eMMC breakout traces

Question

We are designing a picoZed carrier board with an eMMC chip and are using the same eMMC as the picoZed board.

The eMMC on the picoZed has three signals that are shown on the schematic as 'break out' signals that connect from eMMC NC (No Connect) pins to eMMC functional pins. These eMMC NC pins and corresponding connection eMMC functional pins are shown below, where the dashed line represents the breakout connection.

NC (pin Y1) ---- GND (Y2)
NC (pin K1) ---- VDDIM (K2)
NC (pin J1) ---- DAT3 (J2), net EMMC_IO3

The following note is in the schematic.

Layout Note:
These unused pins shorted intentionally to break out of BGA.

Is the intent when routing the PCB to have an outer layer trace connecting each of the above pin pairs?

What is the purpose of these connections? Probing of these signals, I suspect, possibly for characterization?

As an aside, the eMMC part number listed on the picoZed schematic and BOM is a Micron MTFC4GMDEA-4MIT in a 153 pin BGA package. However, the pin numbers on the schematic and actual PCB are for a 169 pin BGA package. I believe this is done to allow a higher capacity eMMC chip - which is only available in a 169 pin package - to be used on the SOM. The 153 pin package is a subset of the 169 pin, and the standard build of the PicoZed has the 153 pin package installed. I couldn't find this information on the schematic or user guide. If this is documented would you please point me to it?

jafoste4 · Accepted Answer

Hello Urbite,

1) Is the intent when routing the PCB to have an outer layer trace connecting each of the above pin pairs?

We utilize the NC pins on the eMMC packages to facilitate layout of the interface with common manufacturing processes. We utilize the NC pins to give us a route out of the BGA pattern. If we didn’t do this, we would have to use more advanced techniques which equates to a more expensive PCB that is harder to fabricate. It’s common for the 0.5mm pitch BGAs.

2) As an aside, the eMMC part number listed on the picoZed schematic and BOM is a Micron MTFC4GMDEA-4MIT in a 153 pin BGA package. However, the pin numbers on the schematic and actual PCB are for a 169 pin BGA package. I believe this is done to allow a higher capacity eMMC chip - which is only available in a 169 pin package - to be used on the SOM. The 153 pin package is a subset of the 169 pin, and the standard build of the PicoZed has the 153 pin package installed. I couldn't find this information on the schematic or user guide. If this is documented would you please point me to it?

The eMMC footprints on PicoZed (both PCB flavors) support higher density eMMC through a shared footprint. If you look at PicoZed PCBs you will see an outline that represents the larger package associated with the higher density eMMC devices. We have not populated the larger device package as this was meant as a customization feature and I don’t believe anyone has asked for it yet so we haven’t proactively validated it.

--Josh