6260 Views 3 Replies Latest reply: Oct 29, 2015 9:07 AM by vikas1
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# 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?

• 22 posts since
Apr 4, 2014
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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?

• 3 posts since
Oct 27, 2015
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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.

• 3 posts since
Oct 27, 2015
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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

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

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