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PCB board design experience sharing

Posted:12:01 PM June 28, 2019 writer: G

Regardless of the software used, the PCB design has a rough program, which saves time and effort in order, so it will be introduced according to the production process (because the protel interface style is close to the WINDOWS window, the operating habits are similar, and there are powerful simulation functions).

The schematic design is a preliminary preparation. It is often seen that beginners can draw PCB boards directly in order to save trouble. This will not be worth the candle. For simple boards, if you are skilled, you may skip. But for beginners, you must follow the process, so that you can develop good habits on the one hand, and the only way to avoid complicated circuits on the other hand.

When drawing the schematic diagram, the hierarchical design should pay attention to the fact that each file should be connected as a whole at the end, which is also important for future work. Due to software differences, some software may appear to be connected to the actual unconnected (electrical performance). If you do not use the relevant detection tools to detect, if there is a problem, it will be late if the board is ready. Therefore, the importance of doing it in order is emphasized again and again.

The schematic diagram is based on the design project, as long as the electrical connection is correct, nothing to say.

Below we focus on the issues in the specific board making process.

1. Making a physical border

The closed physical frame is a basic platform for future component layout and routing, and also plays a role in the automatic layout. Otherwise, the components coming from the schematic will be overwhelmed. But here must pay attention to accuracy, otherwise the trouble of installation problems in the future can be big. There is also the use of arcs in the corners, on the one hand to avoid sharp corners of the workers, but also to reduce the stress. In the past, one of my products always had a case where the surface of the shell PCB was broken during the transportation process, and it was better to use the arc.

2. Introduction of components and networks

The frame that draws the components and the network should be very simple, but there are often problems here. Be sure to carefully follow the error of the prompts, and it will take more effort. The problems here are generally the following: The package form of the component is not found, the component network is faulty, and there are unused components or pins.

3. Component layout

The layout of the components and the traces have a great influence on the life, stability and electromagnetic compatibility of the products, and should be paid special attention. Generally speaking, there should be some principles as follows:

(1) Placement order

First place the components in the fixed position related to the structure, such as power sockets, indicators, switches, connectors, etc. After these devices are placed, they are locked by the LOCK function of the software so that they will not be moved in the future. Then place special components on the line and large components such as heating components, transformers, ICs, etc. Finally place the small device.

(2) pay attention to heat dissipation

Special attention should be paid to the heat dissipation problem. For high-power circuits, those heating elements such as power tubes, transformers, etc. should be placed as far as possible in a distributed layout to facilitate heat dissipation. Do not concentrate in one place, nor put high capacitance too close to avoid premature aging of the electrolyte.

4. Wiring

Wiring principle

The learning of the line is very advanced, and everyone will have their own experience, but there are still some common principles. The high-frequency digital circuit has finer and shorter wiring. The high-current signal, high-voltage signal and small signal should be separated from each other (the isolation distance is related to the withstand voltage to be withstand. Usually, the distance is 2mm on the board at 2KV. On this basis, the ratio should be increased. For example, if you want to withstand the withstand voltage test of 3KV, the distance between the high and low voltage lines should be more than 3.5mm. In many cases, to avoid creepage, it is still printed. Slot between the high and low voltages on the board.)

When two panels are wired, the conductors on both sides should be perpendicular, oblique, or curved, so as to avoid parallelism to reduce parasitic coupling. The printed conductors used as input and output of the circuit should avoid adjacent parallel as much as possible to avoid Feedback occurs, and it is best to add a grounding wire between these wires.

The corner of the trace should be as large as 90 degrees as much as possible, and the corners below 90 degrees should be eliminated. The 90-degree corner should be used as the address line or data line. The difference in the length of the trace should not be too large. Otherwise, the short-term part should be compensated for the bend line. The wire should be as far as possible on the soldering surface. In particular, the PCB of the through-hole process should use as few via holes as possible. The jumper of the single-layer pad must be large. The line connecting the pads must be thick, and the teardrop can be placed on the teardrop. The quality of 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 used in a grid shape to prevent bubbles from being generated during wave soldering and bending due to thermal stress. However, in special occasions, the flow direction and size of GND should be considered, and it is not easy to fill with copper foil. Instead, the components and traces that need to be routed should not be placed too far. The general single-panel is mostly a paper board. It is easy to break after being stressed. If it is connected at the edge or the components will be affected, it must be considered for production and debugging. The convenience of maintenance is very important for the problem of the analog circuit. The noise generated on the ground is often inconvenient, but once it is generated, it will cause great trouble and should be satin.

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 latter stage; in the high-precision A/D conversion circuit, if there is a high-frequency component on the ground line, a certain temperature drift will occur, affecting The work of the amplifier. At this time, you can add the capacitor at the 4 corners of the board, one foot and the ground connection on the board, and one foot to the mounting hole (connected by the screw and the case), so you can take this component away, the amplifier and AD also It is stable. In addition, the issue of electromagnetic compatibility is even more important in the current situation of people paying more attention to environmentally friendly products. Generally speaking, there are three sources of electromagnetic signals: signal source, radiation, and transmission line. The crystal oscillator is a common high-frequency signal source. The energy value of each harmonic of the crystal oscillator on the power spectrum will be significantly higher than the average value. It is feasible to control the amplitude of the signal, the crystal casing is grounded, the interference signal is shielded, and special filter circuits and devices are used. Need to specify is the serpentine routing, because the application is different, its role is also different, in the computer's motherboard used in some clock signals, such as PCIClk, AGP-Clk, it has two functions: 1. Impedance matching ; 2. Filter inductor. For some important signals, such as HUBLink in the INTELHUB architecture, a total of 13 frequencies, up to 233MHZ, must be strictly equal length to eliminate the hidden danger caused by time lag. At this time, serpentine routing is the only solution.

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

5. Adjustment and improvement

After completing the wiring, all you have to do is make some adjustments to the text, individual components, routing, and copper (this work should not be too early, otherwise it will affect the speed and cause trouble to the wiring), also for the convenience of production, Commissioning and maintenance.

Copper application usually refers to the blank area left after filling the wiring with a large area of ??copper foil. It can be used to lay GND copper foil or VCC copper foil. (However, once the short circuit is easy to burn the device, it is better to ground, unless it is necessary To increase the conduction area of ??the power supply to withstand a large current to connect to VCC). The package usually refers to the use of two ground wires (TRAC) to wrap a signal line with special requirements to prevent it from being disturbed or disturbed by others.

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

6. Check the check network

Sometimes the network relationship between the board and the schematic is different because of misuse or negligence. At this time, it is necessary to check and check. Therefore, after painting, you must not rush to the plate-making factory, you should check it first, and then carry out follow-up work.

7. Use simulation

After completing these tasks, software simulation can also be performed if time permits. Especially high-frequency digital circuits, so that some problems can be found in advance, greatly reducing the amount of debugging work in the future.

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