Business Tips: Ingenious Techniques and Process Defects in PCB Layout

PCB (PrintedCircuitBoard), Chinese name is printed circuit board, also known as printed circuit board, printed circuit board, is an important electronic component, is the support of electronic components, is the provider of electrical connections of electronic components. Because it is made by electronic printing, it is called a "printing" circuit board.

As PCB size requirements become smaller and smaller, device density requirements become higher and higher, and PCB design becomes more and more difficult. How to achieve high PCB layout and shorten design time, I will talk about the design skills of PCB planning, layout and wiring.

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

1 Determine the number of layers of the PCB

Board size and number of wiring layers need to be determined early in the design process. The number of wiring layers and the STack-up method directly affect the wiring and impedance of the printed wiring. The size of the board helps determine the stacking and line width to achieve the desired design. At present, the cost difference between the multi-layer boards is very small, and it is better to use more circuit layers and evenly distribute the copper at the beginning of the 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 special required signal lines, each signal class should have priority. The higher the priority, the stricter the rules. The rules relate to the width of the trace, the maximum number of vias, the parallelism, the interaction between the signal lines, and the limitations of the layers. These rules have a large impact on the performance of the routing tool.

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

3 component layout

In the optimal assembly process, the manufacturability design (DFM) rules impose restrictions on the component layout. If the assembly department allows the components to move, the circuit can be properly optimized for easier automatic routing. The defined rules and constraints affect the layout design. The automatic routing tool only considers one signal at a time. By setting the wiring constraints and setting the layers of the signal lines, the routing tool can be routed as the designer envisions.

For example, for the layout of the power cord:

1 In the PCB layout, the power supply decoupling circuit should be designed near the relevant circuits, and should not be placed in the power supply. Otherwise, the bypass effect will be affected, and the ripple current will flow on the power line and the ground line, causing nuisance;

2 For the power supply inside the circuit, the power supply from the last stage should be taken, and the power supply filter capacitor of this part should be arranged near the last stage;

3 For some major current channels, such as disconnecting or measuring current during commissioning and testing, current gaps should be placed on the printed conductors during layout.

In addition, it should be noted that the regulated power supply is arranged on a separate printed board as much as possible. When the power supply and the circuit are used together with the printed circuit board, in the layout, it should be avoided that the regulated power supply is mixed with the circuit components or the power supply and the circuit are used together.

Because this kind of wiring is not only easy to cause interference, but also can not break the load during maintenance, only a part of the printed wiring can be cut at that time, thereby damaging the printed board.

4 fanout design

In the fan-out design phase, each surface of the surface mount device should have at least one via so that the board can perform internal connections, in-circuit testing, and circuit reprocessing when more connections are required.

In order to maximize the efficiency of the automatic routing tool, it is important to use the largest via size and trace as much as possible, with an interval of 50 mils. Use a via type that maximizes the number of routing paths. After careful consideration and prediction, the design of the circuit online test can be carried out at the beginning of the design and later in the production process.

The type of via fanout is determined by wiring paths and circuit in-circuit tests. Power and ground also affect the routing and fanout design.

5 manual wiring and processing of key signals

Manual routing is an important process for printed circuit board design now and in the future. Manual routing helps automated routing tools complete wiring.

By manually routing and fixing the selected network (net), a path that can be relied upon for automatic routing can be formed.

The key signals are routed first, either manually or in combination with an automated routing tool. After the wiring is completed, the relevant engineering technicians check the signal wiring. After the inspection is passed, the wires are fixed and then the remaining signals are automatically routed.

Due to the presence of impedance in the ground, it can cause common impedance interference to the circuit. Therefore, it is not possible to connect any point with a ground symbol at the time of wiring, which may cause harmful coupling and affect the operation of the circuit.

At higher frequencies, the inductive reactance of the wire will be orders of magnitude greater than the resistance of the wire itself. At this time, even if only a small high-frequency current flows through the wire, a certain high-frequency voltage drop is generated. Therefore, for high-frequency circuits, the PCB layout is as compact as possible, making the printed conductors as short as possible.

There is also mutual inductance and capacitance between the printed conductors. When the operating frequency is large, it will cause interference to other parts, which is called parasitic coupling interference. The suppression methods that can be taken are:

1 Try to shorten the signal routing between the levels;

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

3 adjacent two panels of wires should be vertical or intersecting, not parallel;

4 When the signal wires are to be laid in parallel in the board, they should be separated as far as possible by a certain distance, or separated by ground and power lines 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 the automatic wiring tool and the influence of the input parameters on the wiring, the quality of the automatic wiring can be obtained to some extent. Guarantee.

General rules should be used when autorouting signals. By setting constraints and disabling the routing area to define the layers used for a given signal and the number of vias used, the routing tool can be automatically routed according to the engineer's design philosophy. After the constraints and the rules created by the application are set, the automatic routing will achieve similar results as expected. After some of the design is completed, it will be fixed to prevent it from being affected by the rear wiring process.

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

7 wiring finishing

Some signals with few constraints, the length of the wiring is very long, then you can first determine which wiring is reasonable, which wiring is not reasonable, and then manually edit to shorten the signal wiring length and reduce the number of vias.