PCB layout method tips

With the progress of electronic technology, the complexity of PCB (printed circuit board), the scope of application has a rapid development. Designers engaged in high frequency PCB must have the corresponding basic theoretical knowledge, but also should have rich experience in high frequency PCB production. That is to say, both schematic drawing and PCB design should be considered from its high-frequency working environment, in order to design a more ideal PCB.

PCB Layout design

Although Protel has the function of automatic layout, but can not fully meet the needs of high frequency circuit work, often rely on the experience of designers, according to the specific situation, the manual layout method is first used to optimize and adjust the position of some components, and then combined with automatic layout to complete the overall design of PCB. Whether the layout is reasonable or not directly affects the product life, stability, EMC (electromagnetic compatibility), etc., must be from the overall layout of the circuit board, wiring connectivity and PCB manufacturability, mechanical structure, heat dissipation, EMI(electromagnetic interference), reliability, signal integrity and other aspects of comprehensive consideration.

Generally, fixed position components related to mechanical dimensions are placed first, then special and larger components are placed, and finally small components are placed. At the same time, wiring requirements should be taken into account. The placement of high-frequency components should be as compact as possible, so that the wiring of signal lines can be as short as possible, so as to reduce the cross interference of signal lines.

1. Placement of positioning plug-ins related to mechanical dimensions

Power sockets, switches, interfaces between PCBS, indicators, etc., are positioning plug-ins related to mechanical dimensions. Usually, the interface between the power supply and the PCB is placed on the edge of the PCB, and the PCB edge should have 3 mm ~ 5 mm spacing; Indicates that the LED should be placed accurately as required; Switches and some fine-tuning components, such as adjustable inductance, adjustable resistance, etc. should be placed near the PCB edge for easy adjustment and connection; Components that need to be replaced frequently must be placed in places with fewer components for easy replacement.

2. Placement of special components

High-power tubes, transformers, rectifiers and other heating devices generate more heat when working at high frequency, so ventilation and heat dissipation should be fully considered in the layout, and such components should be placed on the PCB circuit board where the air is easy to circulate. High power rectifier tube and adjusting tube should be equipped with radiator, and should be far away from the transformer. Electrolytic capacitors and other components that are afraid of heat should also be far away from the heating device, otherwise the electrolyte will be baked dry, resulting in the increase of resistance, performance deteriorates, affecting the stability of the circuit.

Components prone to failure, such as adjusting tubes, electrolytic capacitors, relays, etc., should also take into account the convenience of maintenance when placed. For test points that need to be measured frequently, care should be taken to ensure that the test rod can be easily touched when laying out components.

Because a 50 Hz leakage magnetic field is generated inside the power supply device, it will interfere with the low frequency amplifier when it is interlinked with some parts of the amplifier. Therefore, they must be isolated or masked. It is best to arrange the amplifier levels in a straight line according to the schematic. The advantage of this arrangement is that the ground current at all levels flows in this level without affecting the work of other circuits. Input stage and output stage should be as far away as possible to reduce the parasitic coupling interference between them.

Considering the signal transmission relationship between the functional circuits of each unit, low frequency circuit and high frequency circuit should also be separated, and analog circuit and digital circuit should be separated. The integrated circuit should be placed in the center of the PCB, so that it is convenient for each pin to connect with the wiring of other devices.

Inductors, transformers and other devices have magnetic coupling, and should be placed orthogonal to each other to reduce magnetic coupling. In addition, they have a strong magnetic field, around which should be appropriately large space or magnetic shielding, in order to reduce the impact on other circuits.

Appropriate high frequency decoupling capacitors should be configured at key parts of the PCB. For example, an electrolytic capacitor of 10μF ~ 100 μF should be connected at the input end of the PCB board power supply, and a ceramic capacitor of about 0.01 pF should be connected near the power pin of the integrated circuit. Some circuits are also equipped with appropriate high or low frequency chokes to reduce the influence between high and low frequency circuits. This point should be considered in schematic design and drawing, otherwise it will affect the performance of the circuit.

The spacing of components should be appropriate, and the spacing should take into account whether there is a possibility of breakdown or ignition between them.

For amplifiers including push-pull circuit and bridge circuit, attention should be paid to the symmetry of electrical parameters and structure of components during layout, so that the distribution parameters of symmetrical components are consistent as far as possible.

After the manual layout of major components is completed, components should be locked so that they will not move during the automatic layout. This is done by executing the Edit change command or by checking Locked on the Properties of the component.

3. Placement of common components

For common components, such as resistors and capacitors, the automatic layout can be adopted to consider the neat arrangement of components, the size of occupied space, the connectivity of wiring and the convenience of welding.