A 40 layer PCB? What for? - Page 1
Forum section: electronics / general

author: Artlav
author_rank: Frequent Contributor
timestamp: September 28, 2014, 08:00:27 am
content: I don't normally read spam.But occasionally a message happens to sound interesting enough to look through.In this case it was advertising a Chinese PCB manufacture house, with a list of capabilities.And right on the top of the list was: Layer Count: 8-40 Layers.Huh?What can you do that would need a40 layerPCB?4 layer is common, 8 layers for something like a motherboard, 16 layers for a all-in-one little cellphone or a 100 IC per board special equipment, but what would it take to need 40?Is that a real thing, or the usual doubling of the advertised numbers?

author: Psi
author_rank: Super Contributor
timestamp: September 28, 2014, 08:03:58 am
content: It's for things heavy in high throughput number-crunching or analysis, where you might have 20x highend FPGA/ASIC densely packed on both sides of a PCB.Milspec stuff can use that many4G LTE cell tower electronicsFiber optic wire tapping and processing of the data, NSA spy shitetc..Some of the really big FPGAs need quite a few layers just to fan out all the BGA pins. If you have a big FPGA on each side of the PCB you might need 4 layers top and bottom to fan out the pins plus 8 layers between those for your signals and power. So you can see how things quickly add up.You can also embed SMT components inside the PCB substrate to save space. This is something you might have to do for decoupling caps if you had two FPGAs directly opposite each other on the pcb.So that's a few more layers again.This company can even do 72 layerhttp://jp.fujitsu.com/group/fict/en/services/high-layer.htmlIf you scroll to the bottom there's some pictures.

author: SeanB
author_rank: Super Contributor
timestamp: September 28, 2014, 08:18:32 am
content: 24 layer boards for high density interconnects and backplanes are common, with around half being power and ground planes, with the rest being controlled impedance signal lines. Some even have ultra thin insulation between power and ground planes to give a higher capacitance so that they can get around 10nF or so of distributed capacitance for the power planes for decoupling, having bulk decoupling along the board as well. They can be somewhat difficult to rework.I can see higher layer counts for having multiprocessor boards, where you need to have large buses fed out to support chips in a small area. That way you can do track matching and via length matching so all are the same length from processor die edge to support die edge to reduce skew. you can probably also bury small components inside the board like series termination resistors using inner layer cutouts that are filled part way during the board stack layup then finally heat cured so the soldered connections are done as well with no clean solder paste preforms.

author: VK3DRB
author_rank: Super Contributor
timestamp: September 28, 2014, 09:28:33 am
content: Old technology. IBM has been doing 30+ layer boards for over 30 years. The famous Thermal Conduction Module used in the 3081 mainframe had up to 33 layers (on a ceramic PCB) way back in the early 1980's. Some RS6000 processor boards in the 90's with their large PGA's had 24 layers if I recall correctly.For simpler low cost boards, from my experience it is far more challenging to route a 2 layer board with complex circuitry than a 4 or 6 layer. Extra layers gives you the added benefit of having ground planes or power planes on the board, freeing up signal lines for a more direct route. With 2 layers, keeping vias to a minimum can be challenging.In any case, the artwork should be treated with attention to detail; laid out using a high level of stubbornness, cognitive ability, knowledge - and of coursePCB Design Standardswhich all good companies should have and adhere to.

author: EEVblog
author_rank: Unknown
timestamp: September 28, 2014, 10:37:32 am
content: Quote from: Psi on September 28, 2014, 08:03:58 amI've done FPGA boards like that. 8 layers is usually an absolute minimum at the big high end fully utilised FPGA level just for fanning out and routing, with 10-12 layers being a most common minimum once you throw in a few split supply and controlled impedance requirements. Add in a few extra requirements and the layer count can rise again.And yes, double that for high density double sided stuff. I've done into the 20's on some military boards, and that's not considered a lot.I've done into the 10's just for backplane power distribution stuff.So yes, there is a big need for this stuff, and it's more common than you might think.

author: Circlotron
author_rank: Super Contributor
timestamp: September 28, 2014, 11:03:01 am
content: Planar transformers as used in fancier SMPSs can have a single turn as one layer, the jump to the next later for the next turn etc. This is for a transformer who's winding is a helix, not a flat spiral.

author: EEVblog
author_rank: Unknown
timestamp: September 28, 2014, 11:15:20 am
content: Quote from: Circlotron on September 28, 2014, 11:03:01 amI did that in a military PSU that went inside a helium filled (for thermal conductivity) metal can. IIRC it used a 20 layer board for the transformer windings, with the bobbins going through slots in the board. Only really needed 2 layers, but the 20 was used to get the winding and that was it. Expensive, but low profile. And cost doesn't matter when you are only making hundreds. Far more expense went into the helium filling and qualification and thermal testing each unit.

