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Platform forum

Tí , 10-18-2024, 10:07 AM

My eMMC is the 153-ball type, with a pin pitch of 0.5mm and a minimum via size of 0.2mm/0.4mm. The PCB manufacturer does not allow vias under the pads, and there is not enough clearance to fan out externally. Can I connect DATA, GND, etc., to the NC pins to fan out externally as shown in this diagram? I did not choose to remove pin B1 or C1 because I was afraid that during SMT, the solder ball in that area might melt and spread uncontrollably, causing short circuits.

Kristóf Kanics , 10-18-2024, 01:01 PM

Is there a layout example in the datasheet? There must be a way to layout it.
NC means do not connect, that means you should not connect it. Some datasheets allow you to connect it to GND or to the positive supply for better thermal behavior, but definitely not to other signals.

Kristóf Kanics , 10-18-2024, 01:02 PM

Either you need vias, or smaller tracks, or smaller clearance, or smaller diameter for the pads. One of them will be the solution.

Kristóf Kanics , 10-18-2024, 01:08 PM

You could use vias between pads. 0.4/0.2mm is a normal via size, which seems to be too large in your picture to place it between pads, but you may use micro-vias (costs more when manufacturing).

Tí , 10-18-2024, 11:08 PM

As I said, the PCB manufacturer does not allow placing vias under the pads of the balls, and there is no space for a normal fanout, so I have to do it this way

Robert Feranec , 10-20-2024, 06:16 AM

I have done this multiple times, not optimal but sometimes necessary and worked ok. Just be sure these pins are really not connected on ANY of the memory models you will be fitting on your board. Also be sure, not connected means really not connected as sometimes the pins that are not used in schematic still may be connected internally - e.g. they can be used for production testing.

Nixy , 10-20-2024, 08:43 AM

Is there a reference board for the chip? Can you build a mini board ala JLC with just this chip and enough parts around it to test if that works

Kristóf Kanics , 10-20-2024, 02:09 PM

No vias under the pad does not mean no vias between them, that's why I wrote it. I don't know if they fit, I don't have your numbers, just for your consideration.

QDrives , 10-21-2024, 08:39 PM

1) Am I the only one that reads NC as Not Connected inside the package?
If it were used for testing purposes by the manufacturer, it should be named "reserved".
NC does not stand for Do Not Connect.
It probably is connected to a lead frame, and placing high speed signals on it will both influence the signal itself as well as potentially other signals internal (cross talk). The question is: how much.

2) The fabricator will **not** state "not allow vias under pads".
What may be, is that you need filled and plated vias, or copper filled vias, or small size drill holes, or micro vias, or SMD vias (Solder Mask Defined)
All of which costs extra money, but "allow" is the wrong word.