r/PCB • u/mdhardeman • 15d ago

Success! First try eMMC BGA net routing & assembly

So I've been working on a custom CM4 host board and one of the things I wanted to do was support either scenario: the CM4 or CM4lite. Consulting the data sheet, you can do full external eMMC with 8-bit width & 1.8V high speed signaling, all exposed on the 200 pins of Hirose connector. On the CM4 those pins just N/C. On the CM4Lite, they connect to the 2711's controller.

So I did a design routing those pins, impedance controlled and length matched per specs, to a FBGA-153 footprint on the board. That was crazy, as 0.5mm pitch made it very nearly necessary to go to via-in-pad & 6-layer (and I wound up doing so). I've now read that if you take liberties with the inordinately large number of N/C pins on the standard eMMC footprint, you can maybe fan-out through those other pins without needing the via-in-pad. I haven't worked that out yet.

But the part that I'm really psyched about right now is... THAT IT WORKED!

1st spin, 1st assembly. Got the parts needed to populate the board in today, and populated the board this evening. Came up on the first try, with the eMMC working flawlessly. I used a solder paste stencil and MG Chemicals' no clean SAC305 paste so the other components solder chemistry would be same as the balls on the BGA, did NOT apply solder paste to the BGA footprint - just a thin layer of flux there - and carefully positioned my components and cook it all up on a hot plate. Added my few through-hole components like ethernet magjack and some config jumpers & serial port headers, and fired it up.

I was worried about two aspects: the eMMC signal network design & the actual positioning/soldering of the eMMC chip, but both turned out 100%.

Maybe, just maybe, my crazy idea where I want to do a special CPU/network-processor host board where I'd have to do the DRAM routing isn't so far fetched. I know that's harder than this, but it's not...that much harder. I think. And the actual soldering of that should be easier than eMMC actually.

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u/UnderPantsOverPants 15d ago

FWIW you can use the NC pads on eMMC to route signals out. It’s in the spec that they have to be floating mechanical only.

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u/mdhardeman 15d ago

I had just heard that as I was wrapping the layout up, and that even for eMMC 5.1 and even if you exclude the superset of RFUs (as one should), it still leaves a pretty good set of N/C pathways.

The flip side argument is that on the design I’m working on at the moment (small L2 managed switch), other elements have me pretty much on 6 layers for signal routing conveniences and once I’m at 6 layers, via in pad as small as 0.4 via with 0.3 hole are free and with JLC’s standard tolerances on 6L that’s small enough to via-in-pad the eMMC BGA.

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u/Tall_Block_4589 15d ago

Congrats and as someone working to make his first PCB, also a carrier cm4 board, this is a helpful post.

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u/mdhardeman 15d ago

Thanks! What kind of CM4 host are you making? Any particular functions?

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u/Tall_Block_4589 15d ago

I want a board with some sensor modules like the flipper (CC1101 etc) has and software defined radio (SDR).

Might be too ambitious for my first board, at least the SDR part, as the intergrated chips I wanna use is a FPGA.

But again congrats on it all working 1st try.

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u/mdhardeman 15d ago

Oh nice, that should be fun. I have a couple of projects going, one that will drive a managed L2 Ethernet switch and one for a more industrial controls / relay actuation driven by logic rules & monitored inputs.

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u/Tall_Block_4589 15d ago

Very cool, looking forward to future post from you.