Is there a limit to how many layers can you get in a PCB board?

[Connecteur]

I don't think so, but I'm not able to visualize an application for a PCB with more than 30 layers.  I don't think it's like building a skyscraper, where doubling the floors multiplies the difficulties almost exponentially.  I think it's just basic addition for the most part.  You should be able to stack the layers as high as you like with few limitations, would be my guess.

Components could be mounted on internal layers, (in notched-out chambers) but heat considerations might necessitate liquid or Peltier cooling.  I suppose there might be a case where circuit-density is everything, (like maximum components per cubic foot or something) but I can't think of it.  Of course practical circuit repair would probably be out of the question.

[Mr.B]

Bookmark.
I am very interested to hear answers to those questions also.

[AlfBaz]

I'm also just chiming in to get this thread showing up in my posts link

I thought I once heard Dave, or somebody, talking on the amp hour about 30+ layer boards

[ez24]

Tom says almost 100

http://www.tomshardware.com/reviews/printed-circuit-board-primer,353-3.html

Edit - These guys can make 50

http://www.7pcbassembly.com/files/pcb-specifications.pdf

Also  "almost 100"
http://www.qualiecocircuits.co.nz/multilayer-pcb.htm

So my guess the number of layers is "almost" 100   

[Harvs]

mikeselectricstuff peeled a 30-layer motherboard.

[T3sl4co1l]

Only by thickness.  Insulating layers can't be made much under 5 thou (0.005") thick, maybe 3 thou at the lowest.  The copper foil has to be correspondingly thin as well, or else the resin won't be able to flow into the gaps left between traces.

Resin is also part of the reason why heavy copper PCBs have looser design rules.  That, and the precision of etching and plating such heavy layers in the first place.  The error in position of an edge, is about equal to the height of the foil.  So, 2oz copper is fine for most applications (i.e., including 0.5mm pitch ICs), but heavier (3oz, 4oz, heavy) need to use coarser design rules and components.

A typical multilayer planar transformer (i.e., one made with copper foil traces for windings) has a winding factor of around 0.2.  That is, there's several times more insulation thickness than conductor thickness, and the conductor width to conductor space ratio is only modest (depending on number of turns -- a single turn doesn't need any gap, while maximum number of turns is fixed by minimum trace width and space).

Tim

[langwadt]

and as the PCB get thicker you need to have bigger holes for vias etc. to reliably get them plated 

[tszaboo]

I think the thickest PCB I ever saw was some jet fighter controller board (F18 maybe). That thing was very thick, as I recall, it might have been 12.7mm (in yankee units). Going from 24 to 30 layers have a very small impact on the price. I guess going from 30 to 60 is already the price does not matter category. At least you dont have to worry about board warping and mechanical stresses anymore.

[Jeffw5555]

I worked designing military electronics for aircraft in the 80's, the thickest board we designed was almost a half inch thick and 24 layers (that was the mil spec limit back then), the only reason it was this thick was because it had buried stripline conductors for the microwave signals, and the dielectric needed to be thicker than you would normally do, and it had mixed dielectric (teflon and epoxy resin)

[CatalinaWOW]

There is a complicated dance here.  The only fundamental limit on layer count is the capacity of the press used to laminate them.  That limit is well over 100 layers.  But there is very little reason to have lots of layers unless you have componenents with lots of connections (BGAs, TQFAs, high density connectors or the like).  But when you have these kinds of components you need fine traces and spacings.  Which makes lots of layers tough, for the reasons mentioned earlier.  There are a few other special cases like the inductive components mentioned before.  Other tricks like partitioning the board into layer groups with small numbers of large geometry connections between the layer groups would make large layer counts possible, but the real problem is finding a good reason to do it.

My experience is a few years out of date, but at that time we were able to get good yields in four digit quantities of 22-26 layer boards.  Our vendors promised us boards from 30-38 layers, but yields were effectively zero.  Different design rules would give different results, but I would say that you need to look very carefully at high layer count.  In my mind high starts somewhere in the 16 range and gets really dicey as you go over 30.

[Mr.B]

I think the thickest PCB I ever saw was some jet fighter controller board (F18 maybe). That thing was very thick, as I recall, it might have been 12.7mm...

Does anyone have any tech-porn pictures they can share?
A google search turned up nothing spectacular for me...

[tszaboo]

I think the thickest PCB I ever saw was some jet fighter controller board (F18 maybe). That thing was very thick, as I recall, it might have been 12.7mm...

Does anyone have any tech-porn pictures they can share?
A google search turned up nothing spectacular for me...

https://blog.adafruit.com/2013/12/10/wierd-military-computer-boards/

I saw the one I was talking about on Embedded world.

[Jeffw5555]

I think the thickest PCB I ever saw was some jet fighter controller board (F18 maybe). That thing was very thick, as I recall, it might have been 12.7mm...

Does anyone have any tech-porn pictures they can share?
A google search turned up nothing spectacular for me...

I have a few wacky boards with technologies that most people outside of the military aircraft world wouldn't have heard about.  Any boards that generated heat had thick metal cores in the center to move the heat to an exchanger (air cooling doesn't work well at 80,000 feet)  I have a few of those boards stashed in a box somewhere, they were part of the power supply for that application.  Very simple boards with few components, just wacky looking.

I don't have any samples of the half-inch thick boards, that design was a large motherboard designed to connect a large number of digital and microwave daughter cards.  I didn't take any of those samples home because some of the buried stripline concepts in that design were considered secret technology at the time.  Perhaps still, I don't know. 

[RobK_NL]

I have a few wacky boards with technologies that most people outside of the military aircraft world wouldn't have heard about.

Pics, or it didn't happen. 

[greyes]

I designed boards 40+ layers .250" thick with blind, buried, and back-drilled vias. That was almost 9 years ago! The high layer count was due to having ground layers between every signal layer. The company, Harbor Electronics in Santa Clara, was a fab house specializing in ATE boards.  Not sure if they are still called that - I think they have been bought out a few times. I have a 30lyr round board in a box somewhere in my garage I plan to make an analog clock out of! LOL!

[montemcguire]

One of the reasons for having a lot of PCB layers is to route ball grid array packaged (BGA) semiconductors. Each row of balls needs its own layer to route a signal away from the package, so this can easily add 4-6 layers right away. These fanouts also require blind vias, so aside from the extra layers, BGA parts also require blind vias, usually very small ones, so the cost of the PCB skyrockets quickly.

Your average smartphone uses PCBs like that, but they can get away with it because of their high manufacturing volumes. For small guys like myself, I cannot afford much more than a 4 layer board with pretty huge 'all the way through' vias. So, I never use BGAs because I can't afford the PCB process they require.