Experience of PCB wiring

    How to put it? We can do it. We can‘t survive or starve. I‘ve worked for four or five years. I‘d like to share some experience. PCB wiring for fourorfive years, enough experience to eat.

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

    First: preliminary preparation. This includes preparing catalogs and schematics. "If you want to do a good job, you must first sharpen your tools". To make a good board, you should not only design the principle, but also draw well. Before PCB design, first prepare the component library of schematic Sch and PCB. The component library can use the library provided by peotel, but it is difficult to find a suitable one in general. It is best to make the component library by yourself according to the standard size data of the selected devices. In principle, make the component library of PCB first, and then the component library of sch. The component library of PCB has high requirements, which directly affects the installation of the board; The component library requirements of SCH are relatively loose. Just pay attention to defining the pin attributes and the corresponding relationship with PCB components. PS: pay attention to the hidden pins in the standard library. Then there is the schematic design. When you are ready, 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 surface in the PCB design environment, and place the required connectors, keys / 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. To put it bluntly, the layout is to put devices on the board. At this time, if all the preparations mentioned above are done, you can generate a network table (Design - > create netlist) on the schematic diagram, and then import a network table (Design - > Load nets) on the PCB diagram. You can see that the devices are all piled up, and there are flying wires between the pins to prompt the connection. Then you can layout the device.

The general layout shall be carried out according to the following principles:
① Reasonable zoning according to electrical performance, generally divided into: digital circuit area (i.e. fear of interference and generate interference), analog circuit area (fear of interference) and power drive area (interference source);

② Circuits that complete the same function shall be placed as close as possible, and all components shall be adjusted to ensure the simplest connection; At the same time, adjust the relative position between the functional blocks to make the connection between the functional blocks the most concise;

③ . for components with high quality, the installation position and installation strength shall be considered; Heating elements shall be placed separately from temperature sensitive elements, and thermal convection measures shall be considered when necessary;

④ . the I / O driver shall be close to the edge of the printed board and the outgoing connector as far as possible;

⑤ The clock generator (such as crystal oscillator or clock oscillator) shall be as close as possible to the device using the clock;

⑥ A decoupling capacitor shall be added between the power input pin of each integrated circuit and the ground (single stone capacitor with good high frequency performance is generally used); When the space of the circuit board is dense, a tantalum capacitor can also be added around several integrated circuits.

⑦ . a discharge diode (1N4148) shall be added at the relay coil;

⑧ The layout shall be balanced, dense and orderly, and shall not be top heavy or heavy

    Special attention should be paid to the actual size (area and height) of components and the relative position between components when placing components, 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 properly modified to make them neat and beautiful, and the like components should be placed neatly and in the same direction on the premise that the above principles can be reflected, We should not put it in a "staggered manner".

    This step is related to the overall image of the board and the difficulty of wiring in the next step, so we should make great efforts to consider it. During layout, preliminary wiring can be made for uncertain places and fully considered.

    Fourth: wiring. Wiring is the most important process in the whole PCB design. This will directly affect the performance of PCB. In the process of PCB design, there are generally three levels of wiring: the first is wiring, which is the most basic requirement of PCB design. If the lines are not connected, and there is a flying line, it will be an unqualified board. It can be said that it has not been introduced yet. The second is the satisfaction of electrical performance. This is the standard to measure whether a printed circuit board is qualified. This is to carefully adjust the wiring after wiring to achieve the best electrical performance. Then there is beauty. If your wiring is connected, there is no place to affect the performance of electrical appliances, but at a glance, it is disordered in the past, coupled with colorful and colorful, even if your electrical performance is good, it is still a piece of garbage in the eyes of others. This brings great inconvenience to testing and maintenance. Wiring should be neat and uniform, not crisscross and disorganized. These should be realized under the condition of ensuring the performance of electrical appliances and meeting other individual requirements, otherwise it will be abandoned.

① Generally, the power line and ground wire shall be wired first to ensure the electrical performance of the circuit board. The width of power supply and ground wire shall be widened as far as possible within the allowable range. It is better that the ground wire is wider than the power line width. Their relationship is: ground wire > power line > signal line. Generally, 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 circuit, a wide ground wire can be used to form a circuit, that is, to form a ground network (the ground of analog circuit cannot be used in this way)

② Wires with strict requirements (such as high-frequency lines) shall be wired in advance, and the side lines of input end and output end shall avoid adjacent parallel to avoid reflection interference. If necessary, ground wire shall be added for isolation. The wiring of two adjacent layers shall be perpendicular to each other and parallel, which is easy to produce parasitic coupling.

