PCB Technical - Analysis of Difficulties in PCB Design and Board Process

The PCB schematic diagram is the preparatory work of PCB design. It is often seen that beginners directly draw the PCB board in order to save trouble. This will outweigh the gain. For simple boards, if you are familiar with the process, you can skip it. When drawing a schematic diagram, pay attention to the final connection of each file into a whole when designing the hierarchy, which is also of great significance for future work. Since the schematic is based on the designed project, there is nothing to say as long as the electrical connection is correct. Next, we will discuss and analyze the problems in the PCB design process.

1. Make a physical border

The closed physical frame is a basic platform for the future component layout and routing, and it also plays a constraining role for the automatic layout. Otherwise, the components coming from the schematic will be at a loss. But you must pay attention to accuracy here, otherwise installation problems in the future may be troublesome. In addition, it is best to use arcs at the corners. On the one hand, it can avoid sharp corners from scratching workers, and at the same time, it can reduce the effect of stress. In the past, one of my products always had the PCB board of the face shell broken during the transportation process, and it was just fine after switching to the arc.

2. The introduction of components and networks

It should be very simple to draw the components and the network in the good frame, but there are often problems here. You must carefully solve the errors one by one according to the prompts. Otherwise, it will take more effort. Generally speaking, the problems here are as follows: the package form of the component cannot be found, the component network problem, and there are unused components or pins. These problems can be solved quickly by comparison.

3. Layout of components

The layout and wiring of the components have a great impact on the life, stability, and electromagnetic compatibility of the product, so special attention should be paid. Generally speaking, there should be the following principles:

(1) Placement order

First place the components related to the structure at a fixed position, such as power sockets, indicator lights, switches, connectors, etc. After these components are placed, use the LOCK function of the software to lock them so that they will not be moved by mistake in the future. Then place special components and large components on the circuit, such as heating components, transformers, ICs, etc. Finally, place the small device.

(2) Pay attention to heat dissipation

Particular attention should be paid to heat dissipation in the component layout. For high-power circuits, the heating elements such as power tubes, transformers, etc. should be placed as close to the side as possible to facilitate heat dissipation. Do not concentrate in one place, and do not have high capacitors too close to avoid premature aging of the electrolyte.

4. PCB wiring principles

The knowledge of routing is very advanced, everyone will have their own experience, but there are still some common principles.

◆High-frequency digital circuit traces should be thinner and shorter.

◆Attention should be paid to isolation between high-current signals, high-voltage signals and small signals (the isolation distance is related to the withstand voltage to be withstood. Normally, the distance between the boards is 2mm at 2KV, and the distance should be increased in proportion to this. For example, if you want to withstand the 3KV withstand voltage test, the distance between the high and low voltage lines should be above 3.5mm. In many cases, to avoid creepage, slots are also slotted between the high and low voltages on the printed circuit board.).

◆When wiring two panels, the wires on both sides should be perpendicular, oblique, or bent to avoid parallel to each other to reduce parasitic coupling; printed wires used as the input and output of the circuit should be avoided as far as possible. In order to avoid feedback, it is best to add a grounding wire between these wires.

◆The cabling corner should be greater than 90 degrees as much as possible, avoid corners below 90 degrees, and use 90 degrees as little as possible.

◆The same address line or data line, the length of the trace should not be too different, otherwise the short line part must be artificially bent for compensation.

◆The trace should be on the soldering surface as much as possible, especially the PCB with through-hole technology.

◆ Minimize the use of vias and jumpers.

◆Single-panel pads must be large, and the line connecting the pads must be thick, and teardrops can be placed when teardrops can be placed. The quality of general single-panel manufacturers will not be very good, otherwise there will be problems with soldering and RE-WORK.

◆A large area of copper should be covered with a grid to prevent bubbles and bending of the board due to thermal stress during wave soldering. However, in special occasions, the flow direction and size of the GND should be considered. You cannot simply fill it with copper foil., But it needs to be routed.

◆The components and wiring should not be placed too close to the side. The general single-sided boards are mostly paper boards, which are easy to break after being stressed. If they are connected or placed on the edge, they will be affected.

◆Convenience of production, debugging and maintenance must be considered.

It is very important for analog circuits to deal with the ground problem. The noise generated on the ground is often inconvenient to predict, but once it is generated, it will bring great troubles. For the power amplifier circuit, the extremely small ground noise will have a significant impact on the sound quality due to the subsequent amplification; in the high-precision A/D conversion circuit, if there is a high-frequency component on the ground wire, a certain temperature drift will occur, which will affect The work of the amplifier. At this time, you can add decoupling capacitors to the 4 corners of the board, one pin is connected to the ground on the board, and the other pin is connected to the mounting hole (connected to the housing by screws), so that this component can be taken into account, and the amplifier and AD are also It's stable.

In addition, the issue of electromagnetic compatibility is even more important when people pay more attention to environmentally friendly products. Generally speaking, there are three sources of electromagnetic signals: signal source, radiation, and transmission line. Crystal oscillator is a common high-frequency signal source, and the energy value of each harmonic of the crystal oscillator on the power spectrum will be significantly higher than the average value. The feasible approach is to control the amplitude of the signal, ground the crystal housing, shield the interference signal, and use special filter circuits and devices.

What needs special explanation is the serpentine trace, because its function is different because of different applications. It is used on some clock signals in the motherboard of the computer, such as PCIClk, AGP-Clk, and its function has two points: 1. Impedance matching 2. Filter inductance. For some important signals, such as the HUBLink in the INTELHUB architecture, there are 13 channels in total, and the frequency can reach 233MHZ. The lengths must be strictly equal to eliminate hidden dangers caused by time lag. At this time, snake-shaped wiring is the only solution.

Generally speaking, the line spacing of the serpentine trace is >=2 times the line width; if it is used in an ordinary PCB board, in addition to the filter inductance, it can also be used as the inductance coil of the radio antenna and so on.