PCB design, how do engineers avoid inflows!

- Dec 19, 2019-

PCB design, how do engineers avoid inflows!

Summary of content: For friends who are determined to be engineers, drawing boards are a hard art, and they will not succeed without practicing. Even if you can write down all the characteristic curves of MOS tubes, they will not enter the market after all.

The general PCB basic design process is as follows:

Preliminary preparation-"PCB structural design-" PCB layout-"wiring-" wiring optimization and silk screen-"network and DRC inspection and structural inspection-" plate-making.

 

1. Preliminary preparation includes preparing component library and schematic diagram. "If you want to be good at work, you must first sharpen it." To make a good board, in addition to designing the principles, you must draw well. Before designing the PCB, first prepare the component library for the schematic SCH and the component library for the PCB. The component library can use the library that comes with peotel, but in general it is difficult to find a suitable one. It is best to make the component library yourself according to the standard size data of the selected device. In principle, the component library of the PCB is made first, and then the component library of the SCH is made.

The component library of PCB has higher requirements, which directly affects the installation of the board. The component library requirements of SCH are relatively loose, as long as the pin properties and the corresponding relationship with PCB components are defined. PS: Note the hidden pins in the standard library. After that is the design of the schematic diagram, ready to start PCB design.

2, PCB structure design. In this step, the PCB surface is drawn in the PCB design environment according to the determined circuit board size and various mechanical positioning, and the required connectors, buttons / switches, screw holes, mounting holes, etc. are placed 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).

3, PCB layout

The layout is plain. Put the device on the board. At this time, if the preparations mentioned above are all done, you can generate the netlist (Design-> Create Netlist) on the schematic diagram, and then import the netlist (Design-> Load Nets) on the PCB diagram. I saw the whole stack of devices went up, and there were flying leads between the pins to connect. You can then place the device. The general layout is based on the following principles:

 

1 Reasonable division according to electrical performance, generally divided into: digital circuit area (that is, afraid of interference, but also interference), analog circuit area (afraid of interference), power drive area (interference source);

 

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

3 For components with large masses, 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 be considered when necessary;

4 I / O drive devices should be as close to the edge of the printed board as possible, and close to the lead-out connector;

5 Clock generator (such as: crystal or clock) should be as close as possible to the device using the clock;

6 Between each IC's power input pin and ground, a decoupling capacitor (generally a monolithic capacitor with good high frequency performance) is required; when the board space is dense, it can also be added around several integrated circuits. A tantalum capacitor.

7 A discharge diode should be added at the relay coil (1N4148 is sufficient);

8 Layout requirements should be balanced, dense and orderly, and not top-heavy or sink-in special attention should be paid to the actual dimensions of the components (occupied area and height), and Relative position to ensure the electrical performance of the circuit board and the feasibility and convenience of production and installation. At the same time, under the premise that the above principles can be reflected, the placement of the device should be appropriately modified to make it neat and beautiful. Neatly arranged and in the same direction.

 

This step is related to the overall image of the board and the ease of wiring in the next step, so it takes a lot of effort to consider it. When laying out, you can make preliminary wiring for the places that are not sure, and give full consideration.

4.Wiring

Layout is the most important process in the entire PCB design. This will directly affect the performance of the PCB. In the PCB design process, there are generally three types of divisions in wiring: firstly, the layout is the most basic requirement in PCB design. If the lines are not available and flying leads are everywhere, it will be an unqualified board. The second is the satisfaction of electrical performance.

This is a measure of the eligibility of a printed circuit board. This is after wiring, carefully adjust the wiring so that it can achieve the best electrical performance. Then there is beauty. If your wiring is well connected, there is no place to affect the performance of the electrical appliances, but at a glance, the past is cluttered, colorful and colorful, then the performance of your electrical appliances is considered good, in the eyes of others, it is a piece of garbage. This brings great inconvenience to testing and maintenance. The wiring must be neat and uniform. These must be achieved under the condition of ensuring the performance of the appliance and meeting other individual requirements, otherwise it will be the end.

5 Main principles when wiring. In general, you should first route the power and ground wires to ensure the electrical performance of the circuit board. Within the range allowed by the conditions, try to widen the width of the power and ground wires as much as possible. It is best that the ground wires are wider than the power wires. Their relationship is: ground wire> power wire>

Signal line, usually 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 PCBs of digital circuits, a wide ground wire can be used to form a loop, that is, a ground network is used (the ground of analog circuits cannot be used in this way)

. Route the more stringent cables (such as high-frequency cables) in advance. The edges of the input end and the output end should be avoided to be adjacent and parallel to avoid reflection interference. When necessary, ground wire isolation should be added, and the wiring of two adjacent layers should be perpendicular to each other, and parasitic coupling is easy to occur in parallel.

. The shell of the oscillator is grounded, the clock line should be as short as possible, and it cannot be everywhere. The area of the special high-speed logic circuit under the clock oscillation circuit should increase the area of the ground, and should not go through other signal lines to make the surrounding electric field approach zero;

Use 45o polyline wiring as much as possible, 90o polyline cannot be used to reduce the radiation of high-frequency signals;

Do not form a loop on any signal line. If it is unavoidable, the loop should be as small as possible; the signal line should have as few vias as possible;

. The key lines are as short and thick as possible, and protective grounds are added on both sides.

. When transmitting sensitive signals and noise field band signals through flat cables, use the "ground-signal-ground" method to lead them out.

. Test points should be reserved for key signals 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, fill the unwired area with a ground wire, use a large area copper layer as the ground wire, and connect the unused places to the ground on the printed board as the ground. Line use. Or make a multilayer board, power supply, and ground each occupy one layer.

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