Did you really understand the PCB design?

- Jan 07, 2020-

Did you really understand the PCB design?

First: preparation in advance. This includes preparing libraries and schematics. "We must first of its profits", to make a good board, in addition to good design principles, it should also draw well. Before making PCB design, the first to prepare libraries of SCH good schematic and PCB libraries. 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 the 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.

 

Second: 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).

 

Third: PCB layout. To put it plainly, put the device on the board. At this time, if the preparations mentioned above are all done, you can generate a 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:

 

Divided according to the electrical performance reasonably, 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);

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;

For high-quality components, the installation position and installation strength should be considered; heating elements should be placed separately from temperature-sensitive components, and thermal convection measures should be considered when necessary;

I / O devices as close as possible drive side of the printing plate, close to the lead connector;

clock generator (eg: Zhong or crystal) is used as close as possible to the clock of the device;

between each integrated circuit and supply pins, the need to add a decoupling capacitor (generally a good high-frequency performance monolithic capacitors); the circuit board dense space, can also be added around the integrated circuits of several a tantalum capacitor.

relay coil to increase the discharge diode (1N4148 can);

 The layout needs to be balanced, dense and orderly, not top-heavy or sinking.

 

- require special attention when placing components must consider the actual component size (occupied area and height), the relative position between the components, in order to ensure the feasibility of the electrical performance of the circuit board production and installation At the same time as convenience, under the premise of ensuring that the above principles can be reflected, the placement of the device should be appropriately modified to make it neat and beautiful. For example, the same device should be placed neatly and in the same direction.

 

This step is related to the degree of difficulty of the board and the overall image of the next line, so they do take great efforts to consider. When the layout, the less definitely the place to be for a preliminary wiring, full consideration.

 

Fourth: wiring. Wiring the entire PCB design is the most important step. This will directly affect the performance of good and bad PCB board. In the PCB design process, so the wiring generally divided three phases: first, through the fabric, the basic requirement of PCB design time. If the line did Bouton, and made a full fly line, it would be a failure of the board, it can be said yet started. Followed by the electrical performance of the meet. It is a measure of a printed circuit board eligibility criteria. This is after Bouton, carefully adjust wiring so that it can achieve the best electrical performance. Followed by appearance. If your wiring through the cloth, there is no place to affect the electrical properties, but a glance chaotic, with colorful, colorful, and that even if your electrical performance how good in the eyes of others still refuse one. So to test and repair cause great inconvenience. Wiring to be uniform, not criss-cross clueless. These will have to ensure that in the case of individual electrical performance and meet other requirements to achieve, otherwise the forest for the trees.

 

The main wiring according to the following principles:

 

In general, the first response to the power supply wiring and ground, to ensure that the electrical performance of the circuit board. In the conditions permit, as far as possible widening power, ground width, preferably ground than the power supply line width, their relationship is: Ground> power cables> signal line, signal line width is typically: 0.2 ~ 0.3mm , the smallest width of up to 0.05 ~ 0.07mm, 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, that is, a ground network is used.

Route the more stringent cables (such as high-frequency cables) in advance, and the side lines of the input end and the output end should not 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 led 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. Do not use 90o polyline 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 vias of the signal line should be as few 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 method of "ground wire-signal-ground wire" 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, the unwired area is filled with ground wires, and a large area copper layer is used as the ground wire. The places where they are used are connected with the ground for the ground wire. Or make a multilayer board, power supply, and ground each occupy one layer.

 

——PCB wiring process requirements

 

line

Under normal circumstances, the signal line width is 0.3mm (12mil), the power line width is 0.77mm (30mil) or 1.27mm (50mil); the distance between the line and the line and the pad is greater than or equal to 0.33mm (13mil ), In practical applications, consideration should be given to increasing the distance when conditions allow;

 When the wiring density is high, you can consider (but not recommended) to use two wires between IC pins, the width of the line is 0.254mm (10mil), and the line spacing is not less than 0.254mm (10mil). In special cases, when the device pins are dense and the width is narrow, the line width and line spacing can be appropriately reduced.

 

Pad (PAD)

The basic requirements for pads (PAD) and transition holes (VIA) are: the diameter of the disk is greater than the diameter of the hole; 0.6 mm; for example, general pin resistors, capacitors and integrated circuits, etc., using a disk / hole size of 1.6 mm / 0.8 mm (63mil / 32mil), socket, pin and diode 1N4007, etc., using 1.8mm / 1.0mm (71mil / 39mil). In practical applications, it should be determined according to the size of the actual component, and if possible, the pad size can be appropriately increased;

 The component mounting aperture designed on the PCB board should be about 0.2 ~ 0.4mm larger than the actual size of the component pins.

 

Vias (VIA)

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

When the wiring density is high, the via size can be appropriately reduced, but it should not be too small, and 1.0mm / 0.6mm (40mil / 24mil) can be considered.

 

Pitch requirements for pads, lines, and vias

PAD and VIA: 0.3mm (12mil)

PAD and PAD: 0.3mm (12mil)

PAD and TRACK: 0.3mm (12mil)

TRACK and TRACK: 0.3mm (12mil)

 

 High density:

PAD and VIA: 0.254mm (10mil)

PAD and PAD: 0.254mm (10mil)

PAD and TRACK: 0.254mm (10mil)

TRACK and TRACK: 0.254mm (10mil)

 

Fifth: to optimize routing and screen printing. "There is no best, only better"! No matter how hard you think about designing, after you finish drawing, you can take a look again, and still feel that there are many places that can be modified. The general design experience is that the time to optimize the wiring is twice that of the initial wiring. After feeling that there is no need to modify it, you can lay copper (Place-> polygon Plane). General plated copper to the ground (note that the analog and digital ground separated), it may also involve laying multilayer power. For screen printing, to be noted that the device can not be blocked or through hole and the pad removed. Meanwhile, a front surface of the design element, the underlying word should be mirrored, to avoid confusion level.

 

Sixth: network and DRC inspection and structural inspections. Firstly, the premise of the determination circuit schematic design correct, the generated PCB network file the schematic network file network checks connection relationship physical (NetCheck), and timely design is corrected according to the output file the results to ensure wirings Correctness of connection relationship;

 After the network check is passed correctly, DRC check the PCB design and correct the design in time according to the output file results to ensure the electrical performance of the PCB wiring. Finally, the mechanical installation structure of the PCB needs to be further checked and confirmed.

 

Seventh: plate making. Prior to this, it is best to have a review process.

PCB design is a test of mind to work, who thought dense, high experience, designed like a board. So the design to be extremely careful and full consideration to factor in all aspects (for example, ease of maintenance and inspection that a lot of people not to consider), the better we will be able to design a good board.

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