Raspberry Pi PCB Layout Revealed 112
An anonymous reader writes "Yesterday, the final Raspberry Pi printed circuit board (PCB) layout was revealed. The word 'packed' comes to mind as this is one very complicated looking board. The reason for that is just how much Raspberry Pi has strived to save money on the machine by using complex routing to keep things small and cheap. The Raspberry Pi team don't believe the design is going to change again unless they missed something. With that in mind, they revealed the final board is exactly the same size as a credit card, measuring 85.65 x 53.98mm."
Re:Features? (Score:5, Informative)
While it's cool that they got the cost so low I'm kind of sad to see all those SMC's, kids today can't get into building electronics because so much stuff has gone to stuff that you just can't solder by hand. Yes, I know you can still use microcontrollers with breadboards, which is cool if you want to make a simple robot, but stuff like building your own computer that you can hook up to your TV and use like any other computer would be very cool as well.
Not so packed (Score:4, Informative)
Really, it's not. I do stuff like this every day. It looks pretty normal for a 4-6 layer board with a BGA or two on it. TFA needs to learn about what modern design standards are. It's only complicated if you still lay boards out with ruby tape or a sharpie.
Re:Amazing (Score:4, Informative)
Re:Features? (Score:5, Informative)
While I'm with you on this on many levels (remember building things with the 4000 series? Yeah, we don't do that anymore. Haven't since PICs. We just write some code that does the job much better), I wouldn't say that kids can't get into it anymore.
SparkFun, for example, regularly organizes PTH and SMD soldering classes as well as offering kits for both. Some SMD you can solder by hand quite easily, others you can get a nifty breakout board that lets you easily seat the SMD IC and melt solder up to its leads ( http://www.youtube.com/watch?v=-32orELxkpE [youtube.com] ), and yet others you get some solder paste, a syringe or a toothpick (seems popular), put the paste on the pads or dip the leads in the paste, put the part on the PCB, and then stick it all into your toaster oven.. or on a skillet.
Of course for most kids, just playing with e.g. Arduino and some shields/sensors is going to be a great way to get into electronics in the first place.. then when they need something that's not on the market they can explore PCB design, soldering, etc.
Re:Complicated? (Score:5, Informative)
I would go as far as to say that looks like one of the simplest and least complicated designs I've seen. Also it should be noted that small and cheap compete with one another. Cheap things they that cost space- fewer routing layers (components & traces often need to be farther apart for impedance & via room), using larger components (0402 or bigger generally), not using blind/buried vias, using routing space for power. Small things they did that added cost- front/back side assembly, through hole components on a mostly SM design.
It looks like a fairly simple design. I'd try to get rid of the through-hole stuff unless it's just debug, that adds a step in mfg which can raise cost and also causes place keepouts to eat up valuable real-estate.
The post should have raid "Board layout review, all slashdotters attend".
Re:Complicated? (Score:5, Informative)
Actually looks pretty slack with lots of space. However to make it inexpensive requires much more care in the design rules and routing. Placing and routing a board with tight component clearances and tight trace and space is easy and expensive. Taking the same components on a small board from 0.1/0.1mm trace and space to 0.15/0.15mm trace and space takes a lot of work, but can significantly reduce the cost to manufacture.
From an initial view, the biggest cost adder I see is components on the solder side. There don't seem to be too many on the bottom side and with a bit more work it could probably be made into a single sided board. I'm working on a cost sensitive board right now, and one of the big things we've done to cut cost is make sure all components are on the top side. (Low cost is relative, this BOM is many many times the projected price of the R-Pi.)
Re:complex routing ? (Score:5, Informative)
As I understand it, the biggest challenge was escaping a 0.65mm BGA without using significant amounts of HDI on a 6-layer board, while keeping good solid power and ground planes and large (i.e. cheap) track and gap specs. Relax more or more of those and it is indeed trivial - our alpha boards were done in about four days by doing exactly that.
Comment removed (Score:5, Informative)
Embiggen... (Score:3, Informative)
a perfectly cromulent word
Re:complex routing ? (Score:5, Informative)
Off the top of my head, we save around a buck at 10K-off through a combination of 6 layer, coarser T&G and limited HDI. Figures for UK manufacture; YMMV in elsewhere, particularly in the far east (where cutting edge volume manufacturing is much easier).
The particular stack-up we've chosen is only one possible cost minimum; I've heard it suggested that 8 layers with zero HDI is quite competitive for 0.65mm BGA.
Re:Complicated? (Score:3, Informative)
It doesn't really matter what the chip costs alone, as you won't even get a fucking pin diagram out of broadcom without a large order and an NDA.
Re:Complicated? (Score:5, Informative)
Generally if the small caps are close to the package and tied to planes (I'm assuming there are planes) with short thick ties to reduce inductance you can get by with it just fine. The bulk caps can be quite a ways away as long as they are also tied directly to the planes. We're running some very high speed stuff this way without problems. Xilinx has some good info on bypass caps and how they can be placed in their Spartan 6 docs.
If there's no planes then you have to have the relatively thick tracks already for current carrying capability, but the inherent inductance could possible give you an edge in filtering as long as you're not yanking the individual pin levels out of tolerance.
Re:How can you even tell? (Score:5, Informative)
Most of those questions are answered in the comment thread on the article. No individual layers released until their PCB designer gets back. The picture shown does not include power or ground planes, so the missing ground is likely hidden. The connectors being used will require some through-hole components. The GPIO headers will be on the final release, but unpopulated.
The biggest omission to my mind is the lack of mounting holes or other fixtures. (I'm not sure where you see "plenty of empty space". Even getting screw holes to fit would require some thought, it seems to me.) The screenshot was also pretty useless for determining the exact mechanical placement and dimensions of the connectors, which is the only important thing for those designing cases. Someone in the comment thread did mark and label the rough outlines of the connectors, though. The connector placement also seems not at all designed for usability, or with any thought to future case design but purely to make the cheapest possible board.