PCB News - PCB wiring engineer design experience of four

Preparation.

This includes preparing component libraries and schematics. "To do good work, must first sharpen its device", to make a good board, in addition to the principle of good design, but also draw well. Before PCB design, the component library of schematic SCH and the component library of PCB should be prepared first. Component library can use Protel (many electronic old birds at that time are ProteL) own library, but in general it is difficult to find the right, the best is to do their own component library according to the standard size of the selected device data. In principle, make PCB component library first, and then SCH component library. PCB component library requirements are high, it directly affects the board installation; SCH's component library requirements are relatively loose, as long as attention is paid to the definition of pin attributes and the corresponding relationship with PCB components. PS: Note the hidden pins in the standard library. Then is the schematic design, ready to do PCB design.

Second: PCB structural design.

In this step, according to the circuit board size and mechanical positioning, PCB board surface is drawn in the PCB design environment, and connectors, buttons/switches, screw holes, assembly holes and so on are placed according to positioning requirements. And fully consider and determine the wiring area and non-wiring area (such as how much of the screw hole around the non-wiring area).

Third: PCB layout.

Layout is basically putting devices on a board. At this point, if all the preparatory work mentioned above is done, the network table can be generated on the schematic diagram (Design- >; Create Netlist), and then import the network table on the PCB (design-gt; The Load Nets). See the device hubbub of the whole pile up, between the pins and fly line prompt connection. You can then lay out the device. The general layout is carried out according to the following principles:

1) According to the reasonable partition of electrical performance, generally divided into: digital circuit area (afraid of interference, and interference), analog circuit area (afraid 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 that the connection is simple; At the same time, adjust the relative position between the functional blocks to make the line between the functional blocks simple;

3) Installation position and installation intensity should be considered for components with large quality; The heating element should be separated from the temperature sensitive element, and if necessary, thermal convection measures should be considered;

4) I/O driver device as close as possible to the edge of the printing plate, close to the outlet connector;

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

6) In each integrated circuit between the power input pin and ground, need to add a decoupling capacitor (generally using high frequency good monolithic capacitor); A tantalum capacitor can also be placed around several integrated circuits when the circuit board space is tight.

7)Relay coil to add discharge diode (1N4148 can be);

8) Layout requirements should be balanced, dense and orderly, not top-heavy or heavy

This step concerns the difficulty of board integral figure and next wiring degree, want to spend big effort to consider so. When layout, can make preliminary wiring first to not quite affirmative place, sufficient consideration.

Fourth: wiring.

Wiring is an important procedure in PCB design. This will directly affect the performance of PCB board. In the process of PCB design, wiring generally has such three levels of division: the first is the distribution, which is the basic requirement of PCB design. If the line is not cloth, get everywhere is flying line, it will be a unqualified board, can say that there is no entry. The second is the satisfaction of electrical performance. This is the standard to measure whether a printed circuit board is qualified. This is after the distribution, carefully adjust the wiring, so that it can achieve good electrical performance. Then there is aesthetics. If your wiring cloth was connected, also do not have the place that what affects electric appliance performance, but look past desultorily, add colourful, brightly colored, that calculates how your electric appliance performance is good, still be rubbish in others eye. This brings great inconvenience to testing and maintenance. Wiring should be neat and uniform, not crisscross without rules. All these should be achieved in the context of ensuring electrical performance and meeting other individual requirements, otherwise it is to abandon the essence. Wiring should be carried out according to the following principles:

1) Under normal circumstances, the power line and ground wire should be wired first to ensure the electrical performance of the circuit board. Within the scope of conditions allow, as far as possible to widen the width of power supply, ground wire, it is better to ground wire is wider than the power line, their relationship is: ground wire & GT; The power cord & gt; Generally, the width of signal line is 0.2 ~ 0.3mm, and the width of fine line is 0.05 ~ 0.07mm. Generally, the power line is 1.2 ~ 2.5mm. The PCB of a digital circuit can be used in a circuit with wide ground conductors, that is, a ground network. (Analog ground cannot be used in this way.)

2) Wire wiring for strict requirements in advance (such as high frequency line), input and output side line should avoid adjacent parallel, so as not to produce reflection interference. When necessary, ground wire should be added to isolate, and the wiring of two adjacent layers should be perpendicular to each other, which is easy to produce parasitic coupling in parallel.

3) The oscillator housing is grounded, and the clock line should be as short as possible, and it can't be everywhere. Below the clock oscillation circuit, the special high-speed logic circuit should increase the area of the ground, and should not go to other signal lines, so that the surrounding electric field tends to zero;

4) Use 45O broken line wiring as far as possible, not 90O broken line, in order to reduce the radiation of high frequency signal; (High requirements of the line also use double arc)

5) Any signal line should not form a loop, if unavoidable, loop should be as small as possible; Signal line through the hole should be as little as possible;

6) The key line should be as short and thick as possible, and protective ground should be added on both sides.

7) Through the flat cable transmission sensitive signal and noise field signal, to use the "ground - signal - ground" way out.

8) Key signals should be reserved for test points to facilitate production and maintenance testing

9) After schematic wiring is completed, wiring should be optimized; At the same time, after the preliminary network check and DRC check is correct, the ground wire is filled in the area without wiring, and a large area of copper layer is used as ground wire, and the unused places are connected with the ground as ground wire on the printed board. Or make it multi-layer board, power supply, grounding line each occupy a layer.