Path: utzoo!attcan!uunet!aplcen!uakari.primate.wisc.edu!zaphod.mps.ohio-state.edu!uwm.edu!lll-winken!quintro!bpdsun1!rmf
From: rmf@bpdsun1.uucp (Rob Finley)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Layered M/B?
Message-ID: <1990Jun7.205435.5071@bpdsun1.uucp>
Date: 7 Jun 90 20:54:35 GMT
References: <6811@ucrmath.ucr.edu> <838@peyote.cactus.org>
Reply-To: rmf@bpdsun1.UUCP (Rob Finley)
Organization: Harris Broadcast Div., Quincy, IL
Lines: 49

In article <838@peyote.cactus.org> woan@peyote.cactus.org (Ronald S. Woan) writes:
>In article <6811@ucrmath.ucr.edu>, lulu@ucrmath.ucr.edu (david lu) writes:
>> What exactly does it mean for a motherboard to be "layered"?

>Most motherboards are indeed 3+ layered these days. Personally I have always
>felt that this is not always a good thing as you have to be aware of possible
>interference and capacitance effects.

I am a pcb designer for Harris Broadcast Products.  A multi-layer board
usually consists of even numbers of layers.

   Here is why:
        The blank PC boards start out with copper on both sides.  
        Our fab house doesn't have single sided stock available.
        It is just as expensive to do a four layer as a three layer board.
        Even numbers of layers also control warpage of the finished 
        board over time as copper on both sides exert similar stress.
        Warped boards will fail eventually and you won't enjoy it!
        The processing is the same.  If you develop one side, you are 
	going to have to put both sides in the solution any way.  

Multi-layer boards are astronomical in cost compared to two layer boards.

For capacitance, the usual stack arrangement is to use one layer to 
connect all of the grounds together.  Another layer can also be used only
for the positive (+5V) connections.  The capacitance between the two 
power supply layers actually cleans on the power supply for decoupling
some of the noise.  Having an entire layer as a conductor can also
prevent noise caused by traces that are too small for the current load 
and cause ringing.

I am not going to say that there aren't any transmission line effects
due to a ground plane running underneath those signal traces.  But,
computer similation during the layout process can point out problems
(for me, it starts at 25Mhz) and then we remove some of the ground plane
underneath that trace.  

Besides, to get the '386 that I am using to write this on a two layer
board, I wouldn't want it.  The traces would have to be so small 
to fit them in that no manufacturing process could cope with them.

Multi-Layers are a necessary part of this industry.  Between a two and
a four, I would ignore the two.  There are too many advantages.

IMHO of course.  I am also speaking for myself (didn't I just mention that...)
-----
 "Lets go kick some Earthling butt!" -- Spaced Invaders

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