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.
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 be Protel (many electronic old birds were Protel at that time), but it is difficult to find a suitable one. It is better to make the component library according to the standard size data of the selected device. 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: note 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.
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 simple wiring; At the same time, adjust the relative position between the functional blocks to make the connection between the functional blocks 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 (single stone capacitor with good high frequency performance is generally used) shall be added between the power input pin of each integrated circuit and the ground; When the circuit board space 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
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——Special attention is required
When placing components, the actual size (area and height) of components and the relative position between components must be considered to ensure the electrical performance of the circuit board and the feasibility and convenience of production and installation. At the same time, on the premise that the above principles can be reflected, the placement of components should be appropriately modified to make them neat and beautiful. Similar components should be placed neatly In the same direction, it can not be “scattered”.
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 an important process in the whole PCB design. This will directly affect the performance of PCB. In the process of PCB design, wiring is generally divided into three realms: the first is wiring, which is the 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 good 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 electrical performance and meeting other individual requirements, otherwise it will be abandoning the basics. The following principles shall be followed during wiring:
① Generally, the power line and ground wire shall be wired first to ensure the electrical performance of the circuit board. Within the allowable range, the width of power supply and ground wire shall be widened as much as possible. 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 fine 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 increased, and other signal lines should not be taken 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 signal and noise field band signal through flat cable, it 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 for pad and via are as follows: the diameter of pad shall be greater than 0.6mm than that of hole; 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 pad size can be appropriately increased;
The component mounting aperture designed on the PCB 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.
“No good, 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 general design experience is that the time to optimize the wiring is twice that of the initial wiring. After you feel that 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 up to 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, on the premise that the circuit schematic design is correct, netcheck the physical connection relationship between the generated PCB network file and the schematic network file, and timely correct the design according to the output file results to ensure the correctness of the wiring connection relationship;
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. The mechanical installation structure of PCB shall be further inspected and confirmed after.
Seventh: plate making.
Before that, there should be an audit process.
PCB design is a test of mind. Whoever has dense mind and high experience, the designed board is good. Therefore, we should be extremely careful in design, fully consider various factors (for example, many people don’t consider the convenience of maintenance and inspection), keep improving, and we will be able to design a good board.