What is the design flow of PCB design?

PCB design Flow

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

1.Netlist input

There are two methods for netlist input. One is to use PowerLogic’s OLE PowerPCB Connection function. Select,Send Netlist, using OLE function, can keep the schematic diagram and PCB diagram consistent at any time, minimizing the possibility of errors.Another method is to load the netlist directly in PowerPCB, select File->Import, and import the netlist generated from the schematic diagram.

2. Rule settings

If the PCB design rules have been set during the schematic design stage, there is no need to set these rules again, because when the netlist is entered, the design rules have already been entered into PowerPCB along with the netlist. If the design rules are modified, the schematic must be synchronized to ensure that the schematic and PCB are consistent. In addition to design rules and layer definitions, there are also some rules that need to be set, such as Pad Stacks, which require modifying the size of standard vias. If the designer creates a new pad or via, Layer 25 must be added.
Notice:
PCB 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 input, the power network and ground are assigned to the power layer and ground layer according to the actual design conditions. , and set other advanced rules. 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 PCB diagram are consistent.

3. Component layout

3.1 After the netlist is input, all components will be placed at the zero point of the work area and overlapped. The next step is to separate these components and place them neatly according to some rules, that is, component layout. PowerPCB provides two methods, manual layout and automatic layout. 2.3.1 Manual layout

1. Draw the board outline based on the structural dimensions of the tool printed board.
2. Disperse Components, and the components will be arranged around the edge of the board.
3. Move and rotate the components one by one, place them within the edge of the board, and arrange them neatly according to certain rules.

3.2 Automatic layout
PowerPCB provides automatic layout and automatic partial cluster layout, but for most designs, the effect is not ideal and is not recommended. 2.3.3 Precautions


a. The first principle of layout is to ensure the routing rate of wiring. When moving devices, pay attention to the connection of flying wires and put connected devices together.
b. Digital devices and analog devices should be separated and kept as far away as possible
c. Place the decoupling capacitor as close to the VCC of the device as possible
d. When placing devices, consider future welding and do not place them too densely.
e. Make more use of the Array and Union functions provided by the software to improve layout efficiency.

4. Cabling

There are also two ways of wiring, manual wiring and automatic wiring. The manual wiring functions provided by PowerPCB are very powerful, including automatic pushing and online design rule checking (DRC). Automatic wiring is performed by Specctra's wiring engine. Usually these two methods are used together. The common steps are manual-automatic-manual.

4.1 Manual wiring
1. Before automatic wiring, some important networks should be laid out manually, such as high-frequency clocks, main power supplies, etc. These networks often have special requirements for trace distance, line width, line spacing, shielding, etc.; other special packages, For example, BGA, automatic wiring is difficult to lay out regularly, and manual wiring is also required.
2. After automatic wiring, manual wiring must be used to adjust the PCB routing.

4.2 Automatic routing
After the manual wiring is completed, the remaining network is handed over to the automatic router for routing. 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 completion, if the routing rate is 100, then you can manually adjust the wiring; if it is not 100, indicating that there is a problem with the layout or manual wiring, and the layout or manual wiring needs to be adjusted until everything is cleared.

4.3 Precautions
a. Make the power cord and ground wire as thick as possible
b. The decoupling capacitor should be directly connected to VCC as much as possible.
c. When setting up the Specctra DO file, first add the Protect all wires command to protect the manually laid lines from being re-routed by the automatic router.
d. If there is a mixed power layer, this layer should be defined as Split/mixed Plane. Split it before routing. After routing, use Pour Manager's Plane Connect for copper pouring.
e. Set all device pins to thermal pad mode by setting Filter to Pins, select all pins, modify the properties, and check the Thermal option.
f. When manually routing, turn on the DRC option and use dynamic routing (Dynamic Route)

5. Inspection

The items to be checked include Clearance, Connectivity, High Speed and Plane. You can select Tools->Verify Design for these items. If a high-speed rule is set, it must be checked, otherwise this item can be skipped. Errors are detected and the layout and routing must be modified.

