How Engineers Can Avoid The Trick Of Not Entering The Stream!
Many people like to design. When designing, people must first master the most basic design skills. Then, they will continue to practice and research. For those who are determined to be engineers, the drawing board is hard martial arts. If you don’t practice, you will not succeed, even if you can Make a note of all the characteristic curves of the MOS tube, and after all, it is not inflow. Today, I will share with you the tips of engineers on how to avoid inflows in PCB design! .
First, preparations include preparing component libraries and schematics
"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, first prepare the component library of the schematic SCH and the component library of the PCB. The 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 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.
Second, 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).
Third, 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 electrical division, it is generally divided into: digital circuit area (that is, 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 drive device as close as possible to the edge of the printed board, close to the lead connector;
5 .The clock generator (such as crystal or clock) should be as close as possible to the device that uses the clock;
6. Add a decoupling capacitor between the power input pin of each integrated circuit and ground (usually a monolithic capacitor with good high frequency performance); when the board space is dense, it can also be added around several integrated circuits. A tantalum capacitor.
7 .Add a discharge diode at the relay coil (1N4148);
8. Layout requirements should be balanced, dense and orderly, not top-heavy or sinking - need special attention, when placing the element, must consider the actual size of the component (occupied area and height), between components Relative position, in order to ensure the electrical performance of the circuit board and the feasibility and convenience of production and installation, the device should be properly modified to make it tidy and beautiful, such as the same device. Put it neatly and in the same direction, you can't put it in a "small patchwork."
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 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. These must be achieved while ensuring the electrical performance and meeting other individual requirements, otherwise it will be the end.
Five main principles of wiring
In general, the power and ground wires 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 cord is generally 1.2~2.5mm. For the PCB of a digital circuit, a wide ground wire can be used to form a loop, that is, a ground net is used (the ground of the analog circuit cannot be used in this way).
1. Pre-wire the more stringent requirements (such as high-frequency lines). The edges of the input and output should avoid adjacent parallel 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.
2. The oscillator case is grounded, the clock line should be as short as possible, and it 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;
3. Use 45o polyline wiring as much as possible, and do not use 90o polyline to reduce the radiation of high frequency signals; (higher lines require double arcs)
4. 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;
5. The key lines are as short and thick as possible, with protective ground on both sides.
6. 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”.
7. The key signal should be reserved for test points to facilitate production and maintenance testing. After the schematic wiring is completed, the wiring should be optimized;
8. 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, and the unused areas are connected to the ground on the printed board. Used as a ground wire. Or make a multi-layer board, power supply, ground line each occupy a layer.