Skip navigation
TechXchange
Digi-Key TechXchange Communities > General > Discussions
6260 Views 3 Replies Latest reply: Oct 29, 2015 9:07 AM by vikas1 RSS
Currently Being Moderated

Aug 18, 2014 1:55 AM

Calculating via Capacitance on PCB Designing

Good day everyone,

I have been reading recently on the effects of when a via acts as a capacitor and somehow affects the qualitty of the signal goingthrought the circuit. Can someone please elaborate on the types of vias, and how much can they really affect the quality of signals running through the circuit?

They are really small, so how much damage could they really do? Also, can these capacitance be measured?

  • boborrob Novice 22 posts since
    Apr 4, 2014
    Currently Being Moderated
    Sep 7, 2014 10:28 PM (in response to Riley)
    Re: Calculating via Capacitance on PCB Designing

    that is somewhat of an advanced topic.. maybe you'd like to read on literature on this...

     

    http://www.altera.com/literature/an/an529.pdf

    http://www.ieice.org/proceedings/EMC09/pdf/21Q3-1.pdf

     

    Generally from what I can gather, they are only ever mentioned in high frequency circuits... What type of project are you working on?

  • vikas1 Novice 3 posts since
    Oct 27, 2015
    Currently Being Moderated
    Oct 28, 2015 11:04 AM (in response to Riley)
    Calculating via Capacitance on PCB Designing

    The Via is predominantly Capacitive. The barrel connecting the top and the bottom layer is inductive and has a length of 0.063 inch which corresponds to an electrical length of about 10 ps ( assuming 170 ps per inch). Whether it will affect your signal or not depends upon the rise time of your signal. Typically if your rise time is longer than 10x or about 100 ps, you may choose to ignore the affect of via inductance discontinuity.

     

    As a first order try to minimize the via pad size ( 16 mils or so) , keep the via drill as small as possible ( typically 8 mils) and keep the antipad large ( like 20 mils), This should take care of most signals under 1 GHz. Use this as a guideline to further your research.

    • vikas1 Novice 3 posts since
      Oct 27, 2015
      Currently Being Moderated
      Oct 29, 2015 9:07 AM (in response to vikas1)
      Calculating via Capacitance on PCB Designing

      As a first order approximation, the capacitance of the via can be calculated using the formula

       

      Cvia = 1.41xεrxD1xT/(D2-D1)

       

      Where,

       

      εr =   relative dielectric constant,
      D1 =  diameter of the via pad in inch,
      D2 =  diameter of the anti-pad in inch,
      T = thickness of the PCB in inch,

       

      You can use the following caculator to calculate the via capacitance

      via capacitance calculator .

       

       

      The calculator assumes a coaxial cylindrical structure and should be used only as a first order approximation.

More Like This

  • Retrieving data ...

Bookmarked By (0)

Legend

  • Correct Answers - 4 points
  • Helpful Answers - 3 points