• pad-to-pad capacitance

    From sea moss@21:1/5 to All on Wed Aug 3 11:03:50 2022
    Two part question (my google skills have failed me for both):

    1. Does anyone know approximate pad-to-pad capacitance for the common SMD packages (0201, 0402, 0603, 0805)? Ever measured it? I can't use the parallel plate capacitor formula, since the plates are adjacent, not parallel. (assume there is no copper
    below the pads)

    2. Given an SMD footprint as above, how close does a ground plane need to be (on the same layer) to decrease the effective pad-to-pad capacitance? Is there a formula which gives a good approximation?

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  • From whit3rd@21:1/5 to sea moss on Wed Aug 3 14:54:38 2022
    On Wednesday, August 3, 2022 at 11:03:53 AM UTC-7, sea moss wrote:
    Two part question (my google skills have failed me for both):

    1. Does anyone know approximate pad-to-pad capacitance for the common SMD packages (0201, 0402, 0603, 0805)? Ever measured it? I can't use the parallel plate capacitor formula, since the plates are adjacent, not parallel. (assume there is no copper
    below the pads)

    But, regardless of the pads, the component's mating metal surface has that capacitance regardless, doesn't it? You don't need always to make the 'plates' larger
    than that component feature.

    2. Given an SMD footprint as above, how close does a ground plane need to be (on the same layer) to decrease the effective pad-to-pad capacitance? Is there a formula which gives a good approximation?

    The use of guard rings can make such capacitance negligible, if that's required, but...
    trying to simply treat the tiny elements as separable capacitances, isn't a good and useful
    model.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From John Larkin@21:1/5 to All on Wed Aug 3 15:14:00 2022
    On Wed, 3 Aug 2022 14:54:38 -0700 (PDT), whit3rd <whit3rd@gmail.com>
    wrote:

    On Wednesday, August 3, 2022 at 11:03:53 AM UTC-7, sea moss wrote:
    Two part question (my google skills have failed me for both):

    1. Does anyone know approximate pad-to-pad capacitance for the common SMD packages (0201, 0402, 0603, 0805)? Ever measured it? I can't use the parallel plate capacitor formula, since the plates are adjacent, not parallel. (assume there is no copper
    below the pads)

    But, regardless of the pads, the component's mating metal surface has that >capacitance regardless, doesn't it? You don't need always to make the 'plates' larger
    than that component feature.

    Most surface-mount part capacitance is in the FR4, not in air.


    2. Given an SMD footprint as above, how close does a ground plane need to be (on the same layer) to decrease the effective pad-to-pad capacitance? Is there a formula which gives a good approximation?

    The use of guard rings can make such capacitance negligible, if that's required, but...
    trying to simply treat the tiny elements as separable capacitances, isn't a good and useful
    model.

    It's often important to know the three capacitances: each pad to the
    ground plane, and 3-terminal equivalent capacitance pad to pad.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Klaus Vestergaard Kragelund@21:1/5 to John Larkin on Thu Aug 4 02:24:55 2022
    On 04/08/2022 00.14, John Larkin wrote:
    On Wed, 3 Aug 2022 14:54:38 -0700 (PDT), whit3rd <whit3rd@gmail.com>
    wrote:

    On Wednesday, August 3, 2022 at 11:03:53 AM UTC-7, sea moss wrote:
    Two part question (my google skills have failed me for both):

    1. Does anyone know approximate pad-to-pad capacitance for the common SMD packages (0201, 0402, 0603, 0805)? Ever measured it? I can't use the parallel plate capacitor formula, since the plates are adjacent, not parallel. (assume there is no copper
    below the pads)

    But, regardless of the pads, the component's mating metal surface has that >> capacitance regardless, doesn't it? You don't need always to make the 'plates' larger
    than that component feature.

    Most surface-mount part capacitance is in the FR4, not in air.


    2. Given an SMD footprint as above, how close does a ground plane need to be (on the same layer) to decrease the effective pad-to-pad capacitance? Is there a formula which gives a good approximation?

    The use of guard rings can make such capacitance negligible, if that's required, but...
    trying to simply treat the tiny elements as separable capacitances, isn't a good and useful
    model.

    It's often important to know the three capacitances: each pad to the
    ground plane, and 3-terminal equivalent capacitance pad to pad.

    turn PCB has a toolkit for a number of different calculations on PCB structures:

    https://saturnpcb.com/saturn-pcb-toolkit/

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From sea moss@21:1/5 to Klaus Kragelund on Wed Aug 3 18:00:31 2022
    On Wednesday, August 3, 2022 at 5:24:55 PM UTC-7, Klaus Kragelund wrote:
    On 04/08/2022 00.14, John Larkin wrote:
    On Wed, 3 Aug 2022 14:54:38 -0700 (PDT), whit3rd <whi...@gmail.com>
    wrote:

    On Wednesday, August 3, 2022 at 11:03:53 AM UTC-7, sea moss wrote:
    Two part question (my google skills have failed me for both):

    1. Does anyone know approximate pad-to-pad capacitance for the common SMD packages (0201, 0402, 0603, 0805)? Ever measured it? I can't use the parallel plate capacitor formula, since the plates are adjacent, not parallel. (assume there is no copper
    below the pads)

    But, regardless of the pads, the component's mating metal surface has that
    capacitance regardless, doesn't it? You don't need always to make the 'plates' larger
    than that component feature.

