How many layers in a multi layer board?

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.

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

On Sat, 8 Oct 2005 23:00:50 -0400 in sci.electronics.basics, "R.Spinks" wrote,

The answers have not changed since last week when you asked this same question.

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.

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

Cheers

PeteS

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

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...

is.

if

looking

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

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

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

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