Printed Circuit Boards

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Printed circuit board design/fabrication

Overview

Software Design

See Software Design Tools below.

Step by Step by using Software Design Tool

  • make sure the dimension and shape of PCB
  • make sure the size and location of Via for PCB stand
  • Make sure each components footprint.
  • each components are placed on suitable place by put on a hardcopy of simulation PCB
  • All components get enough clearance between them.
  • Silkscreen layout is confirmed.
  • PCB is drawn.
  • silkscreen adding the following:
    • version no.
    • organization name
    • board name
  • Netlist is ran and got a no error result.
  • DRC is ran and got a no error result.
  • Overall is checked.
  • generate Gerber and send to PCB Manufacturers.

Manual Design

Somepeople do this with layout on clear film or by directly drawing on a circuit board, of even by scratching, grinding.... For now let them google this.

Homebrew fabrication

Before exploring these techniques, you should understand your options with regard to services such as BatchPCB.com, ExpressPCB.com and PCB123.com. Being able to have several boards fabbed in 2 days for $59 (for example) makes it harder to justify the hassle of etching your own boards at home.

Commercial PCB fabrication

Best Practices for PCB Layout

Theory

  • Provide the easiest path (lowest impedance) for current to flow
    • Return current tends to flow directly under signal trace (for PCB having ground plane)
    • Inductance increases with length of traces
    • Inductance increases with the area enclosed by signal trace and ground
  • Prevent digital currents from contaminating analog currents
  • Decouple high speed components
  • Use ground loop avoidance tehniques

Design

  • Partition PCB into "analog stuff" and "digital stuff".
  • No digital signal traces should cross over analog ground, and vice versa
  • For components having both analog and digital signals (e.g. ADC), orientate components so that the analog signal traces goes only over the analog ground plane, and digital signal traces goes only over the digital ground plane
  • AGND and DGND of ADC must have a small impedance (i.e. separated by short distance)
  • Add decoupling capacitors close to Vcc and DGND of ICs
  • Add ferrite beads and capacitors (PI-filter) to power rail for low-pass filtering (reduce ripples).

Routing

  • Place fixed components first (components location that cannot be changed, e.g. connectors, buttons, etc)
  • Make installing parts onto the PCB fast:
  • Separate components into groups
    • Digital signals only
    • Analog signals only
    • Digital and analog (Mixed) signals
    • High current devices (e.g. led backlight for LCD/buzzer)
  • Do not partition ground into analog and digital planes.
  • Orientate components that have mixed signals according to the orientation of the ground planes, and straddle components over DGND and AGND
  • Place digital only components over DGND
  • Place analog only components over AGND
Grounding Example for PCB
  • Decoupling capacitors should be as close to the ICs as possible
              Vcc
               | | | | |
             +-----------+
 -------+-+--|-+---------|-------- Vcc
        |C|  |    IC     |
 -------+-+--|---------+-|-------- GND
             +-----------+
               | | | | |
                       GND
  • Lay critical (noise-sensitive) traces first (e.g. crystal, analog signals)
    • As short as possible
    • Use 45o turnings instead of 90o
  • Paired signal traces (e.g. TX+, TX- in ethernet chips) should run parallel along each other
    TX+ -----\
    TX- ----\ \
             \ \
              \ \
               \ \
                \ \------------ TX+
                 \------------- TX-

References

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