③ The oscillator shell shall be grounded, and the clock line shall be as short as possible, and it shall not be everywhere. Under the clock oscillation circuit and the special high-speed logic circuit, the area of the earth should be added, and other signal lines should not be used to make the surrounding electric field close to zero;

④ 45o broken line wiring shall be adopted as far as possible, and 90o broken line wiring shall not be used to reduce the radiation of high-frequency signal; (double arc shall also be used for lines with high requirements)

⑤ No signal line shall form a loop. If it is inevitable, the loop shall be as small as possible; The vias of signal lines shall be as few as possible;

⑥ The key lines shall be as short and thick as possible, and protective areas shall be added on both sides.

⑦ When transmitting sensitive signals and noise field band signals through flat cables, they shall be led out in the way of "ground wire signal ground wire".

⑧ Test points shall be reserved for key signals to facilitate production, maintenance and detection

⑨ . after the schematic wiring is completed, the wiring shall be optimized; At the same time, after the preliminary network inspection and DRC inspection are correct, fill the non wired area with ground wire, use a large area of copper layer as the ground wire, and connect the unused places with the ground on the printed board as the ground wire. Or it can be made into a multilayer board, and the power supply and ground wire occupy one floor respectively.

PCB wiring process requirements

① . line

    Generally, 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 application, if conditions permit, increase the distance;

    When the wiring density is high, it can be considered (but not recommended) to use two wires between IC pins. The width of the wires is 0.254mm (10mil), and the wire spacing is not less than 0.254mm (10mil). Under special circumstances, when the device pins are dense and the width is narrow, the linewidth and line spacing can be appropriately reduced.

② . pad

    The basic requirements of pad and via are: the diameter of pad is larger than that of hole by 0.6mm; For example, for general pin resistors, capacitors and integrated circuits, the disk / hole size is 1.6mm/0.8mm (63mil / 32mil), and the socket, pin and diode 1N4007 are 1.8mm/1.0mm (71mil / 39mil). In practical application, it should be determined according to the size of the actual components. If possible, the size of the pad can be appropriately increased;

    The component mounting aperture designed on the PCB board shall be about 0.2 ~ 0.4mm larger than the actual size of the component pin.

③ . 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. 1.0mm/0.6mm (40mil / 24mil) can be considered.

④ . spacing requirements of pad, wire and via

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 high:

PAD and VIA ： ≥ 0.254mm（10mil）

PAD and PAD ： ≥ 0.254mm（10mil）

PAD and TRACK ： ≥ 0.254mm（10mil）

TRACK and TRACK ： ≥ 0.254mm（10mil）

 
  Fifth: wiring optimization and silk screen printing. "There is no best, only better"! No matter how hard you try to design, when you finish painting, you will still feel that many places can be modified. The experience of general design is that the time of optimizing wiring is twice that of initial wiring. After you feel there is nothing to modify, you can lay copper (place - > polygon plane). Copper is generally laid with ground wire (pay attention to the separation of analog ground and digital ground), and power supply may also be laid when laying multilayer boards. For silk screen printing, pay attention not to be blocked by devices or removed by vias and pads. At the same time, the design should face the component surface, and the words at the bottom should be mirrored to avoid confusing the layer.

Sixth: network and DRC inspection and structure inspection. Firstly, check the correctness of the circuit diagram generated according to the physical connection diagram of the PCB, and output the correct wiring diagram according to the principle diagram of the network;

After the network check is passed correctly, DRC check the PCB design, and correct the design in time according to the output file results to ensure the electrical performance of PCB wiring. Finally, it is necessary to further check and confirm the mechanical installation

structure of PCB.

    Seventh: plate making. Before that, it is better to have an audit process.

PCB design is a test of the mind. Whoever has dense mind and high experience, the designed board is good. Therefore, it is necessary to be extremely careful in the design, fully consider various factors (for example, many people do not consider the convenience of maintenance and inspection), keep improving, and we will be able to design a good board.

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