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April 2007

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TechNet E-Mail Forum <[log in to unmask]>, DDFRITZ <[log in to unmask]>
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Mon, 30 Apr 2007 09:14:23 -0400
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Adam, 

You have many parts of the semi-additive process outlined, but not all.  You have the concept of thin copper starting foil OK, but the thin copper can be made by at least four ways:
It can be very thin rolled or electroplated copper, much thinner than the half-ounce 17 micron you point out.  Quarter ounce - 9 micron and 5 micron are pretty common, and I have heard of 3 micron.  Brian Ellis points out that this thin foil can be carried on a peel-off sheet like aluminum.  
Foil can be thinned (and is done that way in Japan) to get down to 3-5 microns.  Etching control is very important here
Un-clad laminate can be catalyzed electrolessly plated to give a 1-2 micron starting copper thickness.  This was common 20 years ago, but the adhesion of the trace to the laminate is created by the fab shop, and most do not like this burden of reliability.  Creating the "tooth" for bonding the copper is tricky. In fact, one of the good ways to create tooth is to laminate thick copper foil, etch it all off, and then start with the bare epoxy and electrolessly plate that
Sputtering - mechanical deposition of copper over a "tie" layer like molybdenum.  This is very standard in flex circuit fabrication for "adhesiveless" foil.  
From here, you pattern, electroplate up the copper foil you want and then "wash etch" the background copper to isolate the traces.  

Direct metallization will work on the holes and microvias with all these techniques, but it will not work well with the third alternative above.  The "knit rate" for copper electroplate over direct metallization is not fast enough for most horizontal platers, and if you have NO foil on the board, where do you start the copper electroplate from?  Direct metallization works fine where you have two starting sheets of copper foil and only have to knit the holes.  But it is exceedingly difficult where the is no starting foil anywhere on the surface. 



In a message dated 04/30/07 04:09:40 Eastern Daylight Time, [log in to unmask] writes:
I am trying to get a basic understanding on the "semi additive" process. 
From what I understand, the copper base foil begins at 5um, and circuit 
traces are electroplated to a desired thickness. Unlike conventional 
"pattern plating" , there is no metal etch resist, but instead the 
entire panel is etched just enough to remove the 5um copper base copper, 
leaving the relatively thicker traces still in tack. 
My question is, what steps are involved in obtaining the 5um copper base  ? 

The thinnest copper cladding I can find is 18um (1/2oz) which suggests 
to me people are deposited 5um of electroless copper on blank PCB 
insulating material. 

If this is the case then will any of the direct metalization systems be 
usable ? Due to the plating propagation effect with direct metalization, 
there would be excessive variations in copper thickness over the whole 
panel. 
The only alternative I can see is to take standard 18um copper clad and 
carefully etch down to 5um. The resulting 5um copper clad laminate could 
then be used with direct metalization systems, however hole walls will 
not have a 5um base, and therefore the hole wall plating finishes up 
being 5um thinner than traces. 


regards, 

Adam Seychell 

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