Author Topic: How many layers can you get in a pcb board. (Read 34551 times)

G7PSK · « **on:** February 03, 2012, 12:24:55 pm »

I was not sure if this should go in the blog specific or general so as not tread on any toes I will put this here. In the late 80's I had an exhibit at a government trade show a few stands down there was an electronics company that made military equipment. What caught my eye was a pcb that they were showing to any one who even hesitated when passing their stand. The board in question had 35 layers and was about 20mm thick and also had components inside it, the salesman was saying that they could make a board to any thickness and with as many layers as you wanted,is this just something that was aimed at the military or are such boards used elsewhere. I have not seen such a board since.

Psi · « **Reply #1 on:** February 03, 2012, 01:14:21 pm »

I can't say i've seen more than 10 layers.

Probably just a military thing.

Or maybe for super high speed circuits with an unusually high number of supply rails.
Where an entire layer for each rail is required with ground layers in between as well as normal signal routing.

Rerouter · « **Reply #2 on:** February 03, 2012, 01:28:57 pm »

up to 16 is the normal maximum i would have thought, more than 4 layers being for bga escape routing, (getting the pads to the outside), if you where hitting as high as 35 layers, then that is either a layout job done wrong, or one heck of a high pin count device,

also i personally hate when components are place inside of layers, nearly impossible to repair,

mariush · « **Reply #3 on:** February 03, 2012, 01:50:26 pm »

Most video cards are 6-8 layers... cards like GTX590 use up to 12 layers, as mentioned here : http://www.tomshardware.com/reviews/geforce-gtx-590-dual-gf110-radeon-hd-6990,2898.html

AFAIK most motherboards are up to 6 layers.

eliocor · « **Reply #4 on:** February 03, 2012, 02:50:42 pm »

I have seen 22 layers boards: they were used as windings for switching power supplies transformers.
Their ferrite core had a very low profile and the two "shells" were mounted using some slots milled on the PCB.

jahonen · « **Reply #5 on:** February 03, 2012, 03:14:53 pm »

I have worked on 16 layer PCBs. It should be noted that usually about half of stated layers are dedicated ground/supply planes, and contain no signal traces.

Practical limit comes from minimum track width and impedance requirements, since more layers require thinner dielectrics and that means that for constant track width, impedance will drop. One won't usually want go much below 50 ohms, since below that starts to be too low for ordinary logic circuits. Maintaining proper impedance requires making traces thinner, which is limited by the minimum track width.

Of course, PCB can be made thicker, but then aspect ratio requirement (via length/via diameter) comes into play.

Also, soldering components on thicker boards can be surprisingly difficult since massive PCB absorbs a lot of heat.

Regards,
Janne

G7PSK · « **Reply #6 on:** February 03, 2012, 03:56:04 pm »

Having had a look at the link and seeing that the cores on the chips contain 3 billion transistors each.It left me wondering how anybody can design such a circuit as I find myself getting lost with half a dozen transistors in a single circuit. Are these sort of things completely designed by computers.
I guess the 35 layer boards are a military thing as they tend not to be bothered with things like cost or serviceability.

mikeselectricstuff · « **Reply #7 on:** February 03, 2012, 04:17:47 pm »

Quote from: G7PSK on February 03, 2012, 03:56:04 pm

Having had a look at the link and seeing that the cores on the chips contain 3 billion transistors each.It left me wondering how anybody can design such a circuit as I find myself getting lost with half a dozen transistors in a single circuit. Are these sort of things completely designed by computers.

Mostly - the vast majority will be memory, and it will all be generated, placed and routed by some very expensive software from the HDL descriptions written by the designers.

Neilm · « **Reply #8 on:** February 03, 2012, 04:29:55 pm »

The highest layer count I have ever used was 24 - It was basically an interconnection board between the outside world and internal PCBs. It did carry quite high currents and so layout was critical to ensure that the transients on these high current layers didn't get anywhere they shouldn't. There were a few layers that were 0V or chassis to act as screens for signals.

I seem to remember that we also used buried capacitance with in that board to provide some measure of decoupling to the few circuits that were on it.

Neil

ejeffrey · « **Reply #9 on:** February 03, 2012, 10:15:35 pm »

Almost every factor in PCB design in the past 20 years has reduced the need for lots of layers. The most significant advance is in increased integration. Between 'system on a chip' style devices and programmable logic the number of chips on a board has decreased dramatically. Surface mount devices also dramatically reduce the need for extra layers since the pads only create obstructions on the top layer. Improved design rules, and the prevalence of serial vs. parallel interconnects have also made a difference. If you are routing a board stuffed with 500 DIP packages placed side to side with design rules that only allow one track between pins you are going to need a lot more layers. With modern circuits you need enough for BGA escape, and that is it. Beyond that, multiple layers are for things like segregating analog and digital layers, not simply the minimum routing requirements.

tekfan · « **Reply #10 on:** February 04, 2012, 12:45:03 am »

Quote from: ejeffrey on February 03, 2012, 10:15:35 pm

If you are routing a board stuffed with 500 DIP packages placed side to side with design rules that only allow one track between pins you are going to need a lot more layers.

Wire wrap anyone?

ChrisW · « **Reply #11 on:** February 04, 2012, 01:09:19 am »

The most I have seen advertised is 26 layers;

http://www.entechelectronics.com.au/product-detail/pcb-fabrication?gclid=CJj_kZyRg64CFYMmpAod1XE92Q

EEVblog · « **Reply #12 on:** February 04, 2012, 06:02:38 am »

Quote from: eliocor on February 03, 2012, 02:50:42 pm

I have seen 22 layers boards: they were used as windings for switching power supplies transformers.
Their ferrite core had a very low profile and the two "shells" were mounted using some slots milled on the PCB.

