Allow an odd layer number

[Hildo Guillardi Júnior]

Allow an odd layer number in Pcbnew. Currently it just allow even copper layer quantity for create the pair logic.

This request/enhancement come up from question https://answers.launchpad.net/kicad/+question/680850

Application: Pcbnew
Version: 6.0.0-unknown-a32bb4e~86~ubuntu16.04.1, release build
Libraries:
    wxWidgets 3.0.2
    libcurl/7.47.0 OpenSSL/1.0.2g zlib/1.2.8 libidn/1.32 librtmp/2.3
Platform: Linux 4.4.0-146-generic x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.2 (wchar_t,wx containers,compatible with 2.8) GTK+ 2.24
    Boost: 1.58.0
    OpenCASCADE Community Edition: 6.8.0
    Curl: 7.47.0
    Compiler: GCC 5.4.0 with C++ ABI 1009

Build settings:
    USE_WX_GRAPHICS_CONTEXT=OFF
    USE_WX_OVERLAY=OFF
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_PYTHON3=OFF
    KICAD_SCRIPTING_WXPYTHON=ON
    KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=ON

[Wayne Stambaugh (stambaughw)]

The other option would be to use the old standby method of creating a four layer board and not adding anything to one of the inner two layers. Of course you have have to be careful not to route on the unused layer and to send the correct inner layer to have your boards made. It's not very elegant but it can and has be done. As for supporting an odd number of layers, the only odd number of layers I've ever seen used on a board design is 3.

[Hildo Guillardi Júnior (hildogjr)]

Yes, @Wayne I am doing this.
It is not elegant but works fine.
I think the DRC could warning the user "there a copper layer created without any element". So in my workaround, I will know if is right and may be useful in other situations (the design forgot something...).

[jean-pierre charras (jp-charras)]

For my info, what is the reason to ask for 3 layers instead 4 layers?
(AFAIK, the price to fabricate the board is the same)

If, for some reason, you do not want tracks and zones on a given layer, just add a keepout zone for tracks and zones on this layer.

No need to change the DRC.

[Hildo Guillardi Júnior (hildogjr)]

My housing in Brazil use different price for 3 and for 4 layers.

The DRC doesn't need to change, but add a checking of empty copper layer to warning the user that could be created empty Gerber files.

[Jon Evans (craftyjon)]

3 layer (and odd or unbalanced layer stacks in general) is also reasonably common in flexible circuits.

[KiCad Janitor (kicad-janitor)]

KiCad bug tracker has moved to Gitlab. This report is now available here: https://gitlab.com/kicad/code/kicad/-/issues/2425