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Seven Steps To Determine PCB Layout and Wiring

by: Dec 28,2021 2038 Views 0 Comments Posted in PCB Design & Layout

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

1. Determine the number of PCB layers 

Circuit board size and wiring layers need to be determined in the early design. The number of wiring layers and the way of STack-up will directly affect the wiring and impedance of printed lines. The size of the board helps to determine the stacking mode and the width of the printing line to achieve the desired design effect. At present, the cost difference between multi-layer plates is very small, and it is best to use more circuit layers and make copper coating evenly distributed at the beginning of design.

2. Design rules and restrictions

To successfully complete the routing task, the routing tool needs to work under the correct rules and constraints. To classify all signal lines with special requirements, each signal class should have priority. The higher the priority, the stricter the rules are. Rules related to the width of printed lines, the maximum number of vias, parallelism, the mutual influence between signal lines and the limitation of layers have great influence on the performance of routing tools. Careful consideration of design requirements is an important step in successful wiring.

3. Layout of components  

In the optimal assembly process, the manufacturability design ( DFM ) rule limits the component layout. If the assembly department allows components to move, the circuit can be properly optimized to facilitate automatic wiring. The defined rules and constraints will affect the layout design. The automatic wiring tool only considers one signal at a time. By setting the constraints of wiring and the layer of the configurable signal line, the wiring tool can complete the wiring as the designer envisages.

For example, for the layout of the power line : 1 In the PCB layout, the power decoupling circuit should be designed in the vicinity of the relevant circuit, rather than placed in the power part, otherwise it will affect the bypass effect, and will flow through the pulse current on the power line and ground wire, causing interference ; 2 For the power supply direction within the circuit, the power supply should be supplied from the last stage to the forward stage, and the power filter capacitor of this part should be arranged near the last stage ; 3 For some main current channels, such as the current should be disconnected or measured in the debugging and testing process, and the current gap should be arranged on the printed wire in the layout.

In addition, it should be noted that the regulated power supply is arranged on a separate printed circuit board as much as possible when it is arranged. When the power supply and circuit share the printed circuit board, in the layout, it is necessary to avoid the mixed layout of the stabilized voltage power supply and circuit components or to make the power supply and circuit share the ground wire. 

Because this wiring is not only easy to produce interference, but also can't disconnect the load during maintenance, and can only cut part of the printed wire, 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 hole so that the circuit board can perform inner connection, online testing and circuit reprocessing when more connections are required. 

In order to maximize the efficiency of automatic wiring tools, it is necessary to use the maximum hole size and printing line as much as possible, and the spacing set to 50 mil is ideal. Use the type of hole that maximizes the number of routing paths. After careful consideration and prediction, the design of circuit on-line test can be carried out in the early stage of design and later in the production process.

According to the wiring path and circuit online test to determine the type of hole fan out, power and grounding will also affect the wiring and fan out design.

5. Manual wiring and key signal processing 

Manual wiring is an important process in the design of printed circuit boards now and in the future. Manual wiring helps automatic wiring tools to complete wiring work. 

By manually wiring and fixing the selected network ( net ), the path can be formed when automatically wiring. 

First, wiring the key signals, manual wiring or automatic wiring tools can be used. After the wiring is completed, the relevant engineers and technicians check the wiring of these signals. After checking, these lines are fixed, and then the signal is automatically wiring. 

Due to the existence of impedance in the ground wire, it will bring co-resistance and anti-interference to the circuit. Therefore, no point with grounding symbols can be arbitrarily connected during wiring, which may produce harmful coupling and affect the work of the circuit.

When the frequency is high, the inductance of the wire will be several orders of magnitude larger than the resistance of the wire itself. At this time, even if only a small high frequency current on the wire, it will also produce a certain high frequency voltage drop. Therefore, for high frequency circuits, PCB layout is as compact as possible, making the printed wire as short as possible. 

There are also mutual inductance and capacitance between printed wires, when the working frequency is large, it will interfere with other parts, known as parasitic coupling interference. The possible suppression methods are : 1 to minimize the signal line between levels ; 2 arranging circuits at all levels according to the sequence of signals to avoid crossing each other ; 3 the wires of adjacent two panels should be perpendicular or crossed, and cannot be parallel ; 4 When the signal wires are arranged in parallel in the board, these wires should be separated as far as possible, or the land lines and power lines should be separated to achieve the purpose of shielding.

6. Automatic wiring 

The wiring of key signals needs to consider controlling some electrical parameters during wiring, such as reducing the distributed inductance. After understanding the input parameters of automatic wiring tools and the influence of input parameters on wiring, the quality of automatic wiring can be guaranteed to a certain extent. 

General rules should be adopted in automatic wiring of signals. By setting constraint conditions and forbidden wiring area to limit the layer used by the given signal and the number of holes used, the wiring tool can automatically wiring according to the engineer's design idea. After setting the constraint conditions and the rules created by the application, the automatic wiring will achieve the expected results. After part of the design is completed, it will be fixed to prevent the impact of the back wiring process.

The number of wiring depends on the complexity of the circuit and the number of general rules defined. Now the automatic wiring tool is very powerful, usually can complete 100 % wiring. However, when the automatic wiring tool does not complete all the signal wiring, it is necessary to manually wiring the signal. 

7. Arrangement of wiring 

Some signals with few constraints have long wiring length. At this time, we can first determine which wiring is reasonable and which wiring is unreasonable, and then manually edit to shorten the signal wiring length and reduce the number of holes.

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