I've done a board exactly for that reason, and from memory it was in the 20+ layer category.

It's not uncommon to have 16-20+ layers on really big and complex boards, not so much for signal escape from 1000+ pin BGA's (you can do that on 8-10 layers usually) but just so that you can keep things entirely separate and have massive copper planes to carry ground and various power rails, and still get tons of signals wherever you want with good signal integrity. It's not uncommon for advanced designs to have 8 or more power rails.
I've done backplane/system board designs that have 12-14 layers and they have nothing more than connectors on them, but other boards chock full of huge FPGA's and massive of other circuitry I squeeze onto 6-8 layers.

Dave.

amspire · « **Reply #13 on:** February 04, 2012, 06:08:24 am »

Just had a look at the PCBCORE website, and they can do up to 30 layers, and board thicknesses between 0.2mm (2 layer) and 8mm.

Maximum copper weight is 20 oz, which sounds impressive.

Richard

SeanB · « **Reply #14 on:** February 04, 2012, 02:53:19 pm »

Military electronics cost so much that they are cost effective to repair. Fun was a board which consisted of DIL packages soldered in to through holes, with chips on both sides of the board. All on a 0.1 spacing, and made so that repair was near impossible. If you traced the fault to a particular IC, the only way was to cut the legs off at the body, and pull it out/ Then you cut the lower sections of the new IC legs off and solder it to the legs left in the board. Only board I ever met where a board you held in the palm of your hand would weigh over 1 kg. Don't remember the number of layers on the board, but there was a flexi board connecting the backplane to the panel sockets. That was made up from over 20 individual boards soldered to the backplane and the various sockets. Not fun if there was a broken pin in a socket.

elmohandis · « **Reply #15 on:** May 24, 2012, 01:06:03 pm »

In the words of a Bungard representative talking about their multi-layer PCB press machine: "It's not about how many layers it can do, it's how experienced you are".

Basically the sky's the limit, but careful work needs to go into the actual PCB fabrication for a many layer board.

Analyzing the signal integrity issues on a 16 layer board is going to be pretty nasty however.

Regards

poorchava · « **Reply #16 on:** May 28, 2012, 04:29:10 pm »

There are companies making commercial PCBs with embedded passives (mostly caps and resistors). This is advertised mostly as improved board packing density, improved signal integrity and lower costs (ya.... sure... i wonder at what quantities...). They also supposedly have superior thermal parameters (which kinda seems true given that any passive can dissipate power in every possible direction). I know that Wurth Elektronik in Germany does something like that (no idea about costs though)

free_electron · « **Reply #17 on:** May 28, 2012, 05:52:54 pm »

you can have resistive layer sinside a pcb. instead of applying a copper foil they apply a metaloxide film and then etch that one...

AlphZeta · « **Reply #18 on:** July 21, 2015, 02:14:41 pm »

I recently took apart an old Cisco switch (the switch was made back in the early 90's), it used a 22-layer PCB. More teardown photos can be found here (http://www.kerrywong.com/2015/07/18/teardown-of-a-cisco-css-11503-content-services-switch/).

It must be very expensive to make these boards back then.

TSL · « **Reply #19 on:** July 22, 2015, 12:15:48 am »

High density boards where the norm in big compute in the late 90's a good example being Sun's E10k computer co-designed with Cray.

The centre plane had 28 layers, and the system boards had 24 layers and each weighed 30kgs!.

Being a sysadmin at the time it was a two person job to swap out a CPU!!

More info for the center plane...

"The centerplane holds the 20 address ASICs and 14 data ASICs that route information between the 16 system-board sockets. It is 27” wide x 18” tall x 141 mils thick, with 14 signal layers and 14 power layers. The net density utilization is nearly 100%. Approximately 95% of the nets were routed by hand. There are 14,000 nets, approximately two miles of wire etch, and 43,000 holes."

More detail here...

http://no.spam.ee/~tonu/starfire.pdf

regards

Tim

nitro2k01 · « **Reply #20 on:** July 22, 2015, 12:25:32 am »

Quote from: AlphZeta on July 21, 2015, 02:14:41 pm

I recently took apart an old Cisco switch (the switch was made back in the early 90's), it used a 22-layer PCB. More teardown photos can be found here (http://www.kerrywong.com/2015/07/18/teardown-of-a-cisco-css-11503-content-services-switch/).

It must be very expensive to make these boards back then.

Is that really 22 copper layers? I see the numbers 22, 20, 18...

AlphZeta · « **Reply #21 on:** July 22, 2015, 12:59:25 am »

Yep, here is the picture of the other side of the board. There are more tear down pictures on the website.

jeroen74 · « **Reply #22 on:** July 24, 2015, 08:24:03 pm »

http://sanmina.com/components/printed-circuit-boards/index.php

edit: they claim 70+ layers

free_electron · « **Reply #23 on:** July 24, 2015, 08:37:13 pm »

doing a 22 right now...

IconicPCB · « **Reply #24 on:** August 01, 2015, 08:03:19 am »

the largest we have manufactured for a client was an eight layer 3 ounce design, a low profile transformer board.


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Author Topic: How many layers can you get in a pcb board. (Read 34551 times)

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