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8 Layer Board MCU problem

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taring77

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Hi all
i have a 8 layer board problem. its an mcu board with tis chip
IC, MCU, MC9328MX21 -MAPBGA-17x17-289pins frm freescale, a couple of DRAM, Flash, Lan chip etc. When i change the address n data to different layer its not werking well. there is no movement in components. Any one can advise?
Top component - MCU chip & 2xSDRAM
Bottom Component - Flash Lan chip
Omitted a resistor netwerk for new stackup


NEW layout stack up (that hangs alot)
L1 - Address + normal traces to other components
L2 - normal traces but nothing (except ground plane) around the address & data areas (blind via 1-2 for other fanning)
L3 - Data Address fanning (BLind via L1-L3)
L4 - normal traces but nothing (except ground plane) around the address & data areas
L5 - Address + normal traces to other components
L6- Mostly gnd plane
L7- Power Plane
L8 - traces to flash + normal traces to other components


Old layout stack up (that werks but needs improvement)
L1 - Address + Data+ normal traces to other components
L2 -Address + Data+ normal traces to other components (Blind via 1-2)
L3 - Pure Ground Plane
L4 - Data+ normal traces to other components
L5 - normal traces but nothing (except ground plane) around the address & data areas
L6- Data+ normal traces to other components
L7- Power Plane
L8 - traces to flash + normal traces to other components

any comments thanks
 

Which is the question?
 

Well, it would be hard to diagnose a problem without seeing how traces are routed (perpendicular lines, mis-matched traces, lack of impedance control, etc...) but there are two things that I see that appear to be better in your first version:

- placement of plane layers (surrounding some signal layers helping with high speed signal return paths)
- the comment "Mostly gnd plane" as GND plane layer should ONLY be just that, although you are probably routing some traces in areas away from high speed area


This is all that I can think of. If you are sure you didn't change a net anywhere then maybe you are forced to order a new board with a stackup similar to the old one?
 

my question is, how come with new stach up there is a problem.

so in another words a fully ground plane 'could' be a major factor?
what if i add additional layer to 10 layers and add a ground plane in btw will it help?
some say 10 layers might not be too good not sure y.
 

A continuous ground plane is important under all signals, but is absolute under any high speed signal. If you were using the ground plane layer for traces that was off to the side and your processor/memory was way on the other side, I'm sure you can get away with it. Two layer boards run without a ground plane, but you can't go fast on them, generally speaking.


10 layers could help, but I also think it is important to analyze the stack up of the board. I think that is the real source of your problems. I forget what speed the i.MX21 tops out at (266MHz? 400MHz?), but I am sure it is fast enough (clock speed, slew rates) that using impedance control on 8 layers would work just as well. Also, trace length matching might be important to re-examine if you are not careful there. I don't know if you were concerned about any of this when you made these boards?

Again, it's hard to judge but I think there was a project that uses the i.MX21 on six layers. I forget the product but it is an open sourced box that sits next to your bed like an alarm clock and it has an LCD. Maybe someone else knows it and if gerber files are available you could see what they did.
 

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