PCB Technical - PCB copy board layout and the skills of selecting components

1. PCB copy board layout

The placement of the positioning plug-in related to the mechanical size of the power socket, switch, interface between PCB boards, indicators, etc. are all related to the mechanical size of the positioning plug-in. Generally, the interface between the power supply and the PCB is placed on the edge of the PCB, 3mm~5mm away from the edge of the PCB; the light-emitting diodes should be placed accurately according to the needs; switches and some fine-tuning components, such as adjustable inductors, adjustable resistors, etc., should be Place close to the edge of the PCB for easy adjustment and connection

Components that need to be replaced frequently must be placed in locations with fewer equipment to facilitate replacement.

The placement of high-power tubes, transformers, rectifiers and other heating devices for special board replication components generates more heat in high-frequency conditions. Therefore, ventilation and heat dissipation should be fully considered during layout. Such components are placed in places where PCB air is easy to circulate.

The high-power rectifier tube and regulating tube should be equipped with a radiator and kept away from the transformer. The fear of electrolytic capacitors and other thermal components should also be kept away from the heating device, otherwise the electrolyte will be dried, resulting in increased resistance and poor performance, which will affect the stability of the circuit. When the installation is convenient, the parts that are prone to failure should also be considered, such as regulating tubes, electrolytic capacitors, relays, etc.

For test points that often need to be measured, care should be taken to ensure easy access to the test rod when arranging the components. Because a leakage magnetic field of 50 Hz is generated inside the power supply device, when the low-frequency amplifier is connected to certain parts of the low-frequency amplifier, it will interfere with the low-frequency amplifier. Therefore, they must be separated or shielded. The amplifiers at all levels can best be arranged in a straight line according to the schematic diagram. Therefore, the advantage of this method is that the ground currents at all levels are closed and flow at the current level without affecting the operation of other circuits.

The input and output levels should be kept as far as possible to reduce the parasitic coupling interference between them. Taking into account the signal transmission relationship between the functional circuits of each unit, the low-frequency circuit and the high-frequency circuit should also be separated, and the analog circuit and the digital circuit should be separated. The integrated circuit should be placed in the center of the PCB so that each pin can be easily connected to the wiring of other devices. Inductors, transformers, and other devices have magnetic coupling and should be placed orthogonally to each other to reduce magnetic coupling.

In addition, they all have a strong magnetic field, and there should be a suitable large space or magnetic shield around to reduce the impact on other circuits. Configure appropriate high-frequency decoupling capacitors in the key parts of the PCB copy board. For example, it should be a 10μf~100μf electrolytic capacitor when the PCB power supply is input, and the power supply pin near the integrated circuit should be a 0.01pF ceramic chip capacitor.

Certain circuits also need to be equipped with appropriate high-frequency or low-frequency chokes to reduce the impact between high-frequency and low-frequency circuits.

This should be considered in the schematic design and drawings, otherwise it will also affect the performance of the circuit. The spacing of the components should be appropriate, and their spacing should consider the possibility of penetration or emission between them.

Amplifiers with push-pull circuits and bridge circuits should pay attention to the symmetry of the electrical parameters of the components and the symmetry of the structure, so that the symmetrical components

The distribution parameters are as consistent as possible. After the manual layout of the main components is completed, the component locking method should be used so that these components will not move in the automatic layout.

In other words, execute the Editchange command or choose to lock the properties of the component to lock it, and no longer move it.

Placement of common components For common components, such as resistors, capacitors, etc., from the neat arrangement of the components, the size of the floor area, the conductivity of the wiring and the convenience of welding, the layout of the board can be automatically read.

Second, how to choose PCB copy board components

Starting from the safety design requirements, first select safety-critical components that contain dangerous voltages. Such as: 220V power sockets, fuses, power modules, etc. must pass safety certification or 3C certification (China Compulsory Certification Committee) components or components.

The general choice of other safety and special low-circuit IC circuits: when the price and function are appropriate, surface mount SMT board equipment is preferred and TTL dual-row in-line equipment, TTL equipment is preferred to discrete components. Regarding the power of the IC circuit and the operating speed of the IC (the rise and fall times of the switching circuit), as long as the reliability of the premise can be met, the higher the IC power, the better, and the faster the switching speed, the better.

Because everything has a multifaceted nature, a certain function goes to extremes, and then other problems will appear, such as sensitivity and anti-interference are a pair of contradictions, which must be compatible with the design indicators to properly solve the problem.

Resistors, capacitors, and inductors can generally also be selected as SMT. Large-capacity capacitors can be considered for other forms of devices.

The selection of components should be carried out under the premise of satisfying the function; 3 reductions and shrinkages have been proposed.

1. Reduce the switching speed of IC circuits and reduce harmonic components.

2. Reduce working current and power.

3. Reduce blood circulation area. SMT device cycle area is the smallest and most suitable, and the integration is high, and the reliability is good, so it has become the first choice.

Figure 7 test results assembled on the same PCB board of three different devices.

The third type of SMT has the lowest radiation.

Summary: The choice of equipment does not advocate that the greater the power the better, the faster the better, but it is recommended that as long as the design function requirements, use compatible design indicators, and reduce costs.

And the design goal is perfectly achieved. This combination design is considered the best combination. Of course, various types and grades of machines have different best combinations.

PCB level design and routing

It is very important to correctly understand the power supply, ground interference and radiation conditions before the printed circuit board wiring. When the power supply, the ground wire in the transient appearance increases or decreases the current due to the effect of inductance and capacitance, the power supply, the ground wire interference state See Figure 8 for power line (VCC, ICC), ground (IG, VG) noise voltage and current waveforms. This is the situation when the IC circuit is working. When many circuits are working, the power supply, grounding interference and radiation are very serious.

Therefore, it is recommended to use a four-layer copy board for a complex number of PCB boards. The advantage is that signal lines can be routed on the top and back, increasing the wiring space, and more importantly, it has a low impedance ground layer and power plane, especially the ground plane, which greatly reduces all The circulation area and ground impedance of the IC circuit. In principle, the top layer is the signal line layer, the second layer is the DC ground layer, the third layer is the DC power layer, and the fourth layer is the signal line layer. When the printed circuit board IC circuits are all switching circuits or all analog circuits, their ground wires do not need to be isolated and separated. Sometimes, there are usually multiple power sources in the DC power layer, and gap isolation methods are often used to separate the solutions.

When there are logic circuits and analog circuits on the printed circuit board, through analysis, the ground line of the logic circuit and the ground of the analog circuit are isolated (isolation width> 3mm) single short circuit or magnetic beads and other methods to obtain the same potential reference.

When there are dozens of rows of logic circuits and analog circuits on the printed circuit board, the situation is very complicated. It must be mastered that they should have a separate power supply and ground area, and the coupling of IC circuits and the principle of minimum flow area must be considered.

Design and ensure that the impedance of the ground wire is very low.

The double-layer slab grounding design is used to form a grating frame, that is, a parallel grounding wire with more cloth on one side of the printed circuit board, a vertical grounding wire on the other side, and then connecting them with metalized holes where they cross (perforation resistance) small). In order to take into account that each IC chip should be equipped with a ground wire, place a ground wire every 1~115cm to make the ground wire dense and make the area of the signal loop smaller, which is beneficial to reduce radiation.