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Raspberry Pi PCB Layout Revealed
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."
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".
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.
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.