The purpose of this document is to explain the process and some precautions of PCB design using powerpcb, the PCB design software of pads, and to provide design specifications for designers of a working group, so as to facilitate communication and mutual inspection among designers.
2、 Design process
PCB design process is divided into six steps: net list input, rule setting, component layout, wiring, inspection, review and output.
2.1 net list 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 to keep the schematic diagram consistent with the PCB diagram at any time, so as to minimize the possibility of errors. Another method is to load net list directly in powerpcb, select file import and input net list generated from schematic diagram.
2.2 rule setting
If the PCB design rules have been set in the schematic design stage, there is no need to set these rules, because when the net list is input, the design rules have been input into powerpcb with the net list. If the design rules are modified, the schematic must be synchronized to ensure the consistency between the schematic and PCB. In addition to design rules and layer definitions, there are also some rules to be set, such as pad stacks, which need to modify the size of standard vias. If the designer creates a new pad or via, be sure to add layer 25.
PCB design rules, layer definition, via settings and cam output settings have been made into default startup files, named default.stp. After inputting the netlist, according to the actual situation of the design, the power network and ground are assigned to the power layer and layer, 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, so as to ensure that the rules of the schematic diagram and PCB diagram are consistent.
2.3 component layout
After inputting the net list, 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 put them in order according to some rules, that is, the layout of components. Powerpcb provides two methods, manual layout and automatic layout.
2.3.1 manual layout
a. Draw the board outline according to the structure size of the tool PCB.
b. Disperse components, and the components will be arranged around the edge of the board.
c. Move and rotate the components one by one, place them within the edge of the board, and place them in order 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, so it is not recommended.
a. The first principle of layout is to ensure the routing rate. When moving devices, pay attention to the connection of flying wires, and put the devices with wiring relationship together
b. Digital devices and analog devices should be separated as far as possible
c. The decoupling capacitance should be as close to the VCC of the device as possible
d. When placing the device, consider the future welding, not too dense
e. Use the array and union functions provided by the software to improve the efficiency of layout
There are also two ways of wiring, manual wiring and automatic wiring. Powerpcb provides powerful manual routing functions, including automatic push and online design rule checking (DRC). Automatic routing is carried out by specctra’s routing engine. Generally, these two methods are used together, and the common steps are manual automatic manual.
2.4.1 manual wiring
a. Before automatic wiring, some important networks, such as high-frequency clock, main power supply, etc., are manually laid. These networks often have special requirements for routing distance, line width, line spacing, shielding, etc; Other special packages, such as BGA,
Automatic wiring is difficult to be arranged regularly, and manual wiring is also needed.
b. After automatic wiring, manual wiring should be used to adjust the PCB routing.
2.4.2 automatic wiring
After the manual wiring is finished, the rest of the network is handed over to the automatic router. Select tools – specctra, start the interface of specctra router, set the do file, and press continue to start the automatic routing of specctra router. After that, if the routing rate is 100%, you can adjust the routing manually; If it is less than 100%, it indicates that there is a problem with the layout or manual wiring, and it is necessary to adjust the layout or manual wiring until all the wiring is completed.
a. The power line and ground wire should be as thick as possible
b. The decoupling capacitor should be directly connected to the VCC as far as possible
c. When setting the do file of specctra, first add the command of protect all wires to protect the wires laid by hand from being laid by the automatic router
d. If there is a hybrid power supply layer, it should be defined as split / mixed plane, and it should be divided before wiring. After wiring, use plane connect of pour manager to cover copper
e. Set all device pins to hot pad mode. The method is to set filter to pins, select all pins, modify properties, and check the thermal option
f. Turn on DRC option and use dynamic route for manual routing
The items to be checked include clearance, connectivity, high speed and plane. These items can be checked by tools – “verify design”. If the high speed rule is set, you must check it, otherwise you can skip this item. If the error is detected, the layout and routing must be modified.
