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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Multilayer PCB:s, HOW?
Date: Thu, 19 Sep 2002 01:10:00 GMT
NNTP-Posting-Date: Wed, 18 Sep 2002 18:10:00 PDT
Bill sloman wrote:
> "Roger Johansson" wrote in message
> > I have never worked with multilayer boards, and it just struck me
> > that it is a bit mysterious, how they make such boards.
> > If I imagine a 4-layer board, with +V and ground on the inner planes,
> > then it is impossible to use through holes unless you want to short at
> > least the two middle layers.
> > There is no way to etch these inner layers.
> > So these planes must be etched before the layers are put together.
> > It is topologically impossible to use two middle layers which cover
> > the whole area.
> > But multilayer boards look like the are moulded in one piece, so I
> > have never before imagined that they are created by gluing
> > ready-etched layers together.
> > But that is the only way I can imagine that it works.
> > Maybe I have missed something?
> > How do you create such multilayer boards?
> > And how can you be sure that a pin in a through hole makes contact
> > with a layer of copper it can only see as a very thin ring of copper
> > inside a hole?
> Multilayer boards are made as multiple pairs of thin double-sided boards
> which are eventually sandwiched with thin (not necessarily equally thin)
> layer of partially cured board material, and the whole thing cured as a
> After you'd got to this point you drill out the via holes, and use
> electroless plating to put a thin "seed" layer of platinum (?) onto the
> sides of the via holes, which are then plated up with a couple of microns of
> copper. where the drill as gone through copper on one of the inner layers,
> the copper is smeared out around the drill hole and makes reliable contact
> with the plated via.
> This is all second-hand, but I think it is right. For loads of extra money
> you can get "blind vias" whch don't go all the way through the board. I've
> no idea how they do that - I'd guess that they glue the boards together in
> stages and drill and plate the blind vias in the sub-stacks, one sub-stack
> at a time.
> We've got a couple of regulars who are in the business of making printed
> circuit boards, and I'm half hoping to be corrected ...
> Bill Sloman, Nijmegen
The fact is that the boards have to glued after etching because design
rules allow routing on any layer. Generally, I do not allow our board
house to route in sublayers but I do allow split planes. We mostly use 5
layers to get two voltage planes and one ground plane. I do not allow
actives on both surfaces but I might for an "in form factor board." In
that case I may allow routing on an underlayer.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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