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  • The general process of PCB LAYOUT
     
    Date :2010-4-30

       

    1: LAYOUT general process:

     

    1 Overview
        The purpose of this document is to describe the use of PADS PCB design software for PCB design process PowerPCB and some considerations for the design of a working group providing design, facilitate communication between designers and mutual inspection. 


        

    2, the design process
        PCB design process is divided into netlist input, rule set, components placement, routing, inspection, review the output of six steps.
        2.1 netlist input
        Netlist input There are two ways, one is to use the OLE PowerPCB Connection PowerLogic function, select Send Netlist, application of OLE function, you can always maintain the schematic and PCB line graph to minimize the possibility of error. Another method is to directly load the netlist in PowerPCB, select File-> Import, to schematic generated netlist input come.
        2.2 Rule set
        If the schematic design phase in the PCB design rules have been set up is good, will not need to set these rules, because the transmission network table, the design rules have been entered into with the PowerPCB the netlist. If you modify the design rules, must be synchronized diagram, schematic and PCB to ensure consistency. In addition to design rules and layer definitions, there are some rules need to set up, such as Pad Stacks, need to modify the standard through-hole size. If the designers built a pad or via holes, must be coupled with Layer 25.
        Note:
        PCB design rules, layer definitions, cross hole set, CAM has already shipped out to set the default startup file, named Default.stp, netlist input comes after the actual situation according to the design, the power distribution network and to power planes and ground , and set other advanced rules. In all of the rules set up after the PowerLogic, use the OLE PowerPCB Connection's Rules From PCB features, updates, set the rules schematic, schematic and PCB to ensure the rules of the same graph.
        2.3 Component Layout
        Netlist entered, all components are zero on the work area, overlapping with, the next step is to separate these components, according to some rules neatly placed, that component layout. PowerPCB provides two methods, manual layout and automatic layout.
        2.3.1 Manual Layout
        a. tool to draw the structure size PCB board side (Board Outline).
        b. the components dispersed (Disperse Components), components will be arranged around the edge of the plate.
        c. the components one by one move, rotate, put plate boundary within the neatly placed in accordance with certain rules.
        2.3.2 Automatic Layout
        PowerPCB provides automatic layout and automatic layout of the local cluster, but most of the design, the effect is not ideal, is not recommended.
        2.3.3 Notes
        a. The first rule is to ensure the layout routing fabric through rate, mobile devices fly line connection when attention to the relationship between a device connected together
        b. digital devices and analog devices to be separated, as far as possible away from
        c. decoupling capacitor as close to the device's VCC
        d. to consider when placing the device after welding, not too dense
        e. multi-use software provided by the Array, and Union functions, improve the efficiency of the layout
        2.4 Cabling
        There are two ways cabling, wiring manual and automatic routing. PowerPCB provide manual wiring is so powerful, including the automatic push, online design rule checking (DRC), automatically routing the wiring from the Specctra engines, usually used in conjunction with the two methods commonly used steps are manual - automatic - manual.
        2.4.1 Hand wiring
        a. automatic routing before hand cloth some important networks, such as high-frequency clock, the main power supply, etc., these networks often take the line from the line width, line spacing, shielding and other special requirements; other special packaging, such as BGA, automatic routing fabric was difficult to have rules, but also by hand wiring.
        b. automatic routing after the routing of the PCB but also by hand to adjust the alignment.
        2.4.2 automatic routing
        After the end of hand-wiring, and the rest of the network on to the automatic router from cloth. Select Tools-> SPECCTRA, start Specctra router interface, set up DO file, press Continue to start the Specctra router automatic routing, if the cloth after the pass rate was 100%, then you can manually adjust the layout of the; if you do not to 100%, indicating a problem with the layout or manual routing, need to adjust the layout or manual routing, until all the fabric through so far.
        2.4.3 Note
        a. power line and ground as bold
        b. decoupling capacitor as a direct connection with the VCC
        c. Set Specctra the DO file, add the Protect all wires first order, the protection line of hand-cloth fabric is not regarded highly automated routing
        d. If there are hybrid power layer, the layer should be defined as the Split / mixed Plane, before their split in the wiring, complete line of fabric, use Pour Manager of Plane Connect to copper
        e. all of the device pin is set to heat pad mode, the practice is to Filter Set Pins, select all of the pins,
    Modify the property, before the tick in the Thermal Options
        f. Manually routing options open when the DRC, with dynamic routing (Dynamic Route)
        2.5 Inspection
        Check the items spacing (Clearance), connectivity (Connectivity), high-speed rules (High Speed) and power planes (Plane), these items can select Tools-> Verify Design for. If you set the speed rule, must be checked, otherwise you can skip this one. Check out the error, you must modify the layout and routing.
    Note:
        Some errors can be ignored, such as the Outline of the part of some connectors on the frame, the inspection interval when the error occurred; revised alignment for each other and through hole, the copper must re-one.
        2.6 Review
        Review under the "PCB Checklist," including design rules, layer definitions, line width, spacing, pad, through-hole set; also focus on reviewing the reasonableness of the device layout, power supply, ground network alignment, high-speed clock network alignment and shielding, decoupling capacitors are placed and the connections. Review unqualified designers to modify the layout and routing, qualified after review and signature designer, respectively.
        2.7 Design output
        PCB design can be output to printer or export of Gerber files. PCB layer printer can print, easy to designers and review those checks; Gerber files to the system board manufacturer, the production PCB. Gerber file output is very important bearing on the success of this design, will highlight the following Output Gerber file note.
        a. need to output layer wiring layer (including the top, bottom, intermediate wiring layers), the power level (including the VCC level and the GND layer), screen printing layer (including the top screen and bottom screen), solder mask layer (including solder solder the top and bottom), while also generating drill files (NC Drill)
        b. if the power level is set to Split / Mixed, then Add Document Window Document option Routing, and Gerber files before each output should plan to use on the PCB Pour Manager of Plane Connect to copper; if set to CAM Plane, then select Plane, Layer items in the set time, we should Layer25 plus, in Layer25 layer selection Pads and Vias
        c. In the Device Settings window (by Device Setup), the value of Aperture to 199
        d. Set each layer in the Layer, it will select the Board Outline
        e. Set the Layer screen layer, do not choose Part Type, select the top (bottom) and the screen layer Outline, Text, Line
        f. Set the Layer solder mask layer, select the through-hole vias that do not add solder, not the election, said through hole solder at home, as the case may determine
        g. generating drill files, use the default settings PowerPCB Do not make any changes
        h. After all the Gerber file output with CAM350 to open and print, are under review by the designer and the "PCB Checklist" Check

