The general PCB design process is as follows:
Pre-preparation – PCB structure design – PCB layout – wiring – wiring optimization and silk screen – network and DRC inspection and structural inspection – plate making.
1. Preparations include preparing component libraries and schematics
To make a good board, in addition to designing the principle, you must draw well. Before proceeding with PCB design, first prepare the component library of the schematic SCH and the component library of the PCB. The electronic component library can use the library that comes with peotel, but it is generally difficult to find a suitable one. It is better to do the component library yourself according to the standard size data of the selected device. In principle, first do the PCB library and then do the SCH component library.
PCB component library requirements are higher, it directly affects the installation of the board; SCH component library requirements are relatively loose, as long as you pay attention to define the tube
The foot attributes and the correspondence with the PCB components are sufficient. PS: Note the hidden pins in the standard library. After that is the design of the schematic, after that, I am ready to start PCB design.
2. PCB structure design.
This step draws the PCB surface in the PCB design environment according to the determined board size and various mechanical positioning, and places 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 the extent of the screw hole is a non-wiring area).
3. PCB layout
The layout is white is to put the device on the board. At this time, if the preparations mentioned above are all done, you can generate a network table (Design-“Create Netlist) on the schematic, and then import the network table (Design-“Load Nets) on the PCB. I saw the full stack of devices, and there are flying lines between the pins. Then you can lay out the device. The general layout is based on the following principles:
1). According to the reasonable division of electrical performance, it is generally divided into: digital circuit area (that is, fear of interference and interference), analog circuit area (fear of interference),power drive zone (interference source);
2). The circuit to complete the same function, should be placed as close as possible, and adjust the components to ensure the most concise connection; At the same time, adjusting the relative position between the function blocks makes the connection the most concise;
3). For the large-quality components, the installation position and installation strength should be considered; the heating elements should be placed separately from the temperature-sensitive components, and if necessary, thermal convection measures should be considered;
4). The I/O driver device is as close as possible to the edge of theprinted board, close to the lead-out connector;
5). The clock generator (such as: crystal or clock) should be as close as possible to the device which will use the clock;
6). A decoupling capacitor (usually a monolithic capacitor with high frequency performance) is required between the power input pin of each integrated circuit and the ground. When the board space is dense, it can also be around several integrated circuits. Add a tantalum capacitor.
7). Addinga discharge diode at the relay coil (1N4148);
8). The layout requirements should be balanced, dense and orderly, can not be actual size of the component (occupied area and height), between components The relative position of the circuit board to ensure the electrical performance of the circuit board and the feasibility and convenience of the production and installation. At the same time, under the premise of ensuring that the above principles can be embodied, the placement of the device should be properly modified to make it neat and tidy, such as the same device. They are placed neatly and in the same direction, and they cannot be placed in a “small and accurate manner”.
9). This step is related to the overall image of the board and the difficulty of wiring in the next step, so it takes a lot of effort to consider. When laying out the layout, you can make preliminary wiring for the place that is not sure.
Wiring is the most important process in the overall PCB design. This will directly affect the performance of the PCB. In the design process of the PCB, the wiring generally has three levels of division: the first is the connection, then the most basic requirements for PCB design. If the line is not laid, and the line is flying, it will be an unqualified board. It can be said that it has not yet started. Second is the satisfaction of electrical performance.
This is a measure of the eligibility of a printed circuit board. This is after the connection, carefully adjust the wiring to achieve the best electrical performance. Then it is beautiful. If your wiring is connected, there is no place that affects the performance of the electrical appliance, but at first glance, it looks like chaos.
Plus colorful, colorful, then how good your electrical performance, in the eyes of others is still a piece of garbage. This brings great inconvenience to testing and maintenance. The wiring should be neat and uniform, and there should be no rules. These must be achieved while ensuring the electrical performance and meeting other individual requirements, otherwise it will be the end.
5. The main principles of wiring
In general, the power and ground wires should be routed first to ensure the electrical performance of the board. As far as the conditions permit, try to widen the power supply and ground line width. It is better to ground the ground line than the power line. Their relationship is: ground line > power line > signal line. Usually the signal line width is 0.2-0.3mm. The finest width can reach 0.05-0.07mm, and the power cord is generally 1.2-2.5mm. For the PCB of the digital circuit, a wide ground wire can be used to form a loop, that is, a ground net is used for use (the ground of the analog circuit cannot be used in this way)
Wires that require more stringent requirements (such as high-frequency lines) are pre-wired. The edges of the input and output ends should be avoided to avoid adjacent reflections to avoid reflection interference. Grounding should be added when necessary. The wiring of two adjacent layers should be perpendicular to each other, and parasitic coupling is easy to occur in parallel. The oscillator case is grounded and the clock line is as short as possible and cannot be pinned everywhere. Below the clock oscillating circuit, the special high-speed logic circuit part should increase the area of the ground, and should not take other signal lines, so that the surrounding electric field approaches zero; as far as possible, use 45o of polyline wiring, not 90o fold line, to reduce high Radiation of frequency signals; (double lines are required for high-line lines) Do not form loops for any signal lines. If it is unavoidable, the loop should be as small as possible; the number of vias of signal lines should be as small as possible; Thick, and add protection on both sides.
When transmitting a sensitive signal and a noise field band signal through a flat cable, it is extracted by means of “ground-signal-ground”.
Key signals should be reserved for test points to facilitate production and maintenance testing. After the schematic wiring is completed, the wiring should be optimized.
At the same time, after the initial network inspection and DRC inspection are correct, the grounding of the unwired area is performed, and a large area of copper is used for the grounding.
The place where the board is not used is connected to the ground as a ground line. Or make a multi-layer PCB board, power supply, ground line each occupy a layer.