    Most surface-mount part capacitance is in the FR4, not in air.


    2. Given an SMD footprint as above, how close does a ground plane need to be (on the same layer) to decrease the effective pad-to-pad capacitance? Is there a formula which gives a good approximation?

    The use of guard rings can make such capacitance negligible, if that's required, but...
    trying to simply treat the tiny elements as separable capacitances, isn't a good and useful
    model.

    It's often important to know the three capacitances: each pad to the ground plane, and 3-terminal equivalent capacitance pad to pad.

    turn PCB has a toolkit for a number of different calculations on PCB structures:

    https://saturnpcb.com/saturn-pcb-toolkit/

    Unfortunately Saturn PCB does not cover this... I tried there first.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From jlarkin@highlandsniptechnology.com@21:1/5 to klauskvik@hotmail.com on Thu Aug 4 07:08:58 2022
    On Thu, 4 Aug 2022 02:24:55 +0200, Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:

    On 04/08/2022 00.14, John Larkin wrote:
    On Wed, 3 Aug 2022 14:54:38 -0700 (PDT), whit3rd <whit3rd@gmail.com>
    wrote:

    On Wednesday, August 3, 2022 at 11:03:53 AM UTC-7, sea moss wrote:
    Two part question (my google skills have failed me for both):

    1. Does anyone know approximate pad-to-pad capacitance for the common SMD packages (0201, 0402, 0603, 0805)? Ever measured it? I can't use the parallel plate capacitor formula, since the plates are adjacent, not parallel. (assume there is no copper
    below the pads)

    But, regardless of the pads, the component's mating metal surface has that >>> capacitance regardless, doesn't it? You don't need always to make the 'plates' larger
    than that component feature.

    Most surface-mount part capacitance is in the FR4, not in air.


    2. Given an SMD footprint as above, how close does a ground plane need to be (on the same layer) to decrease the effective pad-to-pad capacitance? Is there a formula which gives a good approximation?

    The use of guard rings can make such capacitance negligible, if that's required, but...
    trying to simply treat the tiny elements as separable capacitances, isn't a good and useful
    model.

    It's often important to know the three capacitances: each pad to the
    ground plane, and 3-terminal equivalent capacitance pad to pad.

    turn PCB has a toolkit for a number of different calculations on PCB >structures:

    https://saturnpcb.com/saturn-pcb-toolkit/

    It's easy to measure attofarads with cheap instruments that a decent
    home lab should normally have.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Jeroen Belleman@21:1/5 to jlarkin@highlandsniptechnology.com on Thu Aug 4 18:35:45 2022
    On 2022-08-04 16:08, jlarkin@highlandsniptechnology.com wrote:
    On Thu, 4 Aug 2022 02:24:55 +0200, Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:

    On 04/08/2022 00.14, John Larkin wrote:
    On Wed, 3 Aug 2022 14:54:38 -0700 (PDT), whit3rd <whit3rd@gmail.com>
    wrote:

    On Wednesday, August 3, 2022 at 11:03:53 AM UTC-7, sea moss wrote:
    Two part question (my google skills have failed me for both):

    1. Does anyone know approximate pad-to-pad capacitance for the common SMD packages (0201, 0402, 0603, 0805)? Ever measured it? I can't use the parallel plate capacitor formula, since the plates are adjacent, not parallel. (assume there is no copper
    below the pads)

    But, regardless of the pads, the component's mating metal surface has that >>>> capacitance regardless, doesn't it? You don't need always to make the 'plates' larger
    than that component feature.

    Most surface-mount part capacitance is in the FR4, not in air.


    2. Given an SMD footprint as above, how close does a ground plane need to be (on the same layer) to decrease the effective pad-to-pad capacitance? Is there a formula which gives a good approximation?

    The use of guard rings can make such capacitance negligible, if that's required, but...
    trying to simply treat the tiny elements as separable capacitances, isn't a good and useful
    model.

    It's often important to know the three capacitances: each pad to the
    ground plane, and 3-terminal equivalent capacitance pad to pad.

    turn PCB has a toolkit for a number of different calculations on PCB
    structures:

    https://saturnpcb.com/saturn-pcb-toolkit/

    It's easy to measure attofarads with cheap instruments that a decent
    home lab should normally have.


    Femtofarads, sure, but attofarads? Please explain?

