mounting hole diameter designed on the CB board should be about 0.2 to 0.4mm larger than the actual size of the component pin.

③. Vias (VIA)

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. You can consider using 1.0mm/0.6mm (40mil/24mil).

④. Spacing requirements for pads, wires, and vias

PADandVIA: ≥0.3mm(12mil)

PADandPAD: ≥0.3mm(12mil)

PADandTRACK: ≥0.3mm(12mil)

TRACKandTRACK: ≥0.3mm(12mil)

When the density is high:

PADandVIA: ≥0.254mm(10mil)

PADandPAD: ≥0.254mm(10mil)

PADandTRACK: ≥0.254mm(10mil)

TRACKandTRACK: ≥0.254mm(10mil)

Fifth: Wiring optimization and silk screen printing. “There is no best, only better”! No matter how hard you think about designing, after you finish drawing, you will still feel that many places can be modified. The general design experience is: the time for optimizing wiring is twice the time for the first wiring. After you feel that there is nothing to modify, you can lay copper (Place->polygonPlane). Copper is generally laid for ground wires (note the separation of analog ground and digital ground), and it may also be necessary to lay power for multilayer boards. When it comes to silkscreen, be careful not to be blocked by devices or removed by vias and pads. At the same time, when designing, look directly at the component surface, 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 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 corrected in time 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 corrected in time 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 inspected and confirmed.

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

PCB design is a job that requires careful thinking. The more careful and experienced you are, the better the board you will design. Therefore, you must be extremely careful when designing, fully consider all factors (for example, many people do not consider the convenience of maintenance and inspection), and strive for excellence, and you will definitely be able to design a good board.