Printed Circuit Boards Useful Information
  • Contents:
  • Track resistance
  • Track RF Impedances
  • PCB as a heatsink
    • Track resistance:
    Printed circuit copper cladding is normally 35micron  thick "1oz" i.e. 1oz per sq foot, although 75micron  "2oz" is sometimes used on power supply boards.
    there is an excellent PDF file on the Unitrode Web site on using PCB track as resistance elements.

    Current carrying capacity of Printed Circuit Boards:
    A related topic is the current carrying capacity of a track, this is limited by heating effects and so depends somewhat on other heasources on teh board, whether the board hads a ground plane and how much ventilation is avalailable,  This table lists a few useful design values for 35micron (1oz) board  for a single isolated track.
     
    Current carrying capacity of  single 1oz copper track for a 40degC temp rise
    10 thou (0.25mm) 600mA
    20 thou (0.5mm) 1.3Amp
    40 thou (1mm) 2.4Amp
    80 thou (2mm) 3.8Amp
    120 thou (3mm) 5Amp

    So a rule of thumb would be not to exceed 1 amp per 20thou of track. Note you cannot use PCB traces as fuses, the melting point of copper is 800deg C higher than the ignition temperature of the laminate material, the primary function of a fuse after all is to prevent a fire. And an excellent chart on current carrying capacity is  in the Philips "Design of Planar power transformers"  PDF document.
     

  • RF Impedance The RF impedance of printed circuit tracks is a function of the track width and board permittivity


    • PCB as heatsink

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