Notice:
Some errors can be ignored. For example, part of the Outline of some connectors is placed outside the board frame, which will cause errors when checking the spacing. In addition, every time the traces and vias are modified, copper must be refilled.

6. Review

The review is based on the "PCB Checklist", which includes design rules, layer definitions, line widths, spacing, pads, and via settings; it is also important to review the rationality of device layout, routing of power and ground networks, and high-speed clock networks. The wiring and shielding, the placement and connection of decoupling capacitors, etc. If the review fails, the designer will modify the layout and wiring. After passing the review, the reviewer and designer will sign separately.

7. Design output

PCB designs can be output to a printer or output as a light drawing file. The printer can print the PCB in layers for easy inspection by designers and reviewers; the light drawing file is handed over to the board manufacturer to produce printed boards. The output of light painting files is very important and affects the success or failure of this design. The following will focus on the precautions for outputting light painting files.

a. The layers that need to be output include the wiring layer (including the top layer, the bottom layer, and the middle wiring layer), the power layer (including the VCC layer and the GND layer), the silk screen layer (including the top screen screen, the bottom screen screen), and the solder mask layer (including the top solder mask). and bottom solder mask), and also generate drilling files (NC Drill)

b. If the power layer is set to Split/Mixed, then select Routing in the Document item of the AddDocument window, and each time before outputting the light drawing file, use PourManager's PlaneConnect for copper pouring on the PCB diagram; if it is set to CAMPlane, select Plane, when setting the Layer item, add Layer25, select Pads and Viasc in the Layer25 layer. In the device settings window (press Device Setup), change the value of Aperture to 199

c. When setting the Layer of each layer, select Board Outline

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

e. When setting the Layer of the solder mask layer, selecting the via hole means no solder mask is added to the via hole, and not selecting the via hole means adding solder mask. It depends on the specific situation.

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

g. After all light drawing files are output, open and print them with CAM350. The designer and reviewer will check according to the "PCB Checklist". Vias (via) are one of the important components of multi-layer PCB. The cost of drilling usually accounts for PCB board manufacturing costs 30 to 40. Simply put, every hole on the PCB can be called a via. From a functional point of view, vias can be divided into two categories: one is used for electrical connections between layers; the other is used for fixing or positioning devices. From a process perspective, these vias are generally divided into three categories, namely blind vias, buried vias and through vias. Blind holes are located on the top and bottom surfaces of the printed circuit board and have a certain depth. They are used to connect the surface circuits and the inner circuits below. The depth of the holes usually does not exceed a certain ratio (aperture). Buried vias refer to connection holes located on the inner layer of a printed circuit board and do not extend to the surface of the circuit board. The above two types of holes are located in the inner layer of the circuit. They are completed using the through-hole forming process before lamination. During the via hole formation process, several inner layers may be overlapped. The third type is called a through hole, which passes through the entire circuit board and can be used to implement internal interconnections or as mounting positioning holes for components. Because through holes are easier to implement in technology and have lower costs, most printed circuit boards use them instead of the other two via holes. The following via holes are considered as through holes unless otherwise specified.


From a design point of view, a via hole mainly consists of two parts, one is the drill hole in the middle, and the other is the pad area around the drill hole, as shown in the figure below. The size of these two parts determines the size of the via. Obviously, when designing high-speed, high-density PCBs, designers always hope that the vias should be as small as possible, so that more wiring space can be left on the board. In addition, the smaller the vias, the smaller their own parasitic capacitance. The smaller it is, the more suitable it is for high-speed circuits. However, the reduction in hole size also brings about an increase in cost, and the size of the via hole cannot be reduced indefinitely. It is limited by process technologies such as drilling (drill) and electroplating (plating): the smaller the hole, the harder it is to drill. The longer the hole takes, the easier it is to deviate from the center; and when the depth of the hole exceeds 6 times the drill diameter, there is no guarantee that the hole wall will be evenly plated with copper. For example, the current normal thickness (through hole depth) of a 6-layer PCB board is about 50 Mil, so the minimum drilling diameter that the PCB manufacturer can provide can only reach 8 Mil.

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