PCB Technical - Do you know the PCB design process?

1. Design process

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

1.1 Netlist input

There are two ways to enter the netlist. One is to use PowerLogic's OLE PowerPCB Connection function, select Send Netlist, and use 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.

1.2 Rule settings

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 imported 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, be sure to add Layer 25.

Notice:

PCB layout and design rules, layer definitions, via settings, and CAM output settings have been made into the default startup file. After the netlist is input, the power network and ground are assigned to the power layer according to the actual situation of the design. And stratum, and set other advanced rules. All the rules are set up. In PowerLogic, use the Rules From PCB function of OLEPowerPCB Connection to update the rule settings in the schematic to ensure that the rules of the schematic and PCB are consistent.

1.3 Component layout

After the netlist is entered, all the 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, that is, the component layout. PowerPCB provides two methods, Manual layout and automatic layout.

2 Manual layout

2.1. Draw the board outline for the structural dimensions of the printed board of the tool.

2.2 Disperse Components, the components will be arranged around the edge of the board.

2.3 Move and rotate the components one by one, put them inside the edge of the board, and place them neatly according to certain rules.

3. Auto Layout

PowerPCB provides automatic layout and automatic local cluster layout, but for most designs, the effect is not ideal and it is not recommended. 1.3.3 Precautions a. The first principle of layout is to ensure the routing rate and move Pay attention to the connection of the flying leads when connecting the device, put the connected devices together b. Separate digital devices and analog devices as far as possible. c. Keep the decoupling capacitors as close as possible to the VCC of the device.

d. When placing the device, consider future soldering, not too dense

e. Use the Array and Union functions provided by the software more to improve layout efficiency. 1.4 There are also two ways of wiring and 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 carried out by Specctra's wiring engine. Usually these two methods are used together. The common steps are manual-automatic-manual.

4. Manual wiring

4.1 Before automatic wiring, first hand-lay some important networks, such as high-frequency clocks, main power supply, etc., these networks often have special requirements for wiring distance, line width, line spacing, shielding, etc.; in addition, some special packages, such as For BGA, automatic wiring is difficult to arrange regularly, and manual wiring must be used.

4.2 After automatic wiring, manual wiring must be used to adjust the PCB wiring.

4.3 Automatic wiring

After the manual wiring is over, the remaining network is handed over to the automatic router for cloth. Select Tools-》SPECCTRA, start the Specctra router interface, set the DO file, and press Continue to start the Specctra router automatic wiring. After the end, if the wiring rate is 100, then you can manually adjust the wiring; if not 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.

5. 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 up the Specctra DO file, first add the Protect all wires command to protect the manually routed wires from being rerouted by the auto-router. d. If there is a mixed power layer, the layer should be defined as Split/mixed Plane, and the After dividing and wiring, use Plane Connect of Pour Manager for copper pour. e. Set all device pins to thermal pad mode by setting Filter to Pins, select all pins, modify properties, and click Tick the Thermal option f. Turn on the DRC option when manually routing, use Dynamic Route 1.5 Check the items to be checked include Clearance, Connectivity, High Speed and Plane ), these projects can be carried out by choosing Tools-》Verify Design. If the high-speed rule is set, it must be checked, otherwise you can skip this item. If errors are detected, the layout and wiring must be modified.

Notice:

Some errors can be ignored. For example, a part of the outline of some connectors is placed outside the board frame, and errors will occur when checking the spacing; in addition, each time the traces and vias are modified, the copper must be re-coated.

6. Review

PCB manufacturers review according to the "PCB checklist", which includes design rules, layer definitions, line widths, spacing, pads, and via settings; also focus on reviewing the rationality of the device layout, the routing of power and ground networks, and high-speed The routing and shielding of the clock network, the placement and connection of decoupling capacitors, etc. If the recheck is unqualified, the designer shall modify the layout and wiring. After passing, the rechecker and the designer shall sign separately.