PCB News - Don't go wrong in the small and small steps in PCB circuit board design

Generally speaking, the most basic process of designing a circuit board can be divided into the following steps.

(1) The design of the circuit schematic diagram: The design of the circuit schematic diagram is mainly based on PROTEL099's schematic design system (AdvancedSchematic) to draw a circuit schematic diagram. In this process, we must make full use of the various schematic drawing tools and various editing functions provided by PROTEL99 to achieve our goal, that is, to obtain a correct and exquisite schematic circuit diagram.

(2) Generate netlist: netlist is a bridge between circuit schematic design (SCH) and PCB design in PCB factory(PCB), and it is the soul of circuit board automation. The netlist can be obtained from the schematic circuit diagram or extracted from the printed circuit board.

(3) The design of the printed circuit board: The design of the printed circuit board is mainly for another important part of the PCB of PROTEL99. In this process, we use the powerful functions provided by PROTEL99 to realize the layout design of the circuit board. Complete difficult tasks.

However, in practice, the specific steps are mainly based on the following subdivision steps:

1. Preliminary work of PCB design 1. Use the schematic design tool to draw the schematic diagram and generate the corresponding network table. Of course, in some special cases, such as the circuit board is relatively simple, and there is already a network table, etc., it is not necessary to design the schematic diagram and directly enter the PCB design system. In the PCB design system, you can directly use the parts and packaging. Generate a netlist. 2. Manually change the network table to define the pads that are not on the schematic diagram such as the fixed pins of some components to the network connected to it, and the ground or protective ground without any physical connection can be defined. Change the pin names of some devices with inconsistent pin names in the schematic diagram and PCB board package library to be consistent with those in the PCB package library, especially diodes and transistors.

2. Draw the package library of non-standard devices defined by yourself. It is recommended to put all the devices you draw into a special design file for PCB library that you have created.

3. Set the PCB design environment and draw the printed circuit layout frame with a hollow in the middle, etc. 1. The first step after entering the PCB system is to set the PCB design environment, including setting the grid size and type, cursor type, layout parameters, and wiring Parameters and so on. Most of the parameters can use the system default values, and after these parameters are set, they are in line with personal habits and do not need to be modified in the future. 2. Planning the circuit board is mainly to determine the frame of the circuit board, including the size of the circuit board and so on. Put an appropriate size pad on the place where the fixing hole needs to be placed. For 3mm screws, 6.5~8mm outer diameter and 3.2~3.5mm inner diameter pads can be used. For standard boards, they can be imported from other boards or PCBizard. Note-Before drawing the border of the circuit board, the current layer must be set to the KeepOut layer, that is, the wiring layer is prohibited.

4. After opening all the PCB library files to be used, the step of importing the netlist file and modifying the part packaging is a very important step. The netlist is the soul of PCB automatic routing, and it is also the design of the schematic diagram and the impression circuit board. Interface, the wiring of the circuit board can only be carried out after the netlist is loaded. In the process of schematic design, ERC inspection will not involve the packaging of parts. Therefore, when designing the schematic diagram, the packaging of the parts may be forgotten. When the netlist is introduced, the packaging of the parts can be modified or supplemented according to the design situation. Of course, you can manually generate the netlist directly in the PCB, and specify the part package.

5. Arrange the location of the parts package, also known as the part layout. Protel99 can carry out automatic layout or manual layout. For automatic layout, run "AutoPlace" under "Tools". With this command, you need to have enough patience. The key to routing is layout, and most designers use manual layout. Use the mouse to select a component, hold down the left mouse button, drag the component to the destination, release the left button to fix the component. Protel99 has added some new skills in the layout. New interactive layout options include automatic selection and automatic alignment. Using the automatic selection method can quickly collect similar packaged components, and then rotate, unfold and organize them into groups, and then they can be moved to the desired position on the board. When the simple layout is completed, use automatic alignment to neatly expand or shrink a group of similar components. Tip-In automatic selection, use ShiftX or Y and CtrlX or Y to expand and contract the X and Y directions of the selected component. Note-The layout of parts should be comprehensively considered from the aspects of mechanical structure heat dissipation, electromagnetic interference, and the convenience of future wiring. First arrange the devices related to the mechanical size, and lock these devices, then the large position-occupying devices and the core components of the circuit, and then the peripheral small components.

6. Make appropriate adjustments according to the situation and then lock all the devices. If the space on the board allows, you can put some wiring areas similar to the experiment board on the board. For large boards, add more fixing screw holes in the middle. There are heavy components or large connectors on the board and other stressed components should also be fixed screw holes. If necessary, some test pads can be placed in appropriate positions. It is best to add them in the schematic diagram. Increase the size of the via hole of the pad that is too small, and define the network of all the fixed screw hole pads to the ground or the protective ground. After placing it, use the VIEW3D function to check the actual effect and save it.

7. Wiring rules setting Wiring rules are to set the various specifications of wiring (such as the use level, line width of each group, via spacing, wiring topology and other partial rules, which can be exported from other boards through the Menu of Design-Rules, Import this board again) This step does not have to be set every time, just set it once according to personal habits.