PCB Blog - POWER PCB board design specification

1 Overview

The purpose of this document is to explain the PCB board design process and some precautions using PADS' PCB design software PowerPCB, to provide design specifications for designers in a working group, and to facilitate communication and mutual inspection between designers.

2. Design Process

The design process of PCB board 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 SendNetlist, and apply the OLE function to keep the schematic and PCB diagram consistent at any time and minimize the possibility of errors. Another method is to load the netlist directly in the PowerPCB board, select File->Import, and input the netlist generated by the schematic diagram.

2.2 Rule Settings

If the design rules of the PCB board 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 the PowerPCB board along with the netlist. If the design rules are modified, the schematic diagram must be synchronized to ensure that the schematic diagram and the PCB board are consistent. In addition to design rules and layer definitions, there are some rules that need to be set, such as PadStacks, which need to modify the size of standard vias. If the designer creates a new pad or via, be sure to add Layer 25. Note: PCB board design rules, layer definitions, via settings, and CAM output settings have been made into a default startup file named Default.stp. After the netlist is imported, the power network and ground are allocated to the power supply according to the actual design situation. layers and strata, and set other rules. After all the rules are set, in PowerLogic, use the Rules From PCB function of the OLE PowerPCB Connection to update the rule settings in the schematic to ensure that the rules of the schematic and the PCB are consistent.

2.3 Component layout

After the netlist is input, all components will be placed at the zero point of the workspace and overlapped together. The next step is to separate these components and arrange them neatly according to some rules, that is, component layout. PowerPCB board provides two methods, manual layout, and automatic layout.

2.3.1 Manual layout

1) Draw the board outline for the structural size of the tool printed board.

2) Disperse the components (Disperse Components), and the components will be arranged around the edge of the board.

3) Move and rotate the components one by one, put them within the edge of the board, and arrange them neatly according to certain rules.

2.3.2 Auto Layout

PowerPCB board provides automatic layout and automatic partial cluster layout, but for most designs, the effect is not ideal and is not recommended.

2.3.3 Notes

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

2) The digital devices and analog devices should be separated and kept as far away as possible.

3) Place the decoupling capacitor as close as possible to the VCC of the device.

4) Consider future soldering when placing devices, not too dense.

5) Use the Array and Union functions provided by the software to improve the efficiency of the layout.

2.4 Wiring

There are also two ways of wiring, manual wiring, and automatic wiring. The manual routing function provided by the PowerPCB board is very powerful, including automatic pushing and online design rule checking (DRC). The automatic routing is performed by Spectra's routing engine. Usually, these two methods are used together.

2.4.1 Manual wiring

1) Before automatic wiring, manually lay out 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, shielding, etc.; other special packages, Such as BGA, it is difficult to arrange automatic routing in a regular manner, and manual routing is also required.

2) After the automatic wiring, the wiring of the PCB board should be adjusted by manual wiring.

2.4.2 Auto-routing

After the manual routing is over, the rest of the network is handed over to the autorouter. Select Tools->SPECCTRA, start the interface of the Specctra router, set the DO file, and press Continue to start the automatic wiring of the Specctra router. After the end, if the routing rate is 100%, then you can manually adjust the wiring; if not When it reaches 100%, it means that there is a problem with the layout or manual routing, and the layout or manual routing needs to be adjusted until all the layouts are completed.

2.4.3 Notes

1) Make the power and ground wires as thick as possible.

2) The decoupling capacitor should be directly connected to VCC as much as possible.

3) When setting the DO file of Specctra, first add the Protect all wires command to protect the manually routed wires from being rerouted by the autoroute.

4) If there is a mixed power supply layer, this layer should be defined as a Split/mixed Plane, and it should be divided before wiring. After wiring, use PourManager's Plane Connect for copper cladding.

5) Set all device pins to thermal pad mode. The method is to set Filter to Pins, select all pins, modify the properties, and tick the Thermal option.

6) Turn on the DRC option during manual routing and use Dynamic Route.

2.5 Inspection

The items to be checked are Clearance, Connectivity, HighSpeed, and Plane. These items can be selected from Tools->VerifyDesign. If a high-speed rule is set, it must be checked, otherwise, this item can be skipped. Errors are detected and placement and routing must be modified. Note: Some errors can be ignored. For example, part of the Outline of some connectors is placed outside the board frame, and errors will occur when checking the spacing; in addition, after each modification of the traces and vias, the copper must be re-clad.

2.6 Review

The review is based on the "PCB Board Checklist", which includes design rules, layer definition, line width, spacing, pads, and via settings; it is also important to review the rationality of device layout, power supply, ground network routing, and high-speed clocks Network routing and shielding, placement and connection of decoupling capacitors, etc. If the review fails, the designer should revise the layout and wiring. After passing the review, the reviewer and the designer sign respectively.

2.7 Design Output

The PCB board design can be output to a printer or output as a light drawing file. The printer can print the PCB board in layers, which is convenient for designers and reviewers to check; the light drawing file is handed over to the board manufacturer to produce the printed board. The output of light drawing files is very important, which is related to the success or failure of this design. The following will focus on the precautions for outputting light drawing files.

1) The layers that need to be output are the wiring layer (including top layer, the bottom layer, and middle wiring layer), power layer (including VCC layer and GND layer), silk screen layer (including top layer silk screen, bottom layer silk screen), solder mask layer (including top layer solder mask) and bottom solder mask), in addition to generating a drill file (NCDrill).

2) If the power layer is set to Split/Mixed, then select Routing in the Document item of the Add Document window, and use Pour Manager's Plane Connect to copper-clad the PCB board diagram before each output of the light drawing file; if set to CAM.

3) Plane, select Plane. When setting the Layer item, add Layer25, and select Pads and Vias in Layer25. c. In the device setting window (press Device Setup), change the value of Aperture to 199.

4) When setting the Layer of each layer, select Board Outline

5) When setting the Layer of the silkscreen layer, do not select Part Type, select the top layer (bottom layer) and Outline, Text, and Line of the silkscreen layer

6) When setting the Layer of the solder mask layer, select the via hole to indicate that no solder mask is added to the via hole, and not select the via hole to indicate the home solder mask, which is determined according to the specific situation.

7) When generating the drilling file, use the default settings of the PowerPCB board and do not make any changes

8) After all the light drawing files are output, open and print with CAM350, and checked by designers and reviewers according to the "PCB Board Checklist".