16
\$\begingroup\$

This is a question purely out of curiosity.

I have several various PCBs, and I'm curious as to how many layers they use, which helps me decide how many layers to use in my project (for example a cheap competing product using 4 layers suggests to me it is possible to do my product with a 4 layer design).

From looking at the vias (and where they connect to), I can usually determine if they have 2 layers or more, and I can tell apart single layer boards too because they don't have a top or a bottom side, however, I've never been able to actually count the number of layers a more complex board like a motherboard or graphics card uses.

Any clues?

\$\endgroup\$
16
\$\begingroup\$

As far as non-destructive methods, you may be able to shine a bright light into the edge and through a corner see the copper planes. Signal traces may be hiding though, and this only works if the copper comes fairly close to the edge, which it may not.

Using a bright light, it's easy to see if a board has inner layers even if it doesn't have blind vias. Find some place on the board where there aren't traces/planes on the visible, outer layers and see if you can see light through it. If it's blocked in some places, that's probably copper. Even without that, most multi-layer boards I've dealt with using the standard green LPI have a darker appearance than their 2-layer counterparts.

If the board designer had space to kill or the manufacturing engineer wanted for some reason (not sure why), I've also seen boards with a "stair-step"; each layer labeled with a copper number and cutouts in the other layer to be able to see it. This might be unique to my old company though, as our layout tech did all sorts of strange things.

| improve this answer | |
\$\endgroup\$
  • 2
    \$\begingroup\$ one of my past employers did the same stair-step thing. It made it easy to tell how many layers were on the board and (I suppose) if a layer was left off, that would have been obvious as well. Don't know what else it was good for. \$\endgroup\$ – Michael Kohne Mar 3 '11 at 3:15
  • 4
    \$\begingroup\$ The stairstep also makes sure the layers are in their proper order. If you are not using blind/buried vias, the connectivity will be the same regardless of the layer order. But there may be subtle problems like high-frequency traces with the wrong characteristic impedance if the layer stack is messed-up. Still, this is an unlikely thing for the board house to get wrong. Sometimes board designers just add features because they look cool. I know I do. \$\endgroup\$ – markrages Mar 3 '11 at 16:07
  • 1
    \$\begingroup\$ layer stack-up text (also called a "layer name block"). \$\endgroup\$ – davidcary Apr 1 '11 at 14:14
  • 1
    \$\begingroup\$ @markrages - in our case, I wouldn't rule out suspicion of the board house, or the outside layout guy, or simple paranoia on the engineer's part. He used to verify the Gerber files against the schematics by hand before sending to layout. \$\endgroup\$ – Michael Kohne May 12 '11 at 20:06
  • 1
    \$\begingroup\$ I was burnt by layer numbering today, make sure that your "1" can't be mistaken as a marker for pin 1 on some nearby connector \$\endgroup\$ – Toby Jaffey May 12 '11 at 20:19
6
\$\begingroup\$

Cut the board in half and inspect the copper.

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ It's the only way to be sure. \$\endgroup\$ – Nick T Mar 3 '11 at 3:06
  • \$\begingroup\$ @kellenJB, When I saw the question I was coming to write this answer. \$\endgroup\$ – Kortuk Mar 3 '11 at 3:39
  • 3
    \$\begingroup\$ Couldn't you just take a notch off the corner? \$\endgroup\$ – endolith Mar 3 '11 at 4:15
  • \$\begingroup\$ @endolith, yes, the concept is the same, I think kellenjb is just making a point that you can cut into it and know for sure. \$\endgroup\$ – Kortuk Mar 3 '11 at 9:22
  • \$\begingroup\$ @endolith, @Kortuk, if you only notch a corner, you might miss signal planes that don't have traces there. \$\endgroup\$ – Nick T Mar 3 '11 at 20:48
1
\$\begingroup\$

I don't know about other PCB's but all of our boards have little numbers on the corner designating what layer it is, so the top has a 1 and the back has an 8 or something, you can see the hints of other numbers too.

| improve this answer | |
\$\endgroup\$
  • 1
    \$\begingroup\$ Do you have a picture of this? \$\endgroup\$ – user103380 Jul 19 '18 at 20:33
  • 1
    \$\begingroup\$ When I designed 4 and 6 layer PCBs, I added these numbers to each layer. But not every designer does it and not every PCB has some free space to add those numbers. \$\endgroup\$ – Uwe Jul 19 '18 at 21:11
  • \$\begingroup\$ @KingDuken here's an example of a board I was examining, which does appear to have such a designation. This really helps, because almost the whole of front and back surfaces are covered with copper, so bright light wouldn't help seeing internal layers. \$\endgroup\$ – Ruslan Apr 18 at 18:57
0
\$\begingroup\$

Normally all PCB's have a little number in the corner designating what layer it is, L1 in the picture below for the top side of the PCB and L6 for the bottom layer,so this PCB has 6 layers.

enter image description here enter image description here

| improve this answer | |
\$\endgroup\$
  • 3
    \$\begingroup\$ What does "normally all" mean? I could not find this number on any of the boards on my bench. \$\endgroup\$ – Elliot Alderson Feb 12 at 17:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.