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# Need some advice to calculate PCB stray inductance

- To: femm@yahoogroups.com
- Subject: Need some advice to calculate PCB stray inductance
- From: neel_c@club-internet.fr
- Date: Sat, 15 Sep 2001 10:32:20 -0000
- User-agent: eGroups-EW/0.82

    Hello,

    I would like to calculate the stray inductance of a multilayer
    printed circuit board (PCB).

    Basically, I want to modelize a large PCB (50 x 40 cm). It contains
    at least 2 layers of 17.5 micron thickness copper (that covers the
    entire area of the PCB), separated by a standard FR4 (epsilon = 4.5)
    insulating layer, thickness around 0.2 mm.

    I have a power supply, located say at X = X0 and Y= Y0), (X, Y)
    coordinates related to the position in the 50 x 40 cm plane. Each
    lead of the power supply connected to a different copper plane.

    I have a load, located say at X1, Y1 in the PCB plane.Each lead of
    the load connected to the corresponding copper plane.

    My goal is to calculate the inductance between the source and load
    current path.

    I know the problem is basically a tridimensionnal one (Z being the
    axis orthogonal to the PCB. I did a quick 2D simulation with femm, of
    a (X,Z) cross-section (two layers with equal and opposite currents,
    basically like two 17.5µm x 10 cm copper sections, separated by 0.2
    mm air gap). I get around 36 nH/m for a 10 cm wide strip, wich I find
    acceptable as an order of magnitude.

    Now, I need to get further by transposing the results on the real
    current path in the PCB. I imagine that the current density in XY
    plane is similar to a 2 dipole electrostatic E field problem...
    (probably could be obtained by hand ?) Can I get the results with
    femm ? When I get the current density on each XY coordinate, can I
    sum each inductance contributions using the previous femm analysis ?

    Probably this kind of problem is trivial for some of you and I would
    be very gratefull for any hint (using femm or any combinaison of
    tools, or a link) that allows to get closer to the solution.

    Thank you in advance

    Christian Néel

    France

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