author: amyk
author_rank: Super Contributor
timestamp: September 28, 2014, 01:23:50 pm
content: I think what they're saying is "we'll put on as many layers as you want, just don't expect the results to be very good..."Here's another Chinese one I found claiming 50 layers:http://www.china-fastpcb.com/About/?4-1.html...and there's several around, like this one, that are claiming100layershttp://www.kingshengpcba.com/event/133.htmlI wonder if there's a world record for number of layers ever produced on a PCB...

author: SeanB
author_rank: Super Contributor
timestamp: September 28, 2014, 01:33:06 pm
content: Depends, you probably can go up to 1 inch thick and have no problems other than a low yield of good boards, and with a need to have full length leads on resistors and capacitors, and use wire wrap sockets for the IC's and other parts with short legs, unless you are fine with the via only being partially filled with lead. That the board will need reflow soldering with a very slow ramp up to get the inner layers to melting will be an issue with some components and will also have a good chance of warping.

author: Artlav
author_rank: Frequent Contributor
timestamp: September 28, 2014, 01:39:40 pm
content: Quote from: SeanB on September 28, 2014, 01:33:06 pmIsn't warping the more an issue the thinner the PCB, not the other way around?

author: SeanB
author_rank: Super Contributor
timestamp: September 28, 2014, 01:44:15 pm
content: Long heat soak, and then possibly fast cooling means you will get warpage, that will be a bugger to straighten. Though at 1in thick you probably could put everything inside the board and then cure it in a heated press.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2014, 04:14:01 pm
content: I once inquired about multilayer 'heavy' PCB for building a particularly compact, high capacity transformer. The project never went anywhere though.FWIW, even in heavy copper processes, the insulator layers can't be very thin (it needs to be able to squeeze out into gaps between traces, I think, else voids occur), so whether you're doing a conventional multilayer build with 1-2oz foil, or 20+ mil thick copper in a heavy process, your 'winding factor' (ratio of copper cross sectional area to theoretical winding area) remains in the 0.25 range or thereabouts.Also, from what I saw of quotes, you don't save much value in using thicker or more layers of PCB.Tim

author: Circlotron
author_rank: Super Contributor
timestamp: September 29, 2014, 12:19:42 am
content: Seems to me there is something of a parallel between high level programming languages and the need for gigabytes of ram, and autorouters and the need for forty layer boardsIf there was enough time and patience and skill available (or like in the old days when there was no other option) two or maybe four layers would likely get you through almost everything.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 29, 2014, 01:09:15 am
content: I've done a 324 pin BGA (18 x 18) with a few voltage domains on 4-layer, but it wasn't the best, and not all IOs were used (fortunately, but also unfortunately). I wouldn't suggest less than 6 layer for that or larger.Chips designed for consumery purposes (e.g., motherboard system chips) often have rings of pins, so there's enough room to fanout all the pins with fewer layers.And if you need better signal quality, you'll need more layers to keep signals spread out and isolated.That's another thing consumer chips deal with -- SSTL for instance (used for DDR) is a low voltage, terminated standard, like LVDS in that the inputs are differential, but against a bus-wide "REF" instead of "_N"s. The drivers are stronger, and often have pre-drive (overshoot) to compensate for loss and dispersion in the circuit (due to FR-4 losses, stubs going up on memory modules, etc.). The inputs are also designed to not respond to extra transitions for a few nanoseconds, limiting the impact of really terrible signal quality. So, given that termination and stuff is still a good idea, you can get pretty ratty with the stuff and still have a functioning computer.Tim

author: marshallh
author_rank: Unknown
timestamp: September 29, 2014, 03:14:13 am
content: With big devices you absolutely need those layers to properly fanout the signals, and assortment of voltage rails. You also need good return paths for those signals which are usually quite fast. So for every 1-2 signal layers you need a dedicated ground plane. Each layer gets you another ring into the middle of the BGA.Here's the ballout for a 1152pin Stratix IV:

author: djacobow
author_rank: Super Contributor
timestamp: September 29, 2014, 03:26:09 am
content: We used to use 30+ layer boards at my old employer, Quickturn Design Systems (now part of Cadence) where we sold chip emulations systems for ASIC designers. People designing ASICs could go "in-circuit" using our emulators which were nothing more than a bunch of Xilinx parts on a board and a bunch of boards plugged into a fancy backplane that had a bunch of Xilinx parts of its own. The fully configured system was about the size of a large refrigerator, and several could be linked together to make an even bigger system. At the time, maybe the state of the art was 3M ASIC gates equivalent? I don't remember exactly.This was a lot less than the total theoretical capacity of all the FPGAs, but there needed to be a lot of extra capacity for signal routing, debug and trace logic, and of course, just to make the fit jobs run quickly. (If you are fitting 1000s of FPGAs, you really need each of them to complete in a few minutes.Anyway, my point was that we pushed the boundaries of our PCB suppliers at the time.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 29, 2014, 03:54:30 am
content: Yikes, I'll thank you not to post a typical cost of one of those systems.

