PCB Blog - Matters needing attention in PCB board design

Schematic design is the preparatory work in the early stage. It is often seen that beginners directly draw PCB board in order to save trouble. This will not be worth the loss. For simple boards, if you are proficient in the process, you may wish to skip it. But for beginners, you must follow the process, so that on the one hand you can develop good habits, and on the other hand, it is the only way to avoid mistakes in complex circuits.

When drawing a schematic diagram, it is necessary to pay attention that each file should be connected as a whole when designing a hierarchy, which is also of great significance to future work. Due to software differences, some software may appear to be connected but not actually connected (in electrical performance). If you don't use relevant testing tools to test, if there is a problem, it will be too late to find out when the board is ready. Therefore, I have repeatedly emphasized the importance of doing things in order, hoping 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 discussing the problems in the specific plate-making program.

1. Make a physical border

The closed physical border is a basic platform for future component layout and routing, and it also acts as a constraint on the automatic layout. Otherwise, the components coming from the schematic diagram will be overwhelmed. But you must pay attention here, otherwise, installation problems will be troublesome in the future. There is also the use of arcs in the corners, which can avoid sharp corners from scratching workers and reduce stress at the same time. In the past, one of my products always had a case where the PCB board of the face shell was broken during the transportation process. It would be fine after switching to a circular arc.

2. Introduction of components and networks

It should be very simple to draw components and networks into a well-drawn frame, but there are often problems here, and you must carefully follow the prompts to solve them one by one, otherwise, it will take more effort later. Generally speaking, the problems here are as follows: the package form of the components cannot be found, the network problems of the components, and the unused components or pins. The comparison prompts that these problems can be solved quickly.

3. Layout of components

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

3.1 Placement order

First place components in fixed positions related to the structure, 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 line, such as heating elements, transformers, ICs, etc. Place small devices.

3.2 Pay attention to heat dissipation

The component layout should also pay special attention to heat dissipation. For high-power circuits, those heating elements such as power tubes and transformers should be placed as far as possible in a distributed layout to facilitate heat dissipation. Do not concentrate them in one place, and do not have high capacitance too close to prevent premature aging of the electrolyte.

4. Wiring

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

1) The traces of high-frequency digital circuits should be thinner and shorter.

2) 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 endured. Usually, the distance between the board is 2mm at 2kv, and the ratio is calculated on top of this. For example, if you want to withstand a 3kv withstand voltage test, the distance between the high and low voltage lines should be more than 3.5mm. In many cases, in order to avoid creepage, a slot is also made between the high and low voltage on the printed circuit board.)

3) When wiring on two panels, the wires on both sides should be perpendicular to each other, obliquely, or bent to avoid being parallel to each other to reduce parasitic coupling; the printed wires used as the input and output of the circuit should try to avoid adjacent ones. Parallel to avoid feedback, add a ground wire between these wires.

4) The corners of the wiring should be as large as 90 degrees, avoid corners below 90 degrees, and use as few 90-degree corners as possible.

5) The same is the address line or data line, the difference in the length of the line should not be too large, otherwise, the short line should be compensated by artificially bending the line

6) The traces should be on the welding surface as far as possible, especially on the PCB board with a through-hole process

7) Minimize the use of vias and jumpers

8) The single-panel pads must be large, the wires connected to the pads must be thick, and teardrops can be dropped as much as possible. Generally, the quality of single-panel manufacturers will not be very good, otherwise, there will be problems with soldering and re-work.

9) Large-area copper plating should be made of mesh to prevent the board from generating air bubbles and bending due to thermal stress during wave soldering. However, on special occasions, the flow direction and size of GND should be considered, and it cannot be simply filled with copper foil. thing, but need to walk the line

10) Components and wiring should not be placed too far to the side. Generally, single-sided boards are mostly paper boards, which are easy to break after being stressed. If you connect or place components on the edge, it will be affected.

11) The convenience of production, debugging, and maintenance must be considered

For analog circuits, it is very important to deal with the problem of ground. The noise generated on the ground is often unpredictable, but once it occurs, it will bring great trouble and should be avoided. For the power amplifier circuit, very small ground noise will have a significant impact on the sound quality due to the amplification of the post-stage; in the high-precision A/D conversion circuit, if there is a high-frequency component on the ground wire, there will be a certain temperature drift, which will affect the sound quality. Amplifier work. At this time, a decoupling capacitor can be added to the 4 corners of the board, one foot is connected to the ground on the board, and the other foot is connected to the mounting hole (connected to the chassis through screws) so that this component can be taken into account, and the amplifier and AD are also It's stable. In addition, the problem of electromagnetic compatibility is more important in the current situation where people pay more attention to environmental protection products. Generally speaking, there are three types of electromagnetic signals: signal source, radiation, and transmission line. The 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 oscillator shell, shield the interference signal, and use special filter circuits and devices. It should be noted that the serpentine trace has different functions because of different applications. It is used in some clock signals in the motherboard of the computer, such as pick and AGP-Clk. It has two functions: 1) Impedance matching and 2) Filter inductor. For some important signals, such as the hub link in the intel hub architecture, there are a total of 13, and the frequency can reach 233MHz. The requirement must be strictly equal in length to eliminate the hidden danger caused by the time delay. At this time, the serpentine routing is the solution. Generally speaking, the line spacing of the serpentine trace is >= 2 times the line width; if it is in a common PCB board, in addition to the function of a filter inductance, it can also be used as an inductance coil for a radio antenna, etc.

5. Perfect adjustment

After completing the wiring, all you need to do is to make some adjustments to the text, individual components, and traces and apply copper (this work should not be done too early, otherwise it will affect the speed and bring trouble to the wiring), also for the convenience of production, Commissioning and maintenance. A copper deposition usually refers to filling the blank area left after wiring with a large area of copper foil. You can lay GND copper foil or VCC copper foil (but this will easily burn the device in case of a short circuit, and ground it unless you have to use it to add copper foil. The conduction area of the large power supply is connected to VCC in order to withstand a large current). Ground wrapping usually refers to wrapping a bunch of signal lines with special requirements with two ground wires (TRAC) to prevent it from being disturbed or disturbed by others. If copper is used instead of the ground wire, you must pay attention to whether the entire ground is connected, the current size, flow direction, and whether there are special requirements to ensure that unnecessary mistakes are reduced.

6. Check and verify the network

Sometimes due to misoperation or negligence, the network relationship of the drawn board is different from the schematic diagram. At this time, it is necessary to check. Therefore, after painting, you must not rush to the plate maker, you should check it first, and then carry out follow-up work.

7. Use the simulation function

After completing these tasks, software simulations can be performed if time permits. Especially for high-frequency digital PCB board, some problems can be found in advance, which greatly reduces the workload of debugging in the future.