Circuit design skills PCB design process

General PCB basic design process is as follows: preliminary preparation – >; PCB structure design -& GT; PCB layout – & gt; Wiring – & gt; Routing optimization and screen printing – > Network and DRC inspections and structural inspections – > Plate making.

First: 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. Peotel libraries can be used, but in general it is difficult to find a suitable library, it is best to make your own library according to the standard size information of the selected device. 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- >; CreateNetlist), and then import the network table on the PCB diagram (design-gt; LoadNets). 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 electrical performance reasonable partition, generally divided into: digital circuit area (that is, afraid 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). Installation position and installation intensity should be considered for components with large mass; The heating element should be separated from the temperature sensitive element, and if necessary, thermal convection measures should be considered;

(4). I/O drive 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 the 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.

All landowners. Relay coil to add discharge diode (1N4148 can be);

Today. Layout requirements should be balanced, dense and orderly, not top-heavy or heavy

— Special attention should be paid to the actual size (area and height) of the components and the relative position of the components when placing the components to ensure the electrical performance of the circuit board and the feasibility and convenience of production and installation. At the same time, the above principles should be reflected

Under the premise, modify the placement of devices appropriately to make them neat and beautiful. For example, the same devices should be placed neatly and in the same direction, instead of being “strewn at random”. 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 the most important process 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 most 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 the best 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). In general, the power cable and ground cable should be routed first to ensure the electrical performance of the circuit board. In the scope that condition allows, widen width of power supply, ground wire as far as possible, it is best that ground wire is wider than power line, their relation is: ground wire > power line > signal line, usually signal line width is: 0.2 ~ 0.3mm, the thinest width can reach 0.05 ~ 0.07mm, power line is 1.2 ~ 2.5mm generally. 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). In advance, wire strict requirements (such as high frequency line) for wiring, 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 should be grounded, and the clock line should be as short as possible, and not spread all over the place. 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). In order to reduce the radiation of high frequency signal, 45O broken line should be used as far as possible, instead of 90O broken line. (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 short and thick, with protection on both sides.

All landowners. When the sensitive signal and noise field signal are transmitted through flat cable, the method of “ground – signal – ground wire” is used.

Today. Test points should be reserved for key signals to facilitate production and maintenance testing

Pet-name ruby. After schematic diagram 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.

— PCB wiring process requirements

(1). line

Generally, the signal line width is 0.3mm(12mil), and the power line width is 0.77mm(30mil) or 1.27mm(50mil). Line with

The distance between lines and between lines and pads should be greater than or equal to 0.33mm(13mil). In practical application, it should be considered to increase the distance when conditions permit; When the cabling density is high, it is advisable (but not recommended) to use two cables between IC pins. The width of the cables is 0.254mm(10mil), and the distance between the cables is not less than 0.254mm(10mil).

Under special circumstances, when the pin of the device is dense and the width is narrow, the line width and line spacing can be appropriately reduced.

(2). PAD (PAD)

The basic requirements of PAD and transition hole (VIA) are: the diameter of PAD is greater than 0.6mm than the diameter of hole; For example, universal pin type resistors, capacitors and integrated circuits, using disk/hole size 1.6mm/0.8mm (63mil/32mil), socket, pin and diode 1N4007, using 1.8mm/1.0mm (71mil/39mil). In practical application, it should be determined according to the size of the actual components. If conditions are available, the size of the pad can be appropriately increased. The installation aperture of the components designed on the PCB board should be about 0.2 ~ 0.4mm larger than the actual size of the pins.

(3). Through hole (VIA)

Generally 1.27mm/0.7mm(50mil/28mil);

When the wiring density is high, the hole size can be appropriately reduced, but not too small, can consider 1.0mm/0.6mm(40mil/24mil).

(4). Spacing requirements for pads, wires and through-holes

PADandVIA: ≥0.3mm (12mil)

PADandPAD: ≥0.3mm (12mil)

PADandTRACK: ≥0.3mm (12mil)

TRACKandTRACK: ≥0.3mm (12mil)

When density is high:

PADandVIA: ≥0.254mm (10mil)

PADandPAD: ≥0.254mm (10mil)

PADandTRACK: ≥0.254mm (10mil)

TRACKandTRACK: ≥0.254mm (10mil)

Fifth: wiring optimization and screen printing. “There is no best, only better”! No matter how much effort you put into the design, when you’re done, look at it again, and you’ll still feel you can change a lot. A general design rule of thumb is that optimal wiring takes twice as long as initial wiring. Once you feel that nothing needs fixing, you can Place copper. PolygonPlane). Laying copper generally laying ground wire (pay attention to the separation of analog and digital ground), multilayer board may also need to lay power. For screen printing, we should pay attention to not be blocked by the device or removed by the hole and pad. At the same time, design to face the component surface, the bottom of the word should be mirror processing, so as not to confuse the level.

Sixth: network and DRC check and structure check. Firstly, on the premise that the schematic design is correct, the generated PCB network files and schematic network files are NETCHECK for physical connection relationship, and the design is timely amended according to the output file results to ensure the correctness of wiring connection relationship; After the network check is passed correctly, DRC check will be carried out on the PCB design, and the design will be amended according to the output file results in time to ensure the electrical performance of PCB wiring. Finally, the mechanical installation structure of PCB should be further checked and confirmed.

Seventh: plate making. It is best to have a review process before doing so.

PCB design is a test of the mind of the work, who is close to the mind, high experience, the design of the board is good. So the design should be extremely careful, fully consider the factors of all aspects (such as facilitate maintenance and inspection of this a lot of people do not consider), excellence, will be able to design a good board.