Skillful handling of PCB layout and process defects

- Sep 01, 2018-

EdCircuitBoard, known in Chinese as printed circuit board, also known as printed circuit board and printed circuit board, is an important electronic component, a supporting body of electronic components and a provider of electrical connection of electronic components.Because it is made by electronic printing, it is called "printed" circuit board.

As PCB size becomes smaller and device density becomes higher, PCB design becomes more and more difficult.How to achieve a high PCB distribution rate and shorten the design time, the author discusses the design skills of PCB planning, layout and wiring.

Careful analysis of the design and careful setting of tools and software should be done before starting wiring, which will make the design more compliant.

1. Determine the number of layers of PCB

Circuit board size and wiring layer number need to be determined in the early design.The number of wiring layers and stack-up mode will directly affect the wiring and impedance of the printed line.The size of the board helps to determine the lamination mode and print line width to achieve the desired design effect.At present, the cost difference between multilayer plates is very small, so it is better to adopt more circuit layers and make the copper coating evenly distributed at the beginning of design.

2. Design rules and limitations

To successfully complete wiring tasks, wiring tools need to work under correct rules and restrictions.To classify all signal lines with special requirements, each signal class should have a priority, and the higher the priority, the stricter the rules.Rules relate to print line width, maximum number of passes, parallelism, mutual influence between signal lines and layer limitations, and 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 optimized assembly, manufacturability design (DFM) rules place limitations on component layout.If the assembly department allows the component to move, the circuit can be optimized to facilitate automatic wiring.The rules and constraints defined affect the layout design.Automatic wiring tools consider only one signal at a time. By setting the wiring constraints and setting the layer where the signal wires can be laid, the wiring tools can be made as designed as possible.

For example, for the layout of the power cord:

In the PCB layout, the power supply decoupling circuit should be designed in the vicinity of each relevant circuit, and not placed in the power supply part. Otherwise, it will not only affect the bypass effect, but also flow through the pulsating current on the power line and ground line, causing interference.

For the direction of power supply inside the circuit, the power supply from the final stage to the forward stage should be adopted, and the power filter capacitance of this part should be arranged near the final stage.

For some major current channels, such as disconnecting or measuring current during debugging and testing, current gaps should be arranged on the printed conductor during layout.

In addition, attention should be paid to the stable power supply in the layout, as far as possible arranged on a separate board.When the power supply is used in conjunction with the circuit board, the combination of the stabilized power supply and circuit components should be avoided in the layout or the combination of the power supply and circuit ground wire should be avoided.

This kind of wiring is not only easy to cause interference, but also can not disconnect the load during maintenance, when only part of the printed conductor can be cut, thus damaging the board.

4. Fan out design

At the fan out design stage, each pin of the surface-mount device should have at least one via hole so that the circuit board can be interlocked, tested online and reprocessed 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 is set to 50mil.Use the type of crossing that maximizes the number of routing paths.After careful consideration and prediction, the design of online circuit testing can be carried out in the early stage of design and realized in the later stage of production process.

The type of through-hole fan out can be determined according to the wiring path and the circuit online testing.

5. Manual wiring and processing of key signals

Manual wiring is an important process in the design of printed circuit board now and in the future.

By manually wiring and fixing the selected network (net), a path can be established for automatic wiring.

First of all, the key signals can be wired manually or combined with automatic wiring tools.After the wiring is completed, the relevant engineers and technicians will inspect the wiring of these signals. After passing the inspection, the wires will be fixed and the rest signals will be automatically wired.

Due to the existence of impedance in the ground line, common impedance interference will be brought to the circuit.As a result, any point with a ground symbol cannot be connected at will during wiring, which may result in harmful coupling, affecting the operation of the circuit.

At higher frequencies, the inductance of a wire will be several orders of magnitude greater than the resistance of the wire itself.At this time, 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 is as compact as possible, making the printed wires as short as possible.

There are also mutual inductance and capacitance between printed conductors. When the operating frequency is large, it will interfere with other parts, which is called parasitic coupling interference.The possible forms of inhibition are:

Shorten the signal routing between different levels as far as possible;

Arrange the circuits in the order of the signals to avoid the signal lines crossing each other.

The wires of the adjacent two panels should be perpendicular or cross, not parallel;

When the signal wires should be arranged in parallel in the plate, they should be spaced as far as possible, or separated by land line and power line, so as to achieve the purpose of shielding.

6. Automatic wiring

For the wiring of key signals, consideration should be given to controlling some electrical parameters during wiring, such as reducing the distributed inductance, etc. After understanding which input parameters of automatic wiring tools and the influence of input parameters on wiring, the quality of automatic wiring can be guaranteed to some extent.

General rules should be adopted when automatic wiring of signals.By setting constraints and no-wiring areas to limit the layers used by a given signal and the number of holes used, the wiring tools can be automatically wired according to the engineer's design.After the constraints are set and the rules created are applied, the automatic wiring will achieve the expected results, and after part of the design is completed, it will be fixed to prevent the impact of the later wiring process.

The number of wiring depends on the complexity of the circuit and the number of general rules defined.Today's automatic wiring tools are very powerful and can usually complete 100% wiring.However, if the automatic wiring tool does not complete all signal wiring, manual wiring for the remaining signals is required.

7. Finishing of wiring

For some signals with few constraints, the length of wiring is very long. At this point, we can first determine which wiring is reasonable and which is not. Then, we can shorten the length of signal wiring and reduce the number of holes through manual editing.