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How many layers in a multi layer board?

Discussion in 'Electronic Basics' started by R.Spinks, Oct 9, 2005.

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  1. R.Spinks

    R.Spinks Guest

    What are the typical methods for determining how many layers a PCB board is.
    Like after you take something apart and see the PCB ... how can you tell if
    it's multi layer and how many layers it is. I tried looking at the side
    profile of the board ... it's FR4 but that's all I can tell by just looking
    at it.
  2. Not particularly easy to tell just from looking. The most reliable
    method is to contact the manufacturer of the board, and find out how
    many layers they used.

    Assuming you know the thickness of each layer, you could try
    measuring the overall thickness of the board with a digital caliper or

    As a rule of thumb: If you hold the board in question up to a
    brightl light, and you cannot see said light through non-plated areas of
    the board, you're dealing with at least a four-layer piece, possibly
    more. I will add that you may see light around (or through) some of the
    vias or unused component holes. Don't assume that the board is only two-
    layer as a result. Check the unplated areas.

    I hope that helps. Keep the peace(es).

    Dr. Anton T. Squeegee, Director, Dutch Surrealist Plumbing Institute.
    (Known to some as Bruce Lane, ARS KC7GR,
    kyrrin (a/t) bluefeathertech[d=o=t]calm --
    "If Salvador Dali had owned a computer, would it have been equipped
    with surreal ports?"
  3. David Harmon

    David Harmon Guest

    On Sat, 8 Oct 2005 23:00:50 -0400 in sci.electronics.basics,
    The answers have not changed since last week when you asked this
    same question.
  4. Jasen Betts

    Jasen Betts Guest

    assuming you know there's atleast one ground plane... (otherwise hold it up
    to the sun)

    look for clues like "layer 5" written in copper.

    if you can cut it in half do that

    otherwise you could try Xrays etc....

  5. Impmon

    Impmon Guest

    Unfortunately there's no easy way to tell just by looking at it. Some
    boards have come in 4 and 6 layers and there may be some with more
    layers out there.

    Short of destroying the board, it'll be hard.
  6. Don Lancaster wrote in one of his columns back in the '80s that boards with
    as many as 24 layers had been manufactured. No info on how thick these were
    or anything...
  7. PeteS

    PeteS Guest

    I've personally done 16 layers on 2.4mm boards (with 10 of those
    impedance controlled) and 28 layers on a rather thicker board
    (backplane, 5mm).

    The board thickness rarely is a decent clue as to the number of layers
    - 2.4mm, 1.6mm and 0.8mm are standard board thicknesses for single
    sided to however many layers can fit and are necessary.
    In addition, where BGAs are concerned, the board thickness is
    constrained by the drill size required for signal breakout (rule of
    thumb is min drill size >= 1/8 of board thickness, although 1/10 can be
    done at higher cost due to yield issues). So if you have a board that
    requires a maximum drill size of 0.01 inch (10 thou), you are
    constrained to a 0.1 inch thick board or less. That drill size is a
    common requirement on large BGAs (I've put down 1600 ball devices
    before now).

    As noted a week or so ago, the only real way to know is to do a cross
    section preferably at a via.


  8. PeteS

    PeteS Guest

    Dr. Anton T. Squeegee wrote:

    << Assuming you know the thickness of each layer, you could try
    measuring the overall thickness of the board with a digital caliper or

    The layer thickness in impedance controlled boards is not identical
    layer to layer, so that's not really a useful methodology.


  9. PeteS

    PeteS Guest

    As I don't have a huge amount to do while dinner cooks...

    (View with fixed font)

    An introduction to multilayer board fabrication

    For any board 4 layers or more, the stack actually looks like this (4
    layers of copper shown)

    Top Copper -----------------------------------
    prepreg -----------------------------------
    Layer copper -----------------------------------
    core -----------------------------------
    Layer copper -----------------------------------
    prepreg ----------------------------------
    Bottom copper -----------------------------------

    Prepreg is a glue like substance, with different electrical
    characteristics (especially dielectric constant) to the core (FR4 in
    the OP case). What's interesting is that on 4 layer boards or more, the
    copper on the outer layers is literally glued onto the board.

    Boards are usually symmetrical (otherwise they warp due to uneven
    copper content) from the centre outward.

    Here was my 16 layer board:

    Copper layer number to the right. (Z) means it was an impedance
    controlled layer.

    Top(Sig) -------------------------- 1 (Z)
    Prepreg ---------------------------
    Plane --------------------------- 2
    Core ---------------------------
    Sig --------------------------- 3 (Z)
    Prepreg ---------------------------
    Plane --------------------------- 4
    Core ---------------------------
    Sig --------------------------- 5 (Z)
    Prepreg ---------------------------
    Plane --------------------------- 6
    Core ---------------------------
    Sig --------------------------- 7 (Z)
    Prepreg ---------------------------
    Sig --------------------------- 8 (Z)
    Core ---------------------------
    Sig --------------------------- 9 (Z)
    Prepreg ---------------------------
    Sig --------------------------- 10(Z)
    Core ---------------------------
    Plane --------------------------- 11
    Prepreg ---------------------------
    Sig --------------------------- 12(Z)
    Core ---------------------------
    Plane --------------------------- 13
    Prepreg ---------------------------
    Sig --------------------------- 14(Z)
    Core ---------------------------
    Plane --------------------------- 15
    Prepreg ---------------------------
    Sig --------------------------- 16(Z)


  10. R.Spinks

    R.Spinks Guest

    Had someone answered last week I wouldn't have posted it again. Kindly,
    other folks actually gave some constructive answers this time.
  11. R.Spinks

    R.Spinks Guest

    Thanks to all who replied. This is very helpful info.
  12. Rich Grise

    Rich Grise Guest

    Hold it up to the light, and you'll be able to see if it has embedded
    traces or power/ground planes, unless it's already got full pours on
    the outside layers.

    Good Luck!
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