PCB Technical - Common rules for PCB circuit board design LAYOUT wiring

Novices often do not understand some of the rules of PCB LAYOUT in PCB design, which often leads to serious production accidents in the designed PCB. Let's introduce the common rules of PCB layout design.

Common rules for PCB circuit board design LAYOUT wiring

1 The processing of the power supply and ground wire makes the wiring in the entire PCB board very well completed, but the interference caused by the improper consideration of the power supply and ground wire will reduce the performance of the product, and sometimes even affect the product's performance. Success rate. Therefore, the wiring of the electric and ground wires must be taken seriously, and the noise interference generated by the electric and ground wires should be minimized to ensure the quality of the product.

2. Common ground processing of digital circuits and analog circuits Nowadays, many PCBs are no longer single-function circuits (digital or analog circuits), but are composed of a mixture of digital and analog circuits. Therefore, it is necessary to consider the mutual interference between them when wiring, especially the noise interference on the ground wire. The frequency of the digital circuit is high, and the sensitivity of the analog circuit is strong. For the signal line, the high-frequency signal line should be as far away as possible from the sensitive analog circuit device. For the ground line, the whole PCB has only one node to the outside world, so The problem of digital and analog common ground must be dealt with inside the PCB, and the digital ground and analog ground inside the board are actually separate, they are not connected to each other, but only at the interface (such as plugs, etc.) connecting the PCB to the outside world. There is a short connection between the digital ground and the analog ground. Please note that there is only one connection point. There are also non-common grounds on the PCB, which is determined by the system design.

3. When the signal line is laid on the electrical (ground) layer and the multi-layer printed board is laid out, there are not many wires left in the signal line layer that have not been laid out, and adding more layers will cause waste and increase production. For a certain amount of work, the cost has increased accordingly. To solve this contradiction, you can consider wiring on the electrical (ground) layer. The power plane should be considered first, and the ground plane second. Because it is best to preserve the integrity of the formation.

4. Treatment of connecting legs in large-area conductors. In large-area grounding (electricity), the legs of commonly used components are connected to it. The treatment of connecting legs requires comprehensive consideration. In terms of electrical performance, the pads of component legs It is better to be fully connected to the copper surface, but there are some undesirable hidden dangers in the welding and assembly of the components, such as: 1. Welding requires a high-power heater. 2. It is easy to cause virtual solder joints. Therefore, both electrical performance and process requirements are made into cross-patterned pads, which are called heat shields, commonly known as thermal pads. In this way, virtual solder joints may be generated due to excessive cross-section heat during soldering. Sex is greatly reduced. The processing of the power (ground) leg of the multilayer board is the same.

5. The role of the network system in PCB wiring In many CAD systems, the wiring is determined based on the network system. The grid is too dense and the path has increased, but the step is too small, and the amount of data in the field is too large. This will inevitably have higher requirements for the storage space of the device, and also the computing speed of the computer-based electronic products. Great influence. Some vias are ineffective, such as those occupied by the pads of the component legs or by mounting holes or fixed holes. Too sparse grids and too few channels have a great impact on the distribution rate.

2. Design process: The PCB design process is divided into six steps: netlist input, rule setting, component layout, wiring, inspection, review, and output.

2.1 Netlist input There are two methods for netlist input. One is to use PowerLogic's OLE PowerPCB Connection function, select Send Netlist, and apply the OLE function to keep the schematic diagram and PCB diagram consistent at any time to minimize the possibility of errors. Another method is to load the netlist directly in PowerPCB, select File->Import, and input the netlist generated by the schematic diagram.

2.2 Rule setting If the PCB design rules have been set in the schematic design stage, there is no need to set these rules, because when the netlist is input, the design rules have been entered into PowerPCB along with the netlist. If the design rules are modified, the schematic diagram must be synchronized to ensure that the schematic diagram is consistent with the PCB. In addition to the design rules and layer definitions, there are also some rules that need to be set, such as Pad Stacks, which need to modify the size of the standard vias. If the designer creates a new pad or via, you must add Layer

2.3 Component layout After the netlist is entered, all components will be placed at the zero point of the work area and overlapped together. The next step is to separate these components and arrange them neatly according to some rules, namely component layout. PowerPCB provides two methods, manual layout and automatic layout.

2.3.1 Manual layout 1. Draw the board outline for the structure size of the printed board with the tool. 2. Disperse the components (Disperse Components), the components will be arranged around the edge of the board. 3. Move and rotate the components one by one, put them inside the edge of the board, and place them neatly according to certain rules.

2.3.2 Automatic layout PowerPCB provides automatic layout and automatic local cluster layout, but for most designs, the effect is not ideal and it is not recommended.

2.3.3 Matters needing attention

a. The first principle of the layout is to ensure the wiring rate, pay attention to the connection of the flying leads when moving the device, and put the connected devices together

b. Separate digital devices from analog devices and keep them as far away as possible

c. The decoupling capacitor is as close as possible to the VCC of the device

d. Consider future soldering when placing the device, not too dense

e. Use the Array and Union functions provided by the software more to improve layout efficiency

2.4 Wiring There are also two ways of wiring, manual wiring and automatic wiring. The manual wiring function provided by PowerPCB is very powerful, including automatic pushing and online design rule checking (DRC). Automatic wiring is performed by Specctra's wiring engine. Usually these two methods are used together. The common steps are manual-automatic-manual.

2.4.1 Manual wiring

1. Before automatic wiring, first hand-lay some important networks, such as high-frequency clocks, main power supplies, etc. These networks often have special requirements for wiring distance, line width, line spacing, and shielding; in addition, some special packaging, Such as BGA, it is difficult to arrange automatic wiring regularly, and manual wiring must be used. 2. After the automatic routing, the PCB routing needs to be adjusted by manual routing.

2.4.2 Automatic routing After the manual routing is completed, the remaining network will be handed over to the automatic routing device for routing. Select Tools->SPECCTRA, start the Specctra router interface, set the DO file, and press Continue to start the Specctra router automatic wiring. After the completion, if the wiring rate is 100%, then you can manually adjust the wiring; if not If it reaches 100%, it indicates that there is a problem with the layout or manual wiring, and the layout or manual wiring needs to be adjusted until all the connections are made.

2.4.3 Matters needing attention

a. Make the power cord and ground wire as thick as possible

b. Try to connect the decoupling capacitor directly to VCC

c. When setting the DO file of Specctra, first add the Protect all wires command to protect the manually clothed wires from being redistributed by the automatic router

d. If there is a mixed power layer, you should define the layer as Split/mixed Plane, divide it before wiring, and after wiring, use Pour Manager's Plane Connect for copper pour

e. Set all device pins to thermal pad mode. The practice is to set Filter to Pins, select all pins, modify the properties, and tick the Thermal option. f. Turn on the DRC option during manual wiring and use dynamic Wiring (Dynamic Route)

2.5 Inspection The items to be inspected include Clearance, Connectivity, High Speed and Plane. These items can be selected by Tools->Verify Design. If the high-speed rule is set, it must be checked, otherwise you can skip this item. If an error is detected, the PCB layout and wiring must be modified.