PCB News - What should be paid attention to in PCB design

As an electronic engineer designing a circuit is a necessary hard work, but no matter how perfect the principle design is, if the PCB design is unreasonable, the performance will be greatly reduced, and it may even not work properly in severe cases. Based on my experience, I have summarized the following points that should be paid attention to in PCB design. I hope it can be enlightened to you.

l. Make a physical frame

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 using the arc in some PCB manufacturers.

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. The problems here are generally as follows:

The package form of the component can not be found, the component network problem, 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:

3. l 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.

3.2 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. Wiring

Wiring principle

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

◆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 corners of the routing should be as large as possible to be greater than 90 degrees, to eliminate corners below 90 degrees, and to minimize the use of 90 degrees corners.

◆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

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

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

◆The trace should be on the soldering surface as much as possible, especially the PCB of the through-hole process

◆Use as few vias as possible

◆Single-panel pads must be large, and the wire 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.

◆Large area 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 need to route

◆The components and traces 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 amplification of the subsequent stage; 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. 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. 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 in some clock signals in the computer motherboard, such as PCIClk, AGP-Clk. 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 the hidden dangers caused by time lag. At this time, the serpentine trace is the only solution.

Generally speaking, the line spacing of the serpentine trace is more than 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.

5. Adjustment and perfection

After completing the wiring, all you need to do is to adjust the text, individual components, wiring, and apply copper (this work should not be too early, otherwise it will affect the speed and bring trouble to the wiring), also for the convenience of production, Commissioning and maintenance.

Copper coating usually refers to filling the blank area left by the wiring with a large area of copper foil. It can be covered with GND copper foil or VCC copper foil. To increase the conduction area of the power supply, to withstand a larger current before connecting to VCC). Grounding usually refers to wrapping a bunch of signal wires with special requirements with two ground wires (TRAC) to prevent it from being interfered by or interfered with by others.

If you use copper instead of the ground wire, you must pay attention to whether the entire ground is connected, the current flow direction and whether there are special requirements to ensure that unnecessary mistakes are reduced.

6. Check the network

Sometimes the network relationship of the board drawn is different from the schematic diagram due to misoperation or negligence. At this time, it is necessary to check and verify. Therefore, you should not rush to hand it over to the plate maker after you finish the drawing. You should check it first before proceeding with follow-up work.