PCB question

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#1
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This may be a very basic question but why do all PCB's have even number of layers? Is this because each layer should have a "return" layer or is it because of the board as a whole has better compressive/tensile strength and prevents warping?

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Single layer PCBs have ONE layer - I make that "odd" myself ;-)

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Multilayer boards are made by laminating two or more thin double-sided boards together, with insulating sheets between them, hence the even number of layers.

Tom Pappano
Tulsa, Oklahoma

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Early CRAY computers were build with 5 copper layers... But that doesn't mean anything, and nowadays probably they will cost even more than a 6 layer board anyway.

The 'return layer' is not a bad idea, but it is generally GND power plane (usually one or more copper layer devoted to just this).

The warping issue is in fact another matter, but since FR4 and copper have similar thermal expansion coefficients, this is secondary. Aluminum base boards can exhibit a much biger warping, as well as CEM1 and, to a lower exent, CEM3.

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

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That makes sense. Thanks, guys.

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npat_avr,

I once asked my board fab guy if I could mix and match the number of layers on a board. He told my that standard board mfg practices would not allow a multi-layered board to be mfg'd this way. The problem is that the board would warp if one side a had more layers on it. This of coarse drove me nuts because you can certainly have single sided boards. He insisted that the accumulation and curing of the layers on one side would create a problem.

WTFDIK?

A

Last Edited: Wed. Apr 22, 2009 - 12:04 PM
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I've been told that typically the cost of doing a board with 2n-1 layers is about double the cost of doing a board with 2n layers.

I suspect the main reason for the big price difference is just that it's completely non standard.

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which makes you wonder, if you laid out your design so that it had odd number of layers, would you have to add the extra layer to make it even? I suppose an extra GND layer would not hurt. I wonder how Gigabyte or other PC board makers do it with more complex designs.

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They layout out the design with 8/10/12 layers in mind, I think an odd number of layers is so uncommon that it even would not come up in their minds :)

Some interesting articles on stackups and other things:
http://www.hottconsultants.com/tips.html

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nice link, jay. Very informative for a newbie like me. Thanks.