i.MX8M Plus PCB Design

[Wobaffet]

Hello,

In our application we would like to use the reference design for i.MX8M Plus. The evaluation kit has two PCB's. One is a compute module and the other one is base board. For our design, we would like to merge them into a single PCB but the PCB thickness for compute module (CPU) 1.2mm, the other one is 1.6mm. What is your suggestion in our case? Thank you in advance.

[Wobaffet]

There are different packages for RAM as following. I think the difference between them is thickness of the device and ball path diameter (Ø). Can we use these packages in the same layout because depending on the capacity of the RAM, package code changes. We would like to have different variants for the same PCB.

Package Codes
DS = 200-ball WFBGA 10 x 14.5 x 0.8mm (Ø0.35 SMD)
DT = 200-ball VFBGA 10 x 14.5 x 0.95mm (Ø0.35 SMD)
NP = 200-ball WFBGA 10 x 14.5 x 0.8mm (Ø0.28 SMD)
NQ = 200-ball VFBGA 10 x 14.5 x 0.95mm (Ø0.28 SMD)

[Manuel_Salas · Accepted solution]

I could not any documentation of the Path thickness, for IMX8MP is used the "DT = 200-ball VFBGA 10 x 14.5 x 0.95mm (Ø0.35 SMD)".

Any way, the changes are not significant, in all case, the most critical case is the "NP = 200-ball WFBGA 10 x 14.5 x 0.8mm (Ø0.28 SMD) ", but as I said, the changes are a little insignificants.

[Wobaffet]

I'd like to copy CPU board layout into my custom board. But the thickness for CPU board is 1.2mm and our custom board is 1.6mm.

In our case would the impedances change?

Do we need to modify the layout?

[Manuel_Salas]

No problem, the impedance is affected but in a "good way" decreasing, and a little more of heat dissipation.

Best regards @Wobaffet !

[Wobaffet]

• If so is only delay matching of traces important for LPDDR4 and eMMC? Isn't there a trace impedance requirement for these signals?
• Should the material of the PCB (TU768) be same as the NXP evaluation kit?
• For other signals (LVDS, Ethernet etc.) should be impedance controlled? Is this correct?

[Manuel_Salas]

1. The LPDDR4 impedance is controlled for the PHY. So, remember when you have your first custom board you will do the DDR test for be sure that is working.
2. Yes, the material TU768 should be the same.
3. And yes, you are right, the impedance should be controlled.

[Wobaffet]

I've read the document. I'd like to copy CPU board layout into my custom board. But the thickness for CPU board is 1.2mm and our custom board is 1.6mm.

In our case would the impedances change?

Do we need to modify the layout?

[Manuel_Salas]

Hello!

You can refer to the Hardware Developer’s Guide (table 14, PCB recommendations)

The Hardware Developer’s Guide recommendations apply to a board of any size. There is not a defined rule for PCB thickness.

Best regards @Wobaffet !