author: daqq
author_rank: Super Contributor
timestamp: September 29, 2014, 08:04:42 am
content: Holy crap... and here I was, so proud of doing my first 4 layer recentlyIt's really amazing to see what kind of tech is at the top... when I grow up I want to design thoseHow much experience does it take before they let you do that kind of PCB? I'm guessing that "40 layer impedance controlled, 1k$/piece PCB designer" is not exactly an entry/fresh-out-of-college position.By the way, this item is really interesting to the thread:http://www.ebay.com/itm/VINTAGE-CRAY-COMPUTER-CORP-SUPERCOMPUTER-PCB-TECHNOLOGY-PHOTOMASK-SET-/231341967651?pt=US_Vintage_Computing_Parts_Accessories&hash=item35dd0e4923Is it really for PCBs?

author: tszaboo
author_rank: Super Contributor
timestamp: September 29, 2014, 12:04:37 pm
content: You would be surprised. When time to market is more important than the board cost, and after you placed down the decoupling caps, set the design rules and clicked the autoroute button, to make your 30+ layer board, that is when engineering kinda dies.While going from four to 6 layer, the cost is significant, going from 38 to 40 is nothing.

author: Wilksey
author_rank: Super Contributor
timestamp: September 29, 2014, 12:16:56 pm
content: So,Typically, how much does it cost to produce a 10, 12, 16, 24, 32, 40 layer board? I guess quantity is important, but what about for prototyping purposes?I must be honest, the jump from 4 to 6 seemed quite significant, the costings for 8 seems steep to say the least. I have done 6 layers, but never more.I always wonder how people can make these 8 layer super computer modules for <£100 a piece as they must buy an absolute shed load in bulk!

author: Artlav
author_rank: Frequent Contributor
timestamp: September 29, 2014, 03:10:46 pm
content: Quote from: NANDBlog on September 29, 2014, 12:04:37 pmHm.That reminds me of the proof that any graph can exist in 3D space without intersections.It went something along the lines of "give every edge it's own Z layer".Which is kind of a simple solution if the cost does not scale with layers...Just route one net per layer.Anyway, thanks all for commenting - that was... interesting to know.

author: coppice
author_rank: Super Contributor
timestamp: September 29, 2014, 05:13:37 pm
content: Quote from: Wilksey on September 29, 2014, 12:16:56 pmIts amazing what "Quote me for 100k per month for 12 months" can do to board prices. :-)

author: SeanB
author_rank: Super Contributor
timestamp: September 29, 2014, 05:26:56 pm
content: Scary was the aircraft computer that used 26 flat flex boards as interconnects to the circular bayonet connectors on the case. Power was easy, as it used a 9 pin connector and wires in a loom. the other multipole connectors though all were done using multiple layers of FFC soldered to the pins and the big bed of pins on the backplane. Standard with that was " do not break an important pin!", as you had a chance among 400 pins of one broken one being either a NC or a shielded cable ground to the ATE system, so that you could operate with it missing. Otherwise order the 26 FFC sheets ( each with it's own part number and serial number), the whole lot of connectors new, the big pins for the backplane, and a good supply of desolder braid, solder, flux and spare tip for the Weller desolder station, and you spend a week doing that. My major work on my side was easy, all wire wrapped, though to get the crisy power supplies out was major work, and often parts did not survive.BTW, wonder just how many layers it takes to route the modern i7 processor die to the socket LGA1150 that is used to connect it to the motherboard.

author: nukie
author_rank: Frequent Contributor
timestamp: October 07, 2014, 11:31:55 am
content: I know for PC desktop hardware it's usually 4 layers for cost saving, not sure about the die to LGA1150.

author: DutchGert
author_rank: Frequent Contributor
timestamp: October 07, 2014, 11:38:59 am
content: Quote from: nukie on October 07, 2014, 11:31:55 amNo that doenst sound right. I cant imagine a PC motherboard is less than 10 layers.

author: Wilksey
author_rank: Super Contributor
timestamp: October 07, 2014, 12:02:57 pm
content: I must be honest I thought PC motherboards used at least 8 layers these days, if not more.

author: ovnr
author_rank: Frequent Contributor
timestamp: October 07, 2014, 06:17:28 pm
content: Cheap PC motherboards are generally 6-layer, while more expensive ones are 8-layer. It's all designed to be easy to deal with, as opposed to many FPGAs and the like.And LGA1150 is nothing compared to LGA2011.(Something like 70-80% of the pins are power, though)

author: nukie
author_rank: Frequent Contributor
timestamp: October 08, 2014, 01:43:56 pm
content: Hi I looked at a Asus micro atx motherboard it clearly showed 1,2 top side and 3,4 on the other side. Unless I am having a great dream. 6 layer board such huge size would cost some money right?