4 layer pcb, questions.

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It is time for me to do a 4 layer pcb, I have experience with 2 layer but I am a 4 layer virgin.

Where should I put the ground layer? Should I have 2 ground layers, 1 below each signal layer?

What kind of trace directionality (if that is a word) should I use for the middle layers?

In eagle are there certain layer numbers I should use for the middle layers?

Will an autorouter do a really good job at 4 layers? I assume that it would do a better job with 4 layers as opposed to 2 layers because it has lots more options.

Tips and pointers are welcome.

Regards,

Alan To

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The typical stackup for a 4 layer board would be power & ground for the inner 2 layers, and then the signals on the outer 2 layers. Normally one would route the 2 signals layers in perpendicular fashion. It's not as critical if the layers are separated by power and ground, but if you have signals on adjacent layers it becomes more important, so that you minimize crosstalk.

As for numbering one usually goes from 1 to n starting at the top going down to the bottom. This is on;y convention, you can do whatever you like, as you'll provide the stack-up info when you send the files for production. (following the convention, will reduce any risk of production error, if the stackup order is critical)

Writing code is like having sex.... make one little mistake, and you're supporting it for life.

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<< Where should I put the ground layer? Should I have 2 ground layers, 1 below each signal layer? >>

Unless you're designing for stringent EMC problems (probably not), just do what glitch is saying and have one layer be power, the other ground.

<<What kind of trace directionality (if that is a word) should I use for the middle layers? >>

The power and ground layers don't get routed. They are power planes. Eagle lets you define power planes (example $+5V or $gnd). Read about it in the manual if you're not sure how to do this.

<<Will an autorouter do a really good job at 4 layers? I assume that it would do a better job with 4 layers as opposed to 2 layers because it has lots more options. >>
Eagle's autorouter is pretty nice. I use it for all my designs (2 layers to 8 layers). As stated, you won't autoroute the inner layers since they are power planes.

Let us know if you have other questions.

-Paul

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Glitch wrote:

Quote:
The typical stackup for a 4 layer board would be power & ground for the inner 2 layers, and then the signals on the outer 2 layers.

Believe it or not, I jumped right into a 4 layer board with FreePCB and Advanced Circuits on my...second project and did exactly as Glitch recommended (Yeah!)

I am unfamiliar with Eagle but, FreePCB allows you to layout (via highlighted "hatch" or other layout of your choice) the size of the inner planes. Then, you can use "stubby" little via's going just a short way from a pad and right to the ground (or power) plane! My program has default clearance settings for passing a power via through the ground plane but, you can change these settings if you wish.

As easy as four layer boards can be to layout manually (which I always do), I later found that I rarely need the extra expense of the four layers and honestly, have NEVER seen a four layer production board in any piece of equipment I have serviced or installed.

John

Just some guy

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toalan,

This is a good resource for PCB info. They also have a forum there where you can ask PCB questions.

http://www.pcblibraries.com/resources/GEN-docs.asp

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>2 layers are usually needed only if you are using high density/high pincount components (QFN, BGA, etc..) and layout necessitates a small board size...

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4 layers is also necessary sometimes to achieve conformity with EMC requirements.

Leon

Leon Heller G1HSM

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I had to redesign two PCB's from 2 to 4 layers just due EMC problems. And it worked like a charm, and speeds up desig, since it is much easier with less nets (power lines and clean GND planes) and more room.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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I do a lot of four layer boards - my boards are typically small and very tightly packed. I don't use the power plane functions because I can't route any tracks on a plane, and I generally need at least one extra layer to get the signals around. So I create two inner layers, use them as sparingly as possible for track routing, and finish by putting a VCC polygon on one and a GND polygon on the other. With a bit of care I can usually avoid putting any tracks on the ground layer, so it ends up a complete plane anyway.

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One note about multi-layer boards. At least with the board house I've used (pcbcart), the cost goes up fast if you start using buried or mico vias. Those are vias where the hole doesn't go all the way through the board.

-Brad