# How many layers in a multi layer board?

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

1. ### R.SpinksGuest

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. ### Dr. Anton T. SqueegeeGuest

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
micrometer.

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 -- www.bluefeathertech.com
"If Salvador Dali had owned a computer, would it have been equipped
with surreal ports?"

3. ### David HarmonGuest

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 BettsGuest

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.
otherwise

if you can cut it in half do that

otherwise you could try Xrays etc....

Bye.
Jasen

5. ### ImpmonGuest

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. ### Matt J. McCullarGuest

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. ### PeteSGuest

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.

Cheers

PeteS

8. ### PeteSGuest

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
micrometer.>>

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

Cheers

PeteS

9. ### PeteSGuest

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)

Cheers

PeteS

10. ### R.SpinksGuest

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

11. ### R.SpinksGuest

Thanks to all who replied. This is very helpful info.

12. ### Rich GriseGuest

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!
Rich