    Jeroen Belleman

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From John Larkin@21:1/5 to jlarkin@highland_atwork_technology. on Thu Aug 4 10:19:24 2022
    On Thu, 04 Aug 2022 10:15:25 -0700, John Larkin <jlarkin@highland_atwork_technology.com> wrote:

    On Thu, 04 Aug 2022 18:35:45 +0200, Jeroen Belleman
    <jeroen@nospam.please> wrote:

    On 2022-08-04 16:08, jlarkin@highlandsniptechnology.com wrote:
    On Thu, 4 Aug 2022 02:24:55 +0200, Klaus Vestergaard Kragelund
    <klauskvik@hotmail.com> wrote:

    On 04/08/2022 00.14, John Larkin wrote:
    On Wed, 3 Aug 2022 14:54:38 -0700 (PDT), whit3rd <whit3rd@gmail.com> >>>>> wrote:

    On Wednesday, August 3, 2022 at 11:03:53 AM UTC-7, sea moss wrote: >>>>>>> Two part question (my google skills have failed me for both):

    1. Does anyone know approximate pad-to-pad capacitance for the common SMD packages (0201, 0402, 0603, 0805)? Ever measured it? I can't use the parallel plate capacitor formula, since the plates are adjacent, not parallel. (assume there is no
    copper below the pads)

    But, regardless of the pads, the component's mating metal surface has that
    capacitance regardless, doesn't it? You don't need always to make the 'plates' larger
    than that component feature.

    Most surface-mount part capacitance is in the FR4, not in air.


    2. Given an SMD footprint as above, how close does a ground plane need to be (on the same layer) to decrease the effective pad-to-pad capacitance? Is there a formula which gives a good approximation?

    The use of guard rings can make such capacitance negligible, if that's required, but...
    trying to simply treat the tiny elements as separable capacitances, isn't a good and useful
    model.

    It's often important to know the three capacitances: each pad to the >>>>> ground plane, and 3-terminal equivalent capacitance pad to pad.

    turn PCB has a toolkit for a number of different calculations on PCB
    structures:

    https://saturnpcb.com/saturn-pcb-toolkit/

    It's easy to measure attofarads with cheap instruments that a decent
    home lab should normally have.


    Femtofarads, sure, but attofarads? Please explain?

    Jeroen Belleman

    Femtos, just make a voltage divider from a sig gen, Cx, and the usual
    15 pF input of a scope. Signal average a bit or FFT at the low end. A
    few fF resolution is easy. To get to aF, add a jfet or an opamp.

    One could also add an inductor to resonate out most of that 15 pF.

    Lots of ways to do this.

    The old HP selective (tuned) voltmeters were most cool for things like
    this.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From John Larkin@21:1/5 to jeroen@nospam.please on Thu Aug 4 10:15:25 2022
    On Thu, 04 Aug 2022 18:35:45 +0200, Jeroen Belleman
    <jeroen@nospam.please> wrote:

    On 2022-08-04 16:08, jlarkin@highlandsniptechnology.com wrote:
    On Thu, 4 Aug 2022 02:24:55 +0200, Klaus Vestergaard Kragelund
    <klauskvik@hotmail.com> wrote:

    On 04/08/2022 00.14, John Larkin wrote:
    On Wed, 3 Aug 2022 14:54:38 -0700 (PDT), whit3rd <whit3rd@gmail.com>
    wrote:

    On Wednesday, August 3, 2022 at 11:03:53 AM UTC-7, sea moss wrote:
    Two part question (my google skills have failed me for both):

    1. Does anyone know approximate pad-to-pad capacitance for the common SMD packages (0201, 0402, 0603, 0805)? Ever measured it? I can't use the parallel plate capacitor formula, since the plates are adjacent, not parallel. (assume there is no
    copper below the pads)

    But, regardless of the pads, the component's mating metal surface has that
    capacitance regardless, doesn't it? You don't need always to make the 'plates' larger
    than that component feature.

    Most surface-mount part capacitance is in the FR4, not in air.


    2. Given an SMD footprint as above, how close does a ground plane need to be (on the same layer) to decrease the effective pad-to-pad capacitance? Is there a formula which gives a good approximation?

    The use of guard rings can make such capacitance negligible, if that's required, but...
    trying to simply treat the tiny elements as separable capacitances, isn't a good and useful
    model.

    It's often important to know the three capacitances: each pad to the
    ground plane, and 3-terminal equivalent capacitance pad to pad.

    turn PCB has a toolkit for a number of different calculations on PCB
    structures:

    https://saturnpcb.com/saturn-pcb-toolkit/

    It's easy to measure attofarads with cheap instruments that a decent
    home lab should normally have.


    Femtofarads, sure, but attofarads? Please explain?

    Jeroen Belleman

    Femtos, just make a voltage divider from a sig gen, Cx, and the usual
    15 pF input of a scope. Signal average a bit or FFT at the low end. A
    few fF resolution is easy. To get to aF, add a jfet or an opamp.

    One could also add an inductor to resonate out most of that 15 pF.

    Lots of ways to do this.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)