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From: email@example.com (Bob Wilson)
Subject: Re: Multilayer PCB:s, HOW?
Date: Thu, 19 Sep 2002 04:29:22 -0000
Organization: Your Organization
X-Newsreader: WinVN 0.99.9 (Released Version) (x86 32bit)
In article , firstname.lastname@example.org says...
>"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 (?)
Actually, its palladium. Initiated by an initial stannous deposition.
The palladium acts as a catalyst to initiate the autocatalytic deposition of
the electroless copper. Almost none is required (there is certainly not even
a contiguous layer of palladium to begin the process of electroless
deposition). Copper starts depositing at the sites of palladium, and
continues autocatalytically until the copper sites grow into a contiguous
layer. When sufficient copper is depositied to achieve a very thin
contiguous layer, the drilled laminate is transferred to a copper
electroplating bath that builds up the full layer of copper in the holes.
This is because electroless plating is a very VERY slow process, and is let
go only as long as needed to get full copper coverage. After that much
faster electropating is used to speed up things.
>sides of the via holes, which are then plated up with a couple of microns
>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
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