PCB Technical - PCB process drawing pcb board steps

PCB process drawing PCB board steps

1. Preliminary work of circuit board design
1. Use the schematic design tool to draw the schematic and generate the corresponding network table. Of course, in some special cases, such as the circuit board is relatively simple, 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. 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 define the ground or protective ground without any physical connection. Change the pin names of some devices with inconsistent pin names in the schematic diagram and PCB 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 components you have drawn into a special design file for PCB library that you have created.

Three, set up the PCB design environment and draw the printed circuit board with a hollow in the middle, etc.
1. The first step after entering the PCB system is to set up the PCB design environment, including setting the grid size and type, cursor type, layout parameters, 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 PCB izards.
Note: Before drawing the border of the circuit board, the current layer must be set to the Keep Out layer, that is, the wiring layer is prohibited.

4. After opening all the PCB library files to be used, it is a very important step to import the netlist file and modify the part packaging. The netlist is the soul of PCB automatic routing, and it is also the design of schematic diagram and 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 "Auto Place" under "Tools". You need to be patient with this command. 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: During automatic selection, use Shift+X or Y and Ctrl+X 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 devices
If the board space permits, some wiring areas similar to the experiment board can be placed on the board. For large boards, add more fixing screw holes in the middle. There are heavy components or larger 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 various rules of wiring (such as the use level, line width of each group, via spacing, wiring topology and other 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. Choosing Design-Rules generally requires resetting the following points:
1. Safety distance (Clearance Constraint of Routing label)
It specifies the distance that must be maintained between the trace pad vias of different networks on the board. Generally the board can be set to 0.254mm, the empty board can be set to 0.3mm, and the denser patch board can be set to 0.2-0.22mm. The production capacity of very few printed board manufacturers is 0.1-0.15mm. You can set this value with their consent. Below 0.1mm is absolutely prohibited.

2. Routing layer and direction (Routing Layers of the Routing tab)
Here you can set the routing layer used and the main routing direction of each layer. Please note that the single panel of the patch only uses the top layer, and the in-line single panel only uses the bottom layer, but the power layer of the multilayer board is not set here (you can click the top layer or the bottom layer in the Design-Layer Stack Manager, and use Add Plane, double click with the left mouse button to set, click on the layer and then delete it with Delete), the mechanical layer is not set here (you can select the mechanical layer to be used in Design-Mechanical Layer, and select whether it is available Depends on whether and is displayed in single-layer display mode at the same time). Mechanical layer 1 is generally used for the frame of the drawing board; mechanical layer 3 is generally used for mechanical structural parts such as bars on the drawing board; mechanical layer 4 is generally used for drawing rulers and annotations, etc., specifically you can export a PCAT in the PCB Wizard. Take a look at the structure of the board
3. Via shape (Routing Via Style of the Routing label)
It specifies the inner and outer diameters of the vias that are automatically generated during manual and automatic wiring, which are divided into minimum, maximum and preferred values, of which the preferred value is the most important, the same below.

4. Route width (Width Constraint of Routing tab)
It specifies the width of traces during manual and automatic routing. The preference of the entire board range is generally 0.2-0.6mm, and some network or net class line width settings are added, such as ground wire, +5 volt power line, AC power input line, power output line and power pack Wait. The network group can be defined in the Design-Netlist Manager in advance. The ground wire generally has a width of 1mm, and various power cables generally have a width of 0.5-1mm. The relationship between the line width and the current on the printed board is about 1 per millimeter of line width. For the current of ampere, please refer to the relevant information for details. When the preferred value of the wire diameter is too large so that the SMD pad cannot be routed through the automatic routing, it will automatically shrink to a section of trace between the minimum width and the width of the pad when it enters the SMD pad, where the Board is for the entire The line width constraint of the board has the lowest priority, that is, the line width constraint conditions of the network and the network group are first met when wiring. The picture below is an example

5. Setting of copper connection shape (Polygon Connect Style of Manufacturing tab)
It is recommended that the Conductor Width of the Relief Connect method be 0.3-0.5mm for 4 wires of 45 or 90 degrees. The rest of the items can generally use its original default values, and items such as the topology of the wiring, the spacing of the power layer and the length of the network matching the shape of the connection can be set as needed. Select Tools-Preferences, and select Push Obstacle in Interactive Routing in the Options column (push other routes when encountering a route of a different network, Ignore Obstacle is to pass through, Avoid Obstacle is to block) mode and select Automatically Remove (automatically remove) Delete redundant traces). The Track and Via in the Defaults column can also be changed. Generally, there is no need to move them. Place the FILL filling layer in the area where the wiring is not desired, such as the wiring layer under the radiator and the lying two-pin crystal oscillator, and put the FILL in the corresponding place of the Top or Bottom Solder for tinning. Wiring rule setting is also one of the keys to the design of printed circuit boards, which requires a wealth of practical experience.