     

    2: LAYOUT Precautions

     

         1: When all the parts of the circuit arrangement of parts arranged together as much as possible, take the line as short as possible.
         2: IC to decoupling capacitor should be as close as possible to increase the effect of IC legs.
         3: If the voltage difference between the two lines need to pay attention to safety when a larger space.
         4: To consider the current size of each loop, that is the situation to determine the copper foil thickness of the heat.
         5: Ministry of line around the corner as far as possible to have an acute angle, obtuse angle and arc angle is best.
         6: high-frequency circuits, the two line level best not to walk too long in order to reduce the impact of distributed capacitance, the general public to take the top floor entry way.
         7: Gao Ping Circuit to consider the high-frequency ground impedance, generally means a large area of ground, nearest the point of ground, reducing the amount of ground inductance, so close to the street location of the potential.
         8: high-frequency circuit traces to thick and short and long reduced due to the alignment arising from inductance and high frequency impedance of the circuit.
         9: parts ordered, the general should be ranked with the same parts, neat as possible, have the polarity of the components in the same direction as much as possible to reduce the potential cost of production.
         10: The RF models, the most power as far as possible away from the receiving board parts, to reduce interference.
         11: The TF models, the transmitter should be as far away from the PIR number, in order to reduce the firing of the PIR disruption.

     

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