Questions about PCB stackup and parameters for calculating impedance

[zuzu]

Hello friends,

I've designed a lot of PCBs over years, including some simple multilayer (4-6 layers), but never faced problems to dig deep into stacking fabric (let manufacturer handle). But now we have to make a PCB with some diff. sensitive traces @100...200Mbps, so we need to calculate impedances involved. I'll summarize the questions for kindly answers:

1. We need 8 layers, probably more since 1 is GND, and at least other 2 are PWR (I think more GNDs anyhow for crosstalk reasons). There is a magic number of layers from manufacturer perspective, technology? Ie. odd, even, or we can just pick-up 8-10 layers? Can somebody enumerate a stackup example including type (prep/core)?

2. We use OrCAD. I saw 2 technologies proposed: cores and builtup. Which is most common? Some advices please.

3. For calculating impedances, I need to know thickness of inners and er. For thickness I read 1Oz is common value but just to make sure, isn't too thick (1.4 mil)?! I saw in many app. notes a common value for er is 4.2 (FR4), this is correct? (ie. this is correct value for 1-200MHz?).

Thanks very much in advance. Appreciate.

 

[senilicus]

Re: PCB stackup question

Zuzu

The first thing you need to do is to determine how many signal layers you need. Then find out how many Power/Ground layers you need for your Power distribution and impedance layers. Also find out what core and prepeg thicknesses your pcb fabricator has in stock. ( using non-stock materials will increase the price and delivery time)
Also find out what the final pcb thicknes should be.

When you have the number of layers you can define the stackup. Make sure that the BTM half of the stack is a mirror of the TOP half of your stackup.

Now you have to calculate the core/prepeg thicknesses and impedances you require. The most easy way to follow is to ask your pcb manufacturer to do the calculations for you. They have all the info they need to do impedance calculations for you.

The hard way is to do it it yourself, but you need software like Polar's si9000 and you need to know material properties.

Most PCB/SI software also have software to do these calculations.

 

[robertferanec]

Re: PCB stackup question

I don't normally design PCB stackup - PCB manufacturer may have problems to build it.

This is how I work when I do processor board design.
I start with layers like Signal/GND/Signal/Signal/GND/POWER/POWER/GND/Signal/Signal/GND/Signal
I use 0.1mm/0.1mm (Minimum track / minimum gap) track geometry for everything (except powers) and minimum via 0.45mm/0.2mm (via pad / via drill).

I do layout and try to use as few layers as I can. For example my final PCB may look like:
Signal/GND/Signal/PWR/GND/Signal/GND/Signal

I connect all the pins. Once I am sure I have enough layers, I send request to our PCB manufacturer. I provide number of layers with Signal/Reference Plane position, list of required impedances, minimum track/gap/via requirements and ask them for track geometry and complete stackup.

I didn't answer your questions directly, but I hope my answer helps.

Edited 15-Aug-2011: I have written a post which may interest you How to design PCB stackup - (FEDEVEL)

 

[loosemoose]

Re: PCB stackup question

here is a link for 8 layer stack-ups.

PCB Stack-Up - Part 4
might be helpful