Experience sharing of PCB circuit board design
Designing a circuit as an electronic engineer is a necessary hard work, but the principle design is perfect. If the circuit board design is not reasonable, the performance will be greatly reduced, and in severe cases, it may not even work properly. Based on my experience, I have summarized the following points that should be paid attention to in the design, I hope to give you some inspiration.
No matter what software is used, there is a general procedure designed to save time and effort in order, so I will introduce it according to the production process. (Because the interface style of protel is close to that of windows, the operating habits are also similar, and there are powerful simulation functions. Many people use it. This software will be used as an example.)
Schematic design is a preliminary preparation. It is often seen that beginners go to the drawing board in order to save trouble. This will not pay for it. For simple boards, if you are proficient in the process, you may skip it. But for beginners, it is necessary to follow the flow, so that on the one hand you can develop good habits, and on the other hand, only in this way can you avoid errors.
When drawing the schematic diagram, pay attention to the connection of each file as a whole at the time of hierarchical design, which is also of great significance for future work. Because of software differences, some software may appear to be connected but not connected (in terms of electrical performance). If you don't use relevant testing tools to detect it, if something goes wrong, it will be too late to find out when the board is ready. Therefore, the importance of doing things in order has been repeatedly emphasized, and I hope to attract everyone's attention.
The schematic diagram is based on the design project. As long as the electrical connection is correct, there is nothing to say. Below we focus on the problems in the specific board making process.
1. Make a physical border
The closed physical frame is a basic platform for the layout and routing of components in the future, and it also restricts the automatic layout. Otherwise, the components coming from the schematic will be at a loss. But we must pay attention to accuracy here, otherwise the installation problems will be troublesome in the future. There is also the best use of arcs in the corners. On the one hand, sharp corners can avoid scratching the workers, and at the same time, it can reduce the stress. In the past, one of my products always had a case where the surface of the PCB board broke in some machines during transportation, and it was good to use an arc instead.
2. Introduction of components and networks
It should be easy to draw the components and the network into the drawn frame, but there are often problems here. You must carefully solve the errors one by one, and then you will not have to work harder. The problems here are generally the following:
Component packaging is not found, component network problems, unused components or pins, these problems can be quickly solved by comparison.
3.Layout of components
The layout and routing of components have a great impact on the life, stability and electromagnetic compatibility of the product, which should be paid special attention to. In general there should be the following principles:
(1) Placement order
First place components related to the structure in a fixed position, such as power sockets, indicators, switches, connectors, etc. After placing these components, use the software's LOCK function to lock them so that they will not be moved by mistake in the future. Then place special components and large components on the line, such as heating components, transformers, ICs, and so on. Finally, the small device is placed.
(2) Pay attention to heat dissipation
Part layout also requires special attention to heat dissipation. For high-power circuits, those heat-generating components such as power tubes and transformers should be placed side by side as far as possible to facilitate heat dissipation. Do not concentrate in one place and do not place high capacitance too close to prevent the electrolyte from prematurely aging.
The knowledge of wiring is very advanced, everyone will have their own experience, but there are still some common principles.
◆ High-frequency digital circuit traces are thinner and shorter.
◆ Large current signals, high voltage signals and small signals should be isolated. (The isolation distance is related to the withstand voltage. Generally, the board should be 2mm away at 2KV, and the ratio should be increased. 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, there is a slot between the high and low voltages on the printed circuit board.)
◆ When wiring the two panels, the wires on both sides should be perpendicular, oblique, or bent, avoid parallel to each other to reduce parasitic coupling; printed wires used as input and output of the circuit should be avoided as far as possible. In order to avoid feedback, it is best to add a ground wire between these wires.
◆ The corners of the line should be greater than 90 degrees as far as possible, eliminate the corners below 90 degrees, and try to use less 90 degrees.
◆ It is the same as the address line or data line. The difference in the length of the trace should not be too large, otherwise the short line should be compensated artificially by the curved line.
◆ Traces should go as far as possible on the soldering surface, especially the PCB with through-hole process
◆ Use as few vias and jumpers as possible
◆ Single panel pads must be large, the wires connected to the pads must be thick, and teardrops can be dropped. The quality of general single panel manufacturers will not be very good, otherwise there will be problems with welding and RE-WORK.
◆ Large-area copper should be grid-shaped to prevent the board from generating bubbles and bending due to thermal stress during wave welding. However, in special occasions, the flow direction and size of GND must be considered. It cannot be simply filled with copper foil. , But need to go wiring
◆ Components and wiring should not be placed too close to the side. Generally, 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, commissioning and maintenance must be considered
It is very important for analog circuits to deal with the problem of ground. The noise generated on the ground is often inconvenient. However, once it is generated, it will cause great trouble. It should be rain-proof. 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 line, there will be a certain temperature drift, which affects Amplifier works. At this time, you can add a decoupling capacitor at the 4 corners of the board, one foot is connected to the ground on the board, and one foot is connected to the mounting hole (connected by the screw and the case), so that this component can be taken into account, the amplifier and AD also It's stable.
In addition, the problem of electromagnetic compatibility is even more important when people pay more attention to environmental protection products. Generally speaking, there are three electromagnetic signals: signal source, radiation, and transmission line. Crystal oscillator is a common high-frequency signal source. In the power spectrum, the energy value of each harmonic of the crystal oscillator will be significantly higher than the average value. The feasible method is to control the amplitude of the signal, ground the crystal case, shield the interference signal, and use special filter circuits and devices.
It should be particularly noted that the serpentine wiring is different because its application is different in different applications. It is used on some clock signals in the computer's motherboard, such as PCIClk and AGP-Clk. It has two functions: 1. impedance matching 2. Filter inductance.
For some important signals, such as HUBLink in the INTELHUB architecture, there are 13 in total, and the frequency can reach 233MHZ. It is required to strictly equal length to eliminate the hidden danger caused by time delay. At this time, the serpentine wiring is the only solution.
Generally speaking, the wire pitch of the serpentine wire is》 = 2 times the line width; if it is used in a common PCB board, in addition to its role as a filter inductor, it can also be used as an inductor coil of a radio antenna and so on.
5. Perfect adjustment
After completing the wiring, all you have to do is make some adjustments to the text, individual components, and wiring (this work should not be too early, otherwise it will affect the speed and bring trouble to the wiring). It is also to facilitate production, Commissioning and maintenance.
Copper deposition usually refers to the large area of copper foil to fill the empty space left after wiring. You can lay copper foil of GND, or copper foil of VCC (but this way, it is easy to burn the device in case of a short circuit, it is best to ground it, unless you have to use it. To increase the conduction area of the power supply to withstand a larger current before connecting VCC). Land parcel usually refers to using a pair of ground wires (TRAC) to cover a bunch of signal cables with special requirements to prevent it from being disturbed or disturbed by others.
If you use copper to replace the ground wire, you must pay attention to whether the entire ground is connected, the current magnitude, flow direction, and whether there are special requirements to ensure that unnecessary errors are reduced.
6. Check the check network
Sometimes the network relationship of the drawn board is different from the schematic diagram due to misoperation or negligence. At this time, inspection and verification are necessary. Therefore, after drawing, do not rush to the plate-making manufacturer, you should check it first, and then follow-up work.
7. Use the simulation function
After completing these tasks, software simulation can be performed if time permits. Especially high-frequency digital circuits, so that some problems can be found in advance, greatly reducing the workload of future debugging.