Printed Circuit Board (PCB), also known as Printed Circuit Board (PCB), is used to connect and function electronic components and is an important part of power Circuit design. This article will introduce the basic rules of PCB layout and wiring.

Detail the basic rules of PCB board layout and wiring

Basic rules of component layout

1. According to the layout of circuit modules, the related circuit to achieve the same function is called a module, the components in the circuit module should adopt the principle of nearby concentration, and the digital circuit and analog circuit should be separated;

2. Components, devices and screws shall not be installed within 3.5mm (for M2.5) and 4mm (for M3) around the non-mounting holes such as positioning holes and standard holes within 1.27mm;

3. Horizontal resistance, inductor (plug-in), electrolytic capacitor and other components under the cloth hole, so as to avoid the wave soldering hole and component shell short circuit;

4. The outer part of the component is 5mm away from the edge of the plate;

5. The distance between the outer side of the pad of mounting element and the outer side of the adjacent inserting element is greater than 2mm;

6. Metal shell components and metal parts (shielding boxes, etc.) can not touch other components, can not be close to the printed line, pad, the spacing should be greater than 2mm. The size of positioning holes, fastener mounting holes, elliptic holes and other square holes in the plate is greater than 3mm from the plate side;

7. Heating elements should not be close to wires and thermal elements; High-heat devices should be evenly distributed;

8. The power socket should be arranged around the printed board as far as possible, and the wiring terminal of the power socket and the busbar connected to it should be arranged on the same side. In particular, do not place power sockets and other welding connectors between connectors to facilitate the welding of these sockets and connectors and the design and wiring of power cables. The spacing of power sockets and welding connectors should be considered to facilitate the insertion and removal of power plugs;

9. Layout of other components:

All IC components should be aligned unilaterally, and polarity marks of polar components should be clear. Polarity marks on the same printed board should not be more than two directions. When two directions appear, the two directions should be perpendicular to each other.

10, the surface wiring should be properly dense, when the density difference is too large should be filled with mesh copper foil, the grid is greater than 8mil (or 0.2mm);

11, the patch pad can not have through holes, so as to avoid the loss of solder paste resulting in virtual welding components. Important signal line is not allowed to pass through the socket foot;

12, patch unilateral alignment, consistent character direction, consistent packaging direction;

13. Polar devices should be marked in the same direction as far as possible on the same board.

Two, component wiring rules

1. Draw the wiring area within the area ≤1mm from the PCB board edge, and within 1mm around the mounting hole, and forbid wiring;

2. The power line as wide as possible, should not be less than 18mil; Signal line width should not be less than 12mil; CPU incoming and outgoing lines should not be less than 10mil (or 8mil); Line spacing not less than 10mil;

3. The normal hole is not less than 30mil;

4. Double line insert: pad 60mil, aperture 40mil;

1/4W resistance: 51*55mil (0805 sheet); Direct insert pad 62mil, aperture 42mil;

Non-polar capacitor: 51*55mil (0805 sheet); Direct insert pad 50mil, aperture 28mil;

5. Note that power cables and ground cables should be radial as far as possible, and signal cables should not be looped.

How to improve anti-interference ability and electromagnetic compatibility?

How to improve anti-interference ability and electromagnetic compatibility when developing electronic products with processor?

1. Some of the following systems should pay special attention to anti-electromagnetic interference:

(1) microcontroller clock frequency is particularly high, bus cycle is particularly fast system.

(2) The system contains high-power, high-current driving circuit, such as spark generating relay, high-current switch, etc.

(3) system with weak analog signal circuit and high precision A/D conversion circuit.

2. The following measures are taken to increase the anti-electromagnetic interference capability of the system:

(1) Select microcontroller with low frequency:

The microcontroller with low external clock frequency can effectively reduce noise and improve the anti-interference ability of the system. Square wave and sine wave with the same frequency, the high frequency component of square wave is much more than sine wave. Although the amplitude of the high frequency component of the square wave is smaller than that of the fundamental wave, the higher the frequency, the easier it is to emit and become a noise source. The most influential high frequency noise produced by the microcontroller is about 3 times of the clock frequency.

(2) Reduce the distortion in signal transmission

Microcontrollers are mainly manufactured by high-speed CMOS technology. Static input current signal input at about 1 ma, around ten pf in the input capacitance, high input impedance, high speed CMOS circuit outputs are fairly on load capacity, namely the considerable output value, the output end of a door through a very long lead to the high input, the input impedance reflection problem is very serious, it will cause the signal distortion, Increase system noise. When Tpd “Tr”, it becomes a transmission line problem, must consider the signal reflection, impedance matching and so on.

The delay time of the signal on the printed board is related to the characteristic impedance of the lead, that is, to the dielectric constant of the printed board material. Signals can be roughly considered to travel between 1/3 and 1/2 the speed of light over PCB leads. The Tr (standard delay time) of logic telephone elements commonly used in systems made up of microcontrollers is between 3 and 18ns.

On the printed circuit board, the signal passes through a 7W resistor and a 25cm lead, with an on-line delay of approximately 4 to 20ns. That is to say, the signal on the printed line lead as short as possible, the longest should not exceed 25cm. And the number of holes should be as little as possible, preferably not more than 2.

When the signal rise time is faster than the signal delay time, fast electronics is applied. At this point, the impedance matching of the transmission line should be considered. For signal transmission between integrated blocks on a PRINTED circuit board, Td Trd should be avoided. The larger the printed circuit board, the faster the system can not be too fast.