The clever techniques of PCB layout and processing of technological defects

- May 16, 2017-

With the requirement of PCB size becoming smaller and smaller, the requirement of device density is higher and higher, and the design of PCB is becoming more and more difficult. How to achieve PCB high completion rate and shorten the design time, in which I talk about PCB planning, layout and wiring design skills.

Before starting the wiring, the design should be carefully analyzed and the tool software will be carefully set up, which will make the design more in line with the requirements.

1, determine the number of layers of PCB

The circuit board size and wiring number shall be determined at the beginning of design. The number of wiring layers and the mode of cascading (STack-up) will directly affect the wiring and impedance of printed wiring. The size of the plate helps determine the stacking mode and the width of the printed line to achieve desired design results. At present, the cost difference between the multilayer plates is very small. In the beginning, it is better to use more circuit layers and distribute the copper evenly.

2 design rules and limitations

To successfully complete the wiring task, wiring tools need to work under the correct rules and restrictions. To classify all the required signals, each signal class should have a priority, the higher the priority, the tighter the rules. The rules relate to the width of the printed line, the maximum number of vias, parallelism, the influence of signal lines, and the limitations of the layers. These rules have a great impact on the performance of the wiring tool.

3, the layout of components

In the optimization assembly process, the manufacturability design (DFM) rule limits the component layout. If assembly department allows components to move, you can optimize the circuit and make it easier for automatic routing. The defined rules and constraints affect layout design. An automatic routing tool will consider only one signal at a time. By setting up the wiring constraints and setting the layer of the signal line, the routing tool can make the routing tool complete the wiring as the designer envisaged.

In addition, we should pay attention to the power supply in the layout, as far as possible on the separate printed board. When the power supply and the circuit are used in the printed circuit board, the layout of the power supply and the circuit elements should be avoided, or the power supply and the circuit can be shared with the ground wire.

Because this wiring is not only prone to interference, and at the same time in the maintenance of the load can not be broken, and then only part of the printed wire can be cut, thereby damaging the printed circuit board.

4 、 manual wiring and key signal processing

Manual routing is an important process of printed circuit board design both in the present and in the future. Manual routing is helpful for automatic routing tools to complete wiring work.

First of all, wiring of key signals, manual routing or automatic routing tools can be used. When the wiring is complete, the wiring of the signals is checked by the relevant engineers and technicians. After checking, the wires are fixed and then the wiring of the remaining signals is automatically started.

High frequency, wire inductance will be larger than the resistance wire itself several orders of magnitude. At this time, even a small high-frequency current on the wire will produce a certain high frequency voltage drop. Therefore, for high frequency circuits, the PCB layout is as compact as possible so that the printed wires are as short as possible.

When a signal wire is arranged in parallel, the wires shall be spaced at a certain distance as far as possible, or separated by ground wire and power line, so as to reach the goal of shielding.

5. Automatic routing

General rules should be used when routing signals automatically. By setting up the constraints and the forbidden wiring area, the routing tools can be automatically wired according to the engineer's design to limit the layers and the number of vias used for the given signal. After setting up the constraint conditions and the application of the rules, automatic routing and expected similar results will be reached after the completion of a part of the design, will be fixed, to prevent the influence behind the wiring process.

The number of wiring depends on the complexity of the circuit and the number of generic rules defined. Automatic routing tools are now very powerful and usually do 100% of routing. However, when the automatic routing tool does not complete all the signal wiring, it is necessary to manually route the remaining signals.

6 、 wiring arrangement

Some little signal constraints, the wiring length, this can be the first to identify the reasonable wiring, wiring which is not reasonable, then by manually editing to shorten the signal wiring length and reduce the number of vias.

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