Some errors can be ignored. For example, some of the outline of some connectors are placed outside the board frame, so there will be errors when checking the spacing; In addition, after modifying the routing and vias, the copper should be coated again.
Review according to “PCB Checklist”, including design rules, layer definition, line width, spacing, pad and via settings; It is also important to review the rationality of device layout, the wiring of power supply and ground wire network, the wiring and shielding of high-speed clock network, and the placement and connection of decoupling capacitors. If the review is not qualified, the designer shall modify the layout and wiring. After the review is qualified, the reviewer and the designer shall sign separately.
2.7 design output
PCB design can be output to printer or output photo file. The printer can print PCB layer by layer, which is convenient for designers and reviewers to check; The optical drawing documents are handed over to the board manufacturer to produce the printed board. The output of photo file is very important, which is related to the success or failure of this design. The following will focus on the notes of output photo file.
a. The 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 and bottom layer), solder mask layer (including top layer and bottom layer), and NC drill file
b. If the power layer is set to split / mixed, select routing in the document item of the add document window, and copper overlay the PCB with plane connect of pourmanager before outputting the photo file every time; If it is set to cam plane, select plane. When setting the layer item, add layer25 and select pads and vias in layer25
c. In the device setup window (press device setup), change the value of aperture to 199
d. When setting the layer of each layer, select boardoutline
e. When setting the layer of the silk screen layer, do not select part type, but select the top layer (bottom layer) and the outline, text, line of the silk screen layer
f. When setting the layer of solder mask layer, selecting via means no solder mask on the via, and not selecting via means home solder mask, depending on the specific situation
g. When generating the drilling file, use the default settings of powerpcb without any changes
h. After all the photo files are output, they are opened and printed with CAM350, and checked by the designer and reviewer according to the “PCB Checklist”
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3、 Classic Q & A:
Question: is the mil of Protel 99 English? How small centimeter is it?
A: it can be changed to metric system: 1 / 1000inch, 0.00254cm
Question: the package can be found in the library file, but I don’t know the specification of the component corresponding to each package, such as resistance. I just wrote axial0.3 — axial1.0, but I don’t know what kind of resistance is corresponding to each package. What should I do?
A: footprint refers to the package, which is closely related to the size of components and pins. It must be set in advance when making PCB board General 1 / 8 resistor with axial_ 0.3 (related to Protel version, different versions may have differences, please check the relevant PCB: library file): small capacitance: rad_ 0.1，rad_ 0.2: large capacitance: RB 2 /. 4: integrated circuit: dip_ N (n is the number of pins): the rest will not be repeated.
Question: what’s the difference between the versions of Protel
A: there is no essential difference in the operation mode of PROTEL98 / 99 / 99SE, but the interface and the management mode of engineering documents are changing. Just take any book that introduces pt98 / 99 / 99SE and turn it over. In fact, pt98 / 99 / 99SE is very easy to learn. If you just make schematic diagram and printed circuit board, you will be familiar with it if you use it several times.
Question: is the success rate of automatic wiring 100%?
A: how to say, except for a very simple board, the automatic wiring is only the first time in general, which needs to be optimized manually, and it is necessary. There are still many aspects of AI in the current software that need to be improved. Personally, I usually lay out first, then automatically lay out, then adjust the layout, and then automatically lay out. Finally, the “decisive battle” — manual wiring.
Question: if you draw a schematic diagram in Sch, save it, turn it off, and then turn it on, there will be nothing in it. What’s the matter? It seems that it can’t survive.
A: is your Protel demo? I’ve seen a demo version of PROTEL98 that can’t be saved. Maybe your problem is the same. A crack version is recommended.
Question: how to specify the package form of components in Protel for win?
Answer: 1. Specify in advanced schematic
In the schematic diagram, modify the footprint of the component to the required PCB library package name. Then the net list is generated, and the required package library is added to the PCB. When the net list is transferred in, the components can be loaded at the same time.
2. Specify in advanced PCB
Place the required component package directly in the PCB, set the designer to be consistent with the schematic diagram, and then call the network table again.