Precautions for PCB planning
The planning of printed circuit board is based on the circuit schematic diagram to complete the functions required by the circuit planners. The planning of printed circuit board mainly refers to the map planning, which needs to consider various factors such as the layout of external connection, the optimal layout of internal electronic components, the optimal layout of metal wiring and through-hole, electromagnetic maintenance, heat dissipation, etc. Excellent map planning can save production cost and achieve excellent circuit function and heat dissipation function. Simple map planning can be accomplished by craft, and complex map planning needs can be accomplished by CAD.
The purpose of this document is to clarify the process and precautions of PCB planning using the pad's PCB planning software powerpcb, to provide planning specifications for the planners of an operation group, so as to facilitate communication and mutual viewing between planners.
2 planning process
PCB planning process is divided into six processes: grid input, rule setting, meta equipment layout, wiring, view, review and output
2.1 network meter input
There are two ways to input network table. One is to use OLE powerpcb connection function of PowerLogic, select Send netlist, and use ole function, so as to keep the consistency of schematic diagram and PCB diagram at any time and minimize the possibility of mistakes. Another way is to load the netlist directly in powerpcb, select File - > import, and input the netlist generated by schematic diagram.
2.2 rules setting
If the PCB planning rules have been set up in the schematic planning stage, these rules will not need to be set any more, because when inputting the network table, the planning rules have been input into the powerpcb along with the network table. If the planning rules are modified, it is necessary to synchronize the schematic diagram to ensure the consistency between the schematic diagram and PCB. In addition to the planning rules and layer definition, there are also some rules and requirements settings, such as pad stacks, which need to modify the specification of the size of vias. If the planner creates a new pad or via, be sure to add layer 25.
PCB planning rules, layer definition, through hole setting and cam output setting have been made into default start file with the name of default.stp. In the future, the network table is input. According to the actual situation of planning, the power network and ground are allocated to the power layer and layer, and other high-grade rules are set. After all the rules are set, in PowerLogic, use the rules from PCB function of OLE powerpcb connection to update the rules in the schematic to ensure that the rules in the schematic and PCB are consistent.
2.3 equipment layout
In the future, all the meta devices will be placed at the zero point of the operation area and stacked together. The next step is to separate these meta devices and arrange them according to some rules, that is, the layout of meta devices. Powerpcb provides two methods, technology layout and active layout.
2.3.1 technology layout
1. Draw the board outline for the structural dimension of East-West printed boards.
2. Disperse components, which will be placed around the edge of the board.
3. Move and rotate the equipment one by one, and put it in the edge of the board, and put it in order according to certain rules.
2.3.2 active layout
Powerpcb provides active layout and active partial cluster layout, but for most of the planning, the effect is not ideal and is not recommended.
a. The first principle of layout is to ensure the distribution rate of wiring, pay attention to the connection of flying wires when moving equipment, and put the connected equipment together
b. Digital equipment and analog equipment should be separated as far as possible
c. Decoupling capacitance should be as close to the VCC of the equipment as possible
d. When placing equipment, consider the future welding, not too intensive
e. Use more array and union functions provided by software to improve the efficiency of layout
There are two methods of wiring, technical wiring and active wiring. The technical wiring function provided by powerpcb is very strong, including active pushing and online planning rule checking (DRC). The active wiring is carried out by specctra's wiring engine. Generally, these two methods are used together, and the common process is technology active technology.
2.4.1 technical wiring
1. Before active wiring, some important networks, such as high-frequency clock, main power supply, etc., shall be made of technical cloth. These networks often have special requirements for routing interval, line width, line distance, shielding, etc.; other special packages, such as BGA,
It's hard to have rules for active wiring, but it's also necessary to use technical wiring.
2. In the future of active wiring, it is necessary to adjust the PCB wiring with technical wiring.
2.4.2 active wiring
After the completion of technical wiring, the rest of the network will be handed over to the active router from the cloth. Select tools - > specctra, start the interface of specctra router, set the do file, press continue to start the active routing of specctra router, if the distribution rate is 100% after completion, then the technology can be adjusted; if it is less than 100%, clarify that there is a problem in the layout or technology wiring, and need to adjust the layout or technology wiring until all the distribution stops.
a. The power line and ground wire shall be thickened as much as possible
b. Decoupling capacitor shall be directly connected with VCC as much as possible
c. When setting the do file of specctra, first add the protect all wires command to maintain that the wires of the technical cloth are not valued by the active router
d. If there is a mixed power supply layer, it should be defined as split / mixed plane. Before wiring, it should be divided. After wiring, use plane connect of pour manager to copper clad
e. Set all equipment pins to hot pad method by setting filter to pins and selecting all pins,
Fix the attribute by checking the thermal option
f. Turn the DRC option on when routing manually, and use dynamic route
The items to be viewed include clearance, connectivity, high speed and plane. These items can be selected from tools - > verify design. If the high-speed rule is set, it is necessary to check it, otherwise this item can be skipped. It is necessary to correct the layout and wiring if faults are found.
Some faults can be ignored, for example, some connectors' outline parts are placed outside the board frame, and mistakes will be made when viewing the distance; each time the wiring and hole are corrected, the copper should be covered from the beginning once.
The review is based on the "PCB view table", which includes the planning rules, layer definition, line width, distance, pad and through-hole settings. It is also important to review the rationality of equipment layout, the routing of power and ground wire networks, the routing and shielding of high-speed clock networks, and the placement and connection of decoupling capacitors. In case of unqualified recheck, the Planner shall correct the layout and wiring. After qualified, the rechecker and Planner shall sign separately.
2.7 planning output
PCB planning can output to printer or output photo file. The printer can print the PCB layer by layer, which is convenient for planners and reviewers to check; the photo file is handed over to the board manufacturer to produce the PCB. The output of the photo file is very important, which is related to the success or failure of this planning. The following will focus on the notes of the output of the photo file.
a. The required output layers include wiring layer (including top layer, 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 layer solder mask), and NC drill
b. If the power supply layer is set to split / mixed, the document item of the add document window selects routing, and the plane connect of the pour manager is used to copper cover the PCB drawing before outputting the photo document every time; if the power supply layer is set to cam plane, the plane is selected. When setting the layer item, the layer25 is added, and the pads and vias are selected in the layer25 layer
c. In the device setup window (press device setup), change the aperture value to 199
d. When setting the layer of each layer, select board outline
e. When setting the layer of silk screen layer, do not select part type, select the outline, text and line of the top layer (bottom layer) and silk screen layer
f. When setting the layer of the solder mask layer, the selection of vias indicates that there is no solder mask on the vias, and the non selection of vias indicates the home solder mask, which shall be confirmed according to the specific situation
g. When generating drilling files, use the default settings of powerpcb, and do not make any changes
h. In the future, all photo files will be output, opened and printed with CAM350, and viewed by planners and reviewers according to "PCB view table".
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