PCB News - PCB wiring layout planning and design skills

PCB,is called printed circuit board, is an important electronic component, is the support of electronic components, is the provider of electrical connection of electronic components. Because it is made by electronic printing, it is called "printed" circuit board.

As THE PCB size requirements are getting smaller and the device density requirements are getting higher and higher, PCB design is becoming more and more difficult. How to achieve PCB high pass rate and shorten the design time, in this author talk about PCB planning, layout and wiring design skills.

Careful analysis of the design and setup of the tool software should be carried out before cabling to make the design more suitable.

PCB wiring layout planning and design skills

1. Determine the number of PCB layers

The size of the circuit board and the number of wiring layers need to be determined at the beginning of the design. The number of wiring layers and stack-up mode will directly affect the wiring and impedance of printed lines. The size of the plate helps determine the layering pattern and the width of the printed line to achieve the desired design effect. At present, the cost difference between multilaminates is very small, and more circuit layers are used at the beginning of the design and the copper coating is distributed evenly.

2. Design rules and limitations

To successfully complete wiring tasks, wiring tools need to work under the right rules and constraints. To classify all signal lines with special requirements, each signal class should have a priority, and the higher the priority, the more strict the rules. Rules related to printed line width, number of holes, parallelism, interaction between signal lines and layer limits, these rules have a great impact on the performance of wiring tools.

Careful consideration of design requirements is an important step in successful wiring.

3. Component layout

During the assembly process, manufacturability design (DFM) rules impose restrictions on the layout of components. If the assembly department allows components to move, the circuit can be optimized to facilitate automatic wiring. The rules and constraints defined affect the layout design. The automatic routing tool considers only one signal. By setting the routing constraints and setting the layer where the signal line can be laid, the routing tool can complete the routing as the designer imagined.

For example, for the layout of power cables:

(1) In PCB layout, the power decoupling circuit should be designed near the relevant circuit, not placed in the power supply part, otherwise it will affect the bypass effect, and will flow through the pulsating current on the power line and ground wire, resulting in tamper;

(2) For the direction of the power supply inside the circuit, the power supply should be provided from the last stage to the forward stage, and the filter capacitor of this part of the power supply should be arranged near the last stage;

3. For some major current channels, such as in the debugging and testing process to disconnect or measure the current, in the layout should be arranged on the printed wire current gap.

In addition, we should pay attention to the layout of the regulated power supply, as far as possible arranged in a separate printed board. When the power supply and circuit share the PCB, in the layout, it should be avoided to mix the regulated power supply with the circuit element or to share the ground wire with the power supply and circuit.

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

4. Fan out design

During the fan out design phase, each pin of the surface mount device should have at least one through-hole so that the board can be used for inner connection, in-line testing, and circuit reprocessing when additional connections are required.

To make the automatic wiring tool efficient, it is important to use as many hole sizes and printed lines as possible, with an interval of 50mil being ideal. Use a type of through-hole that gives the number of routing paths. After careful consideration and prediction, the design of circuit on-line test can be carried out in the early design stage and later in the production process.

Determine the type of through-hole fan out based on wiring paths and circuit on-line testing. Power supply and grounding also affect wiring and fan out design.

5. Manual wiring and key signal processing

Manual wiring is and will be an important process in PCB design. Manual wiring is helpful for automatic wiring tools to complete wiring.

By manually routing and securing the selected network (NET), a path can be formed upon which to base automatic routing.

Firstly, the key signals should be wired manually or with automatic wiring tools. After wiring is completed, the wiring of these signals will be checked by the relevant engineering and technical personnel. After passing the inspection, these lines will be fixed, and then the rest of the signals will be wired automatically.

Because of the existence of impedance in ground wire, it will bring common impedance interference to the circuit. Therefore, when wiring, it is not possible to connect any point with ground symbol at will, which may produce harmful coupling and affect the work of the circuit.

At higher frequencies, the inductive reactance of the wire will be several orders of magnitude greater than the resistance of the wire itself. Even if only a small high-frequency current is flowing through the wire, there will be a certain high-frequency voltage drop. Therefore, for high-frequency circuits, the PCB layout should be as compact as possible, keeping the printed wires as short as possible.

There are mutual inductance and capacitance between printed wires. When the working frequency is large, interference will occur to other parts, which is called parasitic coupling interference. The suppression methods that can be adopted include:

1. As far as possible to shorten the signal wiring between levels;

(2) Arrange circuits at all levels according to the order of signals to avoid crossing signal lines at all levels;

(3) The wires of the two adjacent panels should be vertical or cross, not parallel;

(4) when parallel signal wires are laid in the board, these wires should be separated as far as possible from a certain distance, or separated by land lines and power lines, so as to achieve the purpose of shielding.

6. Automatic Cabling

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

General rules should be used for automatic routing of signals. By setting restrictions and no-wiring areas that limit the layers used for a given signal and the number of holes to be used, the wiring tool can automatically route the wires as designed by the engineer. After the constraints are set and the rules created are applied, the automatic wiring will achieve similar results as expected, and after a portion of the design is completed, it will be fixed to protect against subsequent wiring processes.

The number of cabling times depends on the complexity of the circuit and how many general rules are defined. Today's automatic routing tools are very powerful and can typically complete 100% wiring. However, when the automatic routing tool has not completed all signal routing, the remaining signals need to be manually routed.

7. Wiring arrangement

For some signals with few constraints, the wiring length is very long. In this case, we can first determine which wiring is reasonable and which wiring is unreasonable, and then shorten the signal wiring length and reduce the number of holes through manual editing.