Shulou(Shulou.com)06/01 Report--

In this issue, the editor will bring you about how to use PADS for PCB design. The article is rich in content and analyzes and narrates it from a professional point of view. I hope you can get something after reading this article.

In the process of using PADS for PCB design, we need to focus on the PCB design process and related matters needing attention, so as to better provide systematic design specifications for designers in the working group, and at the same time facilitate designers to communicate and check each other.

02

Design process

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

2.1 Netlist input

There are two ways to input Netlist, one is to use the OLE PowerPCB ConnecTIon function of PowerLogic, select Send Netlist and apply OLE function, which can keep the schematic diagram and PCB diagram consistent at any time and minimize the possibility of error.

Another method is to load the Netlist directly in PowerPCB, select File- > Import, and enter the Netlist generated by the schematic.

2.2 Rule Settings

If the PCB design rules have been set in the schematic design phase, there is no need to set these rules, because when entering the Netlist, the design rules have been entered into the PowerPCB with the Netlist. If you modify the design rules, you must synchronize the schematics to ensure that the schematics are consistent with the PCB. In addition to the design rules and layer definitions, there are some rules that need to be set, such as Pad Stacks, and the size of the standard via needs to be modified. If the designer has created a new pad or through hole, be sure to add Layer 25.

Note:

PCB design rules, layer definition, via settings, CAM output settings have been made into the default startup file, named Default.stp. After the Netlist is input, according to the actual situation of the design, the power network and ground are assigned to the power layer and stratum, and other advanced rules are set. After all the rules are set, in PowerLogic, use the Rules From PCB function of OLE PowerPCB ConnecTIon to update the rule settings in the schematic diagram to ensure that the rules of the schematic diagram and the PCB diagram are consistent.

2.3 component layout

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

2.3.1 Manual layout

(1) the structural dimensions of the tool PCB draw the edge of the board (Board Outline).

(2) Disperse Components the components, and they 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 automatic layout

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

2.3.3 considerations

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

b. Digital devices and analog devices should be separated and kept as far away as possible

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

d. Later welding should be considered when placing devices, not too dense.

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

2.4 the way of cabling

It is mainly divided into manual wiring and automatic wiring. The manual routing function provided by PowerPCB is very powerful, including automatic push and online design rule check (DRC). Automatic routing is carried out by Specctra's routing engine. Usually these two methods are used together, and the common step is manual-automatic-manual.

2.4.1 Manual cabling

(1) before automatic routing, manually lay out some important networks, such as high frequency clock, main power supply, etc., these networks are often special for line distance, line width, line spacing, shielding and so on.

Other special packages, such as BGA, are difficult to be routed regularly and have to be routed manually.

(2) after automatic routing, the routing of PCB should be adjusted by manual routing.

2.4.2 automatic routing

After the manual wiring is finished, the rest of the network is handed over to the automatic router for cloth. Select Tools- > SPECCTRA, start the interface of the Specctra router, set the DO file, and press ConTInue to start the automatic routing of the Specctra router. If the routing rate is 100%, you can manually adjust the routing. If it is less than 100%, there is a problem with the layout or manual routing, and you need to adjust the layout or manual routing until it is fully routed.

2.4.3 considerations

a. Thicken the power cord and ground wire as much as possible

b. The decoupling capacitor is directly connected to VCC as far as possible.

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

d. If you have a hybrid power layer, you should define it as Split/mixed Plane, split it before cabling, and use Pour Manager's Plane after cabling

Connect for copper cladding

e. Set all device pins to hot pad mode by setting Filter to Pins, selecting all pins, modifying properties, and ticking before the Thermal option

f. Turn on the DRC option when manual cabling, using dynamic cabling (Dynamic Route)

2.5 items inspected

It mainly includes spacing (Clearance), connectivity (ConnecTIvity), high-speed rule (High Speed) and power layer (Plane). These projects can be carried out by selecting Tools- > Verify Design. If a high-speed rule is set, it must be checked, otherwise this item can be skipped. Errors are detected and the layout and wiring must be modified. Note: some errors can be ignored, for example, a part of the Outline of some connectors is placed outside the plate frame, which will make an error when checking the spacing; in addition, the copper should be clad again every time the wiring and hole are modified.

2.6 Review

According to the "PCB Checklist", the contents include design rules, layer definition, linewidth, spacing, pad, through-hole setting, as well as the rationality of device layout, the wiring of power supply and ground network, the alignment and shielding of high-speed clock network, the placement and connection of decoupling capacitors, etc. If the review is unqualified, the designer should modify the layout and wiring, and after passing the inspection, the reviewer and the designer will sign respectively.

2.7 Design output

PCB designs can be output to a printer or output light drawing file. The printer can print the PCB layer by layer, which can be easily checked by designers and reviewers, and the light drawing documents can be handed over to the board manufacturer to produce the printed board. The output of the light drawing file is very important, which is related to the success or failure of this design. The following will focus on the matters needing attention in the output of the light drawing file.

a. The layers that need to be output are wiring layer (including top layer, bottom layer, middle wiring layer), power layer (including VCC layer and GND layer), screen printing layer (including top layer and bottom layer), solder resistance layer (including top layer and bottom layer), and drilling files (NC Drill).

b. If the power layer is set to Split/Mixed, select Routing in the Document item of the Add Document window, and use Pour Manager's Plane Connect to clad the PCB drawing before each output of the light drawing file; if set to CAM Plane, select Plane, add Layer25 when setting the Layer item, and select Pads and Vias in the Layer25 layer

c. In the device settings window (press Device Setup), change the value of Aperture to 199

d. When setting the Layer for each layer, select Board Outline

e. When setting the Layer of screen printing layer, do not select Part Type, select the Outline, Text, Line of the top layer (bottom layer) and screen printing layer

f. When setting the Layer of the solder resistance layer, selecting the through hole means that there is no resistance welding on the through hole, and not selecting the through hole means home resistance welding, which is determined according to the specific situation.

g. When generating drilling files, use the default settings of PowerPCB and do not make any changes

h. After all the light drawing files are output, they are opened and printed with CAM350 and checked by the designer and reviewer according to the "PCB check list".

The above is the editor for you to share how to use PADS for PCB design, if you happen to have similar doubts, you might as well refer to the above analysis to understand. If you want to know more about it, you are welcome to follow the industry information channel.

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