4 Layer PCB: How do via's work?

Discussion in 'General Electronics Chat' started by bagarwal, Mar 18, 2009.

  1. bagarwal

    Thread Starter New Member

    Mar 18, 2009
    1
    0
    Hi All,

    This might be a noob question but still its extremely confusing for me. I am designing a 4 layer PCB and unfortunately dont understand how via's work. In a 2 layer PCB, a via connects the signal traces from the top to bottom layer. I get that part.

    Now if I have a 4 layer PCB with 2 signal planes (top & bottom, layer 1 and 16) and 2 power planes (layer 2:GND, layer15:+3.3V). Layers 1&2 form a core and layers 15&16 form another core.

    In this case, if I used only through via's how does that work? So if I wanted to connect a signal trace from layer 1(top) to layer 2(gnd), the via would go through all four layers. Doesnt that short out layers 2 and 15, thereby shorting out GND and VCC?

    Might sound really trivial but any help is much appreciated.

    Thanks!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    The vias have a clear space around them that is automatically taken care of by the CAD program. They will only connect the layers they are supposed to. If you examine the photoplots, you will see the clearance holes in the power and ground planes that allow the vias to pass.
     
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