PCB Blog - To shorten PCB board design time, master skills

At present, with the requirement of PCB board size becoming smaller and smaller, and the requirement of device density becoming higher and higher, the difficulty of PCB design is gradually increasing. How to shorten the design time while ensuring the quality? This requires engineers to have excellent technical knowledge and master some design skills.

1. Determine the number of layers of PCB board

The size of the circuit board and the number of wiring layers need to be determined at the initial stage of design. The number of wiring layers and the way of stacking (STack up) will directly affect the wiring and impedance of the printed wire. The size of the board is helpful to determine the stacking mode and the width of the printed line to achieve the desired design effect. At present, the cost difference between multilayer boards is very small. At the beginning of the design, more circuit layers are used and copper coating is evenly distributed.

2. Design rules and limitations

To successfully complete the routing task, the routing tool needs to work under the correct rules and restrictions. To classify all signal lines with special requirements, each signal class should have priority. The higher the priority, the stricter the rules. The rules involve the width of printed wire, the number of vias, parallelism, the interaction between signal wires, and layer restrictions. These rules have a great impact on the performance of wiring tools. Careful consideration of design requirements is an important step for successful cabling.

3. Layout of components

During the assembly process, the design for manufacturability (DFM) rules will restrict the component layout. If the assembly department allows the components to move, the circuit can be properly optimized to facilitate automatic wiring.

For example, for power cord layout:

1) In the PCB layout, the power decoupling circuit should be designed near each related circuit, rather than placed in the power part, otherwise it will not only affect the bypass effect, but also flow pulsating current on the power line and ground wire, causing interference;

2) For the internal power supply direction of the circuit, power shall be supplied from the last stage to the next stage, and the power filter capacitor of this part shall be arranged near the last stage;

3) For some main current channels, such as disconnecting or measuring the current during commissioning and testing, the current gap shall be arranged on the printed wire during layout.

In addition, it should be noted that the regulated power supply should be arranged on a separate printed board as far as possible. When the power supply and circuit share a printed board, in the layout, it should be avoided to mix the regulated power supply and circuit components or make the power supply and circuit share a ground wire. Because this kind of wiring is not only easy to cause interference, but also cannot disconnect the load during maintenance, so only part of the printed wire can be cut, thus damaging the printed board.

4. Fan out design

At the fan out design stage, each pin of the surface mount device should have at least one via, so that when more connections are needed, the circuit board can carry out internal connection, online testing and circuit reprocessing.

5. Manual wiring and key signal processing

Manual wiring is an important process of PCB design now and in the future, and it is helpful for automatic wiring tools to complete wiring. By manually routing and fixing the selected network (net), a path that can be used for automatic routing can be formed. First, route the key signals, either manually or with automatic routing tools. After the wiring is completed, relevant engineers and technicians shall check the signal wiring. After passing the inspection, fix these lines, and then start automatic wiring for other signals. The existence of impedance in the ground wire will bring common impedance interference to the circuit.

6. Automatic routing

For the wiring of key signals, it is necessary to consider controlling some electrical parameters during the wiring, such as reducing the distributed inductance. After knowing what input parameters the automatic wiring tool has and the impact of the input parameters on the wiring, the quality of the automatic wiring can be guaranteed to a certain extent. General rules should be used when routing signals automatically. By setting limiting conditions and prohibited wiring areas to limit the layers used for a given signal and the number of vias used, the wiring tool can automatically wire according to the design idea of the engineer. After the constraint conditions are set and the rules created are applied, the automatic routing will achieve a result similar to that expected. After part of the design is completed, fix it to prevent it from being affected by the subsequent routing process. The number of wiring depends on the complexity of the circuit and the number of defined general rules. Today's auto routing tools are very powerful, and can usually complete 100% routing. However, when the automatic wiring tool does not complete all signal wiring, it is necessary to manually wire the remaining signals.

7. Arrangement of wiring

For signals with few constraints and long wiring length, you can first determine which wiring is reasonable and which is unreasonable, and then manually edit to shorten the signal wiring length and reduce the number of vias on the PCB board.