PCB board design needs to provide information:
1). Schematic: A complete electronic document format that can produce a correct netlist (netlist);
2). Mechanical size: provide the specific position and direction identification of the positioning device, and the identification of the specific height limit position area;
3). BOM list: mainly to determine and check the specified package information of the device on the schematic;
4). Wiring Guidelines: Descriptions of specific signal specific requirements, as well as design requirements for impedance, lamination, etc.
The basic design flow of PCB board design is as follows:
Pre-preparation -> PCB structure design -> PCB layout -> wiring -> wiring optimization and silk screen -> network and DRC inspection and structural inspection -> PCB board.
1: Preliminary preparation
1) This includes preparing the component library and schematic. “If you want to do something good, you must first sharpen your tools.” To make a good board, in addition to designing the principles, you must draw well. Before proceeding with PCB design, you must first prepare the electronic component library of the schematic SCH and the component library of the PCB (this is the first step – very important). The component library can use the library that comes with Protel, 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 component library of the PCB, and then do the component library of the SCH. PCB component library requirements are higher, it directly affects the board installation; SCH component library requirements are relatively loose, as long as you pay attention to define the pin properties and the corresponding relationship with the PCB components.
PS: Note the hidden pins in the standard library. Then there is the design of the schematic, and when it is ready, it is ready to start PCB design.
2) When making the library of the schematic diagram, pay attention to whether the pin is connected to the output/output PCB board and check the library.
2. PCB structure design
This step draws the PCB surface in the PCB design environment according to the determined board planar size and various mechanical positioning, and places the required connectors, buttons/switches, digital tubes, indicators, inputs, and outputs according to the positioning requirements. , screw holes, mounting holes, etc. 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).
Special attention should be paid to the actual size of the component (occupied area and height), the relative position between the components – the size of the space, and the surface on which the device is placed to ensure the electrical performance of the board. At the same time as the feasibility and convenience of production and installation, the device should be properly modified to make it neat and tidy on the premise of ensuring that the above principles can be embodied. If the same device is placed neatly and in the same direction, it cannot be placed. It’s a patchwork.”
3. PCB layout
1) Make sure the schematic is correct before layout – this is very important! —–Very important!
The schematic drawing is completed and the inspection items are: power network, ground network, etc.
2) The layout should pay attention to the surface placed by the device (especially the plug-in, etc.) and the way the device is placed (straight plug is placed horizontally or vertically) to ensure the feasibility and convenience of the installation.
3) 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->CreateNetlist) on the schematic, and then import the network table (Design->LoadNets) on the PCB. I saw the full stack of devices, and there are flying line prompt connections between the pins, and then the device layout can be made.
The general layout is based on the following principles:
When laying out the layout, you should determine the surface on which the device is placed: Generally speaking, the patch should be placed on the same side, and the plug-in should look at the specific situation.
1) According to the reasonable division of electrical performance, generally divided into: digital circuit area (that is, fear of interference, and interference), analog circuit area (fear of interference), power drive area (interference source);
2) Complete the same function of the circuit, should be placed as close as possible, and adjust the components to ensure the most simple 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 high quality components, the installation position and installation strength should be considered; the heating elements should be placed separately from the temperature sensitive components, and thermal convection measures should also be considered when necessary;
4) I/O driving device as close as possible to the edge of the printed circuit board, close to the lead connector;
5) Clock generator (such as: crystal or clock) should be as close as possible to the device that uses the clock;
6) Layout requirements should be balanced, sparse and orderly, not top-heavy or sinking.
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, followed by aesthetics. If your wiring is connected, there is no place that affects the performance of the electrical appliances, but at a glance, in the past, with a lot of colorful, colorful, then how good your electrical performance is, 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 and regulations. These must be realized while ensuring the electrical performance and meeting other individual requirements.
The wiring is mainly carried out according to the following principles:
1) Under normal circumstances, the power line and ground line should be routed first to ensure the electrical performance of the board. Within the scope of the conditions, 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 line is generally 1.2~2.5mm. For the digital circuit PCB, a wide ground wire can be used to form a loop, which is used to form a ground net (the ground of the analog circuit cannot be used like this) );
2) Pre-wire the more demanding lines (such as high-frequency lines), and the edges of the input and output ends should avoid adjacent parallel to avoid reflection interference. If necessary, add grounding, and the wiring of two adjacent layers should be mutually Vertical and parallel are prone to parasitic coupling;
3) The oscillator case is grounded, and the clock line should be as short as possible and cannot be cited 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;
4) Use 45° fold line wiring as much as possible, and do not use 90° fold line to reduce the radiation of high frequency signals; (require high lines to use double arc lines);
5) Do not form a loop on any signal line. 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 lines are as short and thick as possible, and the protection is added on both sides;
7) When transmitting the sensitive signal and the noise field band signal through the flat cable, it should be extracted by means of “ground line-signal-ground line”;
8) key signals should be reserved for test points to facilitate debugging, production and maintenance testing;
9) 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 unwiring area is performed, and a large area copper layer is used for the ground line, and the printed circuit board is used. The places that are not used are connected to the ground as ground. Or make a multi-layer board, power supply, ground line each occupy a layer.
5. Add teardrops
The teardrop is the drip connection between the pad and the wire or between the wire and the guide hole. The purpose of setting the teardrop is to avoid the contact point between the wire and the pad or the wire and the guide hole when the circuit board is subjected to a large external force. Disconnected, in addition, the setting of teardrops can make the PCB circuit board look more beautiful.
In the circuit board design, in order to make the pad stronger, to prevent the break between the pad and the wire when the mechanical plate is made, a transition zone is often arranged between the pad and the wire with a copper film, and the shape is like a teardrop, so it is often called Teardrops.
6. The first item of inspection, in turn, look at the Keepout layer, the top layer, the bottom layer topoverlay, and the bottomoverlay.
7. Electrical rules check: through hole (0 through hole – very incredible; 0.8 boundary line), whether there is a disconnected netlist, minimum spacing (10mil), short circuit (for each parameter to analyze one by one)
8. Power line and ground line inspection – interference. (The filter capacitor should be close to the chip)
9. After the completion of the PCB, reload the network mark to check whether the netlist has been modified – it works.
10. After the PCB is completed, check the line of the core device